C9orf72 Frontotemporal Dementia and/or Amyotrophic Lateral Sclerosis

Helena Gossye; Sebastiaan Engelborghs; Christine Van Broeckhoven; Julie van der Zee

C9orf72 Frontotemporal Dementia and/or Amyotrophic Lateral Sclerosis

Synonym: C9orf72-FTD/ALS

Gossye H, Engelborghs S, Van Broeckhoven C, et al.

Publication Details

Estimated reading time: 33 minutes

Summary

Clinical characteristics.

C9orf72 frontotemporal dementia and/or amyotrophic lateral sclerosis (C9orf72-FTD/ALS) is characterized most often by frontotemporal dementia (FTD) and upper and lower motor neuron disease (MND); however, atypical presentations also occur. Age at onset is usually between 50 and 64 years (range: 20-91 years) irrespective of the presenting manifestations, which may be pure FTD, pure amyotrophic lateral sclerosis (ALS), or a combination of the two phenotypes. The clinical presentation is highly heterogeneous and may differ between and within families, causing an unpredictable pattern and age of onset of clinical manifestations. The presence of MND correlates with an earlier age of onset and a worse overall prognosis.

Diagnosis/testing.

The diagnosis of C9orf72-FTD/ALS is established in a proband with suggestive findings and a heterozygous abnormal G₄C₂ (GGGGCC) hexanucleotide repeat expansion in C9orf72 identified by molecular genetic testing.

Management.

Treatment of manifestations: Care is often provided by a multidisciplinary team that includes a neurologist, specially trained nurses, pulmonologist, speech therapist, physical therapist, occupational therapist, respiratory therapist, nutritionist, psychologist, social worker, and genetic counselor.

Surveillance: Routine follow up by multidisciplinary specialists to monitor neurologic findings, mobility and activities of daily living, psychiatric/behavioral manifestations, nutrition and safety of oral feeding, respiratory and bladder function, and needs of affected individuals and care providers for psychosocial support.

Genetic counseling.

C9orf72-FTD/ALS is inherited in an autosomal dominant manner. Almost all individuals diagnosed with C9orf72-FTD/ALS inherited a C9orf72 G₄C₂ repeat expansion from a heterozygous parent. In most families the heterozygous parent is affected; however, a heterozygous parent may not have clinical manifestations of the disorder due to age-dependent reduced penetrance. Each child of an individual with C9orf72-FTD/ALS has a 50% chance of inheriting the C9orf72 G₄C₂ repeat expansion. Once a C9orf72 G₄C₂ repeat expansion has been identified in an affected family member, prenatal and preimplantation genetic testing for the presence of the C9orf72 G₄C₂ repeat expansion are possible. (Note: The presence of a C9orf72 G₄C₂ repeat expansion cannot predict the disease course in any given individual.)

Diagnosis

In this GeneReview, C9orf72 frontotemporal dementia and/or amyotrophic lateral sclerosis (C9orf72-FTD/ALS) refers to the spectrum of phenotypes caused by C9orf72 G₄C₂ pathogenic repeat expansions, also sometimes referred to as the C9orf72 FTD/ALS complex.

Suggestive Findings

C9orf72-FTD/ALS should be suspected in probands with the following clinical and neuroimaging findings and family history [Van Mossevelde et al 2018, Cammack et al 2019, Moore et al 2020].

Clinical Findings

Age at onset ranges from 20 to 91 years, with a mean of 58 ± 8-10 years [Van Mossevelde et al 2017a].

Neurologic findings (See also Table 1.)

Frontotemporal dementia (FTD), the most common clinical presentation, is characterized by progressive behavioral impairment, decline in executive function, and/or language impairment (see Table 1). Of the three FTD clinical syndromes, behavioral variant FTD (bvFTD) is more often present than the two language variants (collectively identified as primary progressive aphasia [PPA]: semantic variant PPA [svPPA] and non-fluent variant PPA [nfvPPA]).
Manifestations specific to C9orf72-FTD include prominent neuropsychiatric symptoms, such as hallucinations and delusions. Often, some parkinsonian features are present.
Motor neuron disease includes both the upper and lower motor neuron involvement that characterizes amyotrophic lateral sclerosis (ALS) as well as upper and/or lower motor neuron dysfunction that may or may not fulfill criteria for the full ALS phenotype.
Atypical presentations mimicking other neurodegenerative disorders

Table 1.

C9orf72-FTD/ALS: Frequency of Diagnoses Based on Clinical Findings Alone

Neuroimaging

Brain MRI. The pattern of atrophy in C9orf72-FTD is remarkably symmetric and generalized. Cortical atrophy can be seen in the frontal and temporal regions, also the insular and cingulate regions, as well as more posterior cortical areas. Also notable is involvement of subcortical structures and the cerebellum [Cash et al 2018, Van Mossevelde et al 2018].

Brain FDG-PET. The pattern of predominant frontotemporal hypometabolism is mostly congruent with the atrophy patterns seen on brain MRI; however, FDG-PET abnormalities can usually be detected earlier than suggestive brain MRI findings [Greaves & Rohrer 2019].

CSF biomarkers. Nonspecific abnormalities in Alzheimer disease CSF biomarkers, such as (slightly) increased levels of total tau or decreased levels of amyloid beta 1-42, may or may not be present [Niemantsverdriet et al 2018].

Family History

Family history may be positive and consistent with autosomal dominant inheritance (e.g., males and females in multiple generations with ALS, FTD, and/or other manifestations within the C9orf72-FTD/ALS spectrum) or the family history may be negative; absence of a known family history does not preclude the diagnosis. C9orf72 G₄C₂ repeat expansions are to date the most frequent cause of ALS and FTD in individuals representing simplex cases (i.e., a single occurrence within a family) [Van Mossevelde et al 2018].

Note: Simplex cases are sometimes referred to as "sporadic cases"; however, because the term "sporadic" can imply a non-recurring (non-genetic) cause, the term "simplex" is preferred.

Establishing the Diagnosis

The diagnosis of C9orf72-FTD/ALS is established in a proband with suggestive findings and a heterozygous abnormal G₄C₂ (GGGGCC) hexanucleotide repeat expansion in C9orf72 identified by molecular genetic testing [DeJesus-Hernandez et al 2011, Renton et al 2011, Gijselinck et al 2012] (see Table 2).

Note: Pathogenic G₄C₂ repeat expansions in C9orf72 cannot be detected by sequence-based multigene panels, exome sequencing, or genome sequencing.

Repeat sizes

Normal. Range from 2 to 24 G₄C₂ repeats
Uncertain significance. Range from 25 to 60 G₄C₂ repeats
- Repeats in this range are rare in the general population and typically do not segregate in families with C9orf72-FTD/ALS.
- The shortest G₄C₂ repeat identified in white blood cells and reported to cosegregate with the disorder in a family with C9orf72-FTD/ALS was 47 G₄C₂ repeats [Gijselinck et al 2016]. However, in different brain regions of the individual who was heterozygous for the short expansion, a pool of short and long expansion sizes (>1100 repeat units) was apparent, pointing to somatic mosaicism [Gijselinck et al 2016].
Pathogenic. Range from 61 to >4000 of G₄C₂ repeats
Pathogenic expansions >60 to hundreds or thousands of G₄C₂ repeats show age-dependent reduced penetrance [Murphy et al 2017].

Molecular genetic testing relies on targeted analysis to characterize the number of C9orf72 G₄C₂ repeats (see Table 8).

Table 2.

Molecular Genetic Testing Used in C9orf72-FTD/ALS

Clinical Characteristics

Clinical Description

C9orf72 frontotemporal dementia and/or amyotrophic lateral sclerosis (C9orf72-FTD/ALS) is characterized most often by frontotemporal dementia (FTD) and upper and lower motor neuron disease (MND); however, atypical presentations also occur. Mean age at onset is usually 50-64 years (range: 20-91 years) irrespective of the presenting manifestations, which may be pure FTD, pure ALS, or a combination of the two phenotypes. The clinical presentation is highly heterogeneous and may differ between and within families, causing an unpredictable pattern and age of onset of clinical manifestations (see Table 3). The presence of MND correlates with an earlier age of onset and a worse overall prognosis [Van Mossevelde et al 2018, Moore et al 2020].

Like the age of onset, life expectancy is highly variable and mainly associated with the clinical manifestations.

Table 3.

C9orf72-FTD/ALS: Frequency of Disease Features

Initial manifestations may be pure FTD or ALS; additional manifestations in the C9orf72-FTD/ALS spectrum may appear during the disease course [Van Mossevelde et al 2018, Moore et al 2020].

FTD

The three main FTD clinical syndromes are behavioral variant FTD (bvFTD), semantic variant primary progressive aphasia (svPPA), and nonfluent/agrammatic variant PPA (nfvPPA). Most individuals with C9orf72-FTD and C9orf72-FTD/ALS present with bvFTD.

Cognitive deficits associated with FTD are mostly early loss of executive functions, memory impairment, and language problems (mostly dynamic aphasia). Other findings, such as parietal lobe involvement (dyscalculia, apraxia), are common as the disease progresses.

C9orf72-bvFTD includes most of the typical bvFTD behavioral changes: early disinhibition, early apathy or inertia, early loss of empathy, as well as repetitive and ritualistic behaviors. Although sweet food preference occurs, it is less common. Other prominent neuropsychiatric symptoms include early delusions and hallucinations, psychosis, and anxiety.

C9orf72-PPA presents with early and prominent language deficits – speech apraxia and frequent grammatical errors in the more common nfvPPA or decreased understanding of language in svPPA.

ALS

The entire clinical spectrum of ALS (which includes abnormal muscle tone and tendon reflexes, fasciculations, muscle cramps, and gait disturbances) may be present in C9orf72-ALS and C9orf72-FTD/ALS. Spinal onset (involving limb muscles) is more frequent than bulbar onset (including involvement of swallowing and speech) in C9orf72-ALS (54% vs 39%). Some early cognitive impairment may be present even in individuals who had been diagnosed with pure ALS [Cammack et al 2019]. See Amyotrophic Lateral Sclerosis Overview for a definition of this phenotype.

Atypical Presentations

Atypical presentations of the C9orf72-FTD/ALS spectrum mimicking other neurodegenerative brain diseases – including Alzheimer disease, Parkinson disease, Huntington disease (see linked GeneReview chapters for definitions of these phenotypes), and dementia with Lewy bodies – are common. In addition to MND, motor manifestations may also include extrapyramidal signs, most commonly as a symmetric akinetic-rigid syndrome. Clinical diagnoses of atypical parkinsonian syndromes are also relatively common.

A C9orf72 pathogenic G₄C₂ repeat expansion is seen in fewer than 1% of individuals with clinically diagnosed Alzheimer disease (AD). In most of these individuals, the underlying pathology is frontotemporal lobar degeneration (FTLD) [Murray et al 2011, Dobson-Stone et al 2012, Majounie et al 2012a, Cacace et al 2013, Harms et al 2013, Kohli et al 2013]. Similar observations were made in individuals with clinical Parkinson disease (PD) [Lesage et al 2013, Theuns et al 2014, Wilke et al 2016].

The association of C9orf72 G₄C₂ pathogenic repeat expansions with AD, PD, and atypical parkinsonian syndromes may be due to relatively common AD or PD co-pathology occurring in an individual with primary C9orf72-related disease. Moreover, most of the studies reporting on this association defined cohorts of affected individuals solely on clinical diagnoses, leaving the possibility of misclassification of an individual with C9orf72-FTD [Lesage et al 2013, Theuns et al 2014, Wilke et al 2016].

A C9orf72 G₄C₂ repeat expansion was observed in 6.5% of individuals with a diagnosis of depressive pseudodementia [Bieniek et al 2014] and in 2% of Huntington disease phenocopies lacking an HTT CAG trinucleotide repeat expansion [Beck et al 2013, Hensman Moss et al 2014].

Rarely, C9orf72-related corticobasal syndrome, progressive supranuclear palsy, and olivopontocerebellar degeneration have also been reported [Snowden et al 2012, Lesage et al 2013, Lindquist et al 2013, Schottlaender et al 2015, Wilke et al 2016, Bourinaris & Houlden 2018, Cali et al 2019].

Life Expectancy

Life expectancy for individuals with C9orf72-FTD/ALS is highly variable and mainly associated with an individual's clinical features. Overall disease duration averages 6.4 years (range 0-36), which is significantly lower than in individuals with GRN frontotemporal dementia and MAPT frontotemporal dementia [Moore et al 2020].

For C9orf72-ALS, G₄C₂ repeat expansions are associated with an average disease duration of 2.9 ± 2.8 years [Cammack et al 2019, Moore et al 2020].
For C9orf72-FTD, disease duration averages between 7.5 and 14 years, depending on the cohort. As expected, survival in FTD is markedly compromised (on average 1.8 years) when ALS manifestations become apparent [Van Mossevelde et al 2018, Moore et al 2020].

Genotype-Phenotype Correlations

Heterozygous expanded G₄C₂ repeats. Clinical findings cannot predict the presence or size of a C9orf72 G₄C₂ repeat expansion, nor can the presence of a G₄C₂ repeat expansion predict the disease course in any given individual.

Biallelic expanded G₄C₂ repeats. To date, one individual homozygous for an expanded C9orf72 G₄C₂ repeat has been reported. This individual (whose parents were consanguineous) was homozygous for >800 G₄C₂ repeats and presented with early-onset bvFTD at age 43 years followed by rapid deterioration that was nonetheless within the range of the usual disease spectrum [Fratta et al 2013].

Another individual, compound heterozygous for two expanded alleles (one with ±50 G₄C₂ repeats and one with >2000 G₄C₂ repeats), had onset age of 58 years of bvFTD associated with parkinsonism [Cooper-Knock et al 2013].

Penetrance

Heterozygosity for a pathogenic C9orf72 G₄C₂ repeat expansion is associated with age-dependent reduced penetrance, with the youngest individuals developing disease in their twenties and a small number of heterozygotes remaining asymptomatic in their nineties. Age-dependent penetrance is estimated as follows [Benussi et al 2015, Murphy et al 2017]:

~0% at age 35 years
50% at age 58 years
Near 100% at age 80 years

Anticipation

A decreasing age of onset in consecutive generations of family members heterozygous for a C9orf72 G₄C₂ repeat expansion has been reported by some investigators [Van Mossevelde et al 2017b, Moore et al 2020] but not others [DeJesus-Hernandez et al 2011, Renton et al 2011, Barbier et al 2017]. Explanations for this discrepancy could include the following: (1) an apparent earlier age of onset due to observational or recall bias in families experienced with the disorder that prompted earlier medical attention and diagnosis; and (2) technical difficulties as well as age-related and tissue-related issues in correctly sizing the G₄C₂ repeat (see Molecular Genetics).

Thus, to date, G₄C₂ repeat size as measured in leukocyte DNA does not provide prognostic information, such as predicted presence or absence of clinical manifestations of C9orf72-FTD/ALS in an individual, or – if manifestations do develop – the age of onset or severity [Van Mossevelde et al 2017a].

Prevalence

Detailed epidemiologic studies of the prevalence of the C9orf72 G₄C₂ repeat expansion have not been performed. Based on the estimated prevalence of FTD and ALS in the general population and the frequency of a C9orf72 G₄C₂ pathogenic repeat expansion in cohorts of individuals with FTD and ALS, the following rough estimates of C9orf72-FTD/ALS spectrum have been calculated.

With the prevalence of FTD estimated at 1-461:100,000 [Hogan et al 2016] and with an average frequency of 4%-29% of C9orf72 G₄C₂ repeat expansions in FTD cohorts [Van Mossevelde et al 2018], a rough estimate of C9orf72-FTD is 0.04-134:100,000.
The prevalence of ALS is estimated at 5-12:100,000. Among individuals with ALS, about 10% have a family history consistent with autosomal dominant inheritance and about 90% have no family history of the disorder [Oskarsson et al 2018, Masrori & Van Damme 2020]. A pathogenic C9orf72 G4C2 repeat expansion is observed on average in 30%-50% of individuals with familial ALS and 4%-10% of individuals with no family history of ALS [Majounie et al 2012b, Oskarsson et al 2018, Masrori & Van Damme 2020].

It is important to note that the frequency of C9orf72 G₄C₂ repeat expansions greatly depends on ethnicity and geographic region.

The highest repeat expansion frequencies are observed in individuals of northern European heritage.
Markedly elevated expansion frequencies were reported in Scandinavian countries [Majounie et al 2012b, Lindquist et al 2013, van der Zee et al 2013, Smith et al 2013].
By contrast, in Asian populations, expansion frequency is much lower [Majounie et al 2012b, Tsai et al 2012, Konno et al 2013, Zou et al 2013].
Few studies have investigated the effect of repeat expansions in cohorts of African heritage [Nel et al 2019].

Genetically Related (Allelic) Disorders

In the vast majority of individuals, C9orf72 G₄C₂ pathogenic repeat expansions cause phenotypes within the C9orf72-FTD/ALS spectrum; however, increased susceptibility to phenotypic aspects of other neurodegenerative and psychiatric disorders has been reported [Van Mossevelde et al 2017a, Marogianni et al 2019].

Differential Diagnosis

The frequency of C9orf72 G₄C₂ repeat expansions significantly exceeds that of pathogenic variants in any other gene causing frontotemporal dementia (FTD) or amyotrophic lateral sclerosis (ALS).

Family history. The frequency of pathogenic C9orf72 G₄C₂ repeat expansions is about twice as high in individuals with a family history of FTD and/or ALS compared to those without a family history of these disorders. A C9orf72 G₄C₂ repeat expansion is found in:

25% of familial FTD;
30%-50% of familial ALS (Of note, only 10% of individuals with ALS have a positive family history and simplex cases [i.e., a single occurrence in a family] outnumber familial cases among individuals with C9orf72-ALS.);
Up to 88% of individuals with manifestations of both FTD and ALS and a positive family history of these disorders [Cruts et al 2013, Masrori & Van Damme 2020].

Differential diagnosis for C9orf72-FTD

Other types of dementia, especially with behavioral changes. Differential diagnosis includes "frontal variant" Alzheimer disease (see Alzheimer Disease Overview), diffuse Lewy body disease, Huntington disease, other forms of FTD (see GRN Frontotemporal Dementia), prion disease, corticobasal degeneration, and progressive supranuclear palsy.
Some individuals with C9orf72-FTD/ALS have a choreiform movement disorder which (especially when combined with behavioral abnormalities) may be confused with Huntington disease (see Clinical Description, Atypical Presentations) [Hensman Moss et al 2014].
Psychiatric disorders. Especially in C9orf72-bvFTD with prominent behavioral manifestations, often in young individuals, a psychiatric diagnosis such as depression, obsessive compulsive disorder, bipolar disorder, and schizophrenia may be considered. Diagnostic workup and longitudinal clinical follow up are likely to distinguish between psychiatric disorders and FTD; however, they may exist concomitantly.
Age of onset of C9orf72-FTD (mean 58.2 years) was later than in MAPT-FTD (mean 49.5 years) and earlier than in GRN-FTD (mean 61.3 years) [Moore et al 2020]. Most studies report a similar onset age in individuals with C9orf72-FTD and those with FTD of unknown cause [Van Mossevelde et al 2018].

Differential diagnosis for C9orf72-ALS

Isolated upper motor signs. Differential diagnosis includes compressive (cervical) myelopathy, hereditary spastic paraplegia, adrenomyeloneuropathy (see X-Linked Adrenoleukodystrophy), and cerebrotendinous xanthomatosis in individuals with isolated manifestations of upper motor involvement.
Lower motor signs. Plexopathy, chronic inflammatory polyradiculoneuropathy, as well as multifocal motor, toxic, infectious, or metabolic neuropathies or myopathies including inclusion body myositis (see Inclusion Body Myopathy with Paget Disease of Bone and/or FTD) and polymyositis can mimic lower motor signs [Masrori & Van Damme 2020].
Other forms of upper and lower motor neuron disorders. See ALS Overview, Differential Diagnosis.
A rare ALS/FTD variant of prion disease [Vicente-Pascual et al 2018]

Management

Consensus clinical management recommendations for C9orf72 frontotemporal dementia and/or amyotrophic lateral sclerosis (C9orf72-FTD/ALS) have not been published.

Evaluations Following Initial Diagnosis

To establish the extent of disease and needs in an individual diagnosed with C9orf72-FTD/ALS, the evaluations summarized in Table 4 (if not performed as part of the evaluation that led to the diagnosis) are recommended.

Table 4.

Recommended Evaluations Following Initial Diagnosis in Individuals with C9orf72-FTD/ALS

Treatment of Manifestations

Many individuals benefit from care by a multidisciplinary team that includes a neurologist, specially trained nurses, pulmonologist, speech therapist, physical therapist, occupational therapist, respiratory therapist, nutritionist, psychologist, social worker, and genetic counselor.

For ALS-related treatment options, see also Amyotrophic Lateral Sclerosis Overview.

Table 5.

Treatment of Manifestations in Individuals with C9orf72-FTD/ALS

Surveillance

Table 6.

Recommended Surveillance for Individuals with C9orf72-FTD/ALS

Evaluation of Relatives at Risk

See Genetic Counseling for issues related to testing of at-risk relatives for genetic counseling purposes.

Therapies Under Investigation

Although results in a preclinical setting only are available to date, antisense oligonucleotide therapy may be promising as a disease-modifying therapy in C9orf72 repeat expansion heterozygotes. A Phase I clinical trial testing such an agent was commenced in 2018 (NCT03626012).

Other potential RNA therapies include the use of duplex and single-stranded small interfering RNAs to silence C9orf72 transcripts, as well as adeno-associated virus-delivered artificial microRNAs targeting C9orf72 [Panza et al 2020].

Promising results have been achieved in a Phase II/III clinical trial with the selective tyrosine kinase inhibitor masitinib, as an add-on therapy to riluzole in persons with ALS [Mora et al 2020]. An additional Phase III clinical trial to verify and further specify these effects is being set up (NCT03127267).

Search ClinicalTrials.gov in the US and EU Clinical Trials Register in Europe for access to information on clinical studies for a wide range of diseases and conditions.

Genetic Counseling

Genetic counseling is the process of providing individuals and families with information on the nature, mode(s) of inheritance, and implications of genetic disorders to help them make informed medical and personal decisions. The following section deals with genetic risk assessment and the use of family history and genetic testing to clarify genetic status for family members; it is not meant to address all personal, cultural, or ethical issues that may arise or to substitute for consultation with a genetics professional. —ED.

Mode of Inheritance

C9orf72 frontotemporal dementia and/or amyotrophic lateral sclerosis (C9orf72-FTD/ALS) is inherited in an autosomal dominant manner.

Risk to Family Members

Parents of a proband

To date, almost all individuals diagnosed with C9orf72-FTD/ALS inherited a C9orf72 G₄C₂ repeat expansion from a heterozygous parent.
In most families the heterozygous parent is affected; however, a heterozygous parent may not have clinical manifestations of the disorder due to age-dependent reduced penetrance (i.e., the parent may be too young to manifest the disorder) (see Penetrance).
Molecular genetic testing is recommended for the parents of a proband who appears to be the only affected family member.
If the C9orf72 G₄C₂ repeat expansion is not identified in either parent, the following possibilities should be considered:
- The proband has a de novo C9orf72 G₄C₂ repeat expansion. Note: A pathogenic variant is reported as "de novo" if: (1) the pathogenic variant found in the proband is not detected in parental DNA; and (2) parental identity testing has confirmed biological maternity and paternity. If parental identity testing is not performed, the variant is reported as "assumed de novo" [Richards et al 2015].
- The proband inherited a pathogenic variant from a parent with germline (or somatic and germline) mosaicism for a C9orf72 G₄C₂ repeat expansion. (Testing of parental leukocyte DNA may not detect all instances of somatic mosaicism.) Parental mosaicism for a C9orf72 G₄C₂ repeat expansion has not been reported to date.
The family history of some individuals with C9orf72-FTD/ALS may appear to be negative because of failure to recognize the disorder in family members, early death of the parent before the onset of symptoms, or late onset of the disease in the affected parent. Therefore, an apparently negative family history cannot be confirmed unless molecular genetic testing has demonstrated that neither parent is heterozygous for a C9orf72 G₄C₂ repeat expansion.

Sibs of a proband. The risk to the sibs of the proband depends on the genetic status of the proband’s parents:

If a parent of the proband is affected and/or is known to have a C9orf72 G₄C₂ repeat expansion, the risk to the sibs of inheriting the expansion is 50%.
The clinical presentation of the C9orf72-FTD/ALS spectrum is highly heterogeneous and may differ between sibs who are heterozygous for the repeat expansion; whether – and at what age – manifestations will become apparent in a sib who inherits a C9orf72 G4C2 repeat expansion cannot be predicted by the age of onset in other family members [Van Mossevelde et al 2017a].
If both parents are clinically unaffected but their genetic status is unknown, sibs are still at increased risk for C9orf72-FTD/ALS because of the possibility of age-dependent reduced penetrance in a heterozygous parent or the theoretic possibility of parental germline mosaicism.

Offspring of a proband. Each child of an individual with C9orf72-FTD/ALS has a 50% chance of inheriting the C9orf72 G₄C₂ repeat expansion.

Other family members. The risk to other family members depends on the genetic status of the proband's parents: if a parent has the C9orf72 G₄C₂ repeat expansion, the parent's family members may be at risk.

Related Genetic Counseling Issues

Predictive testing (i.e., testing of asymptomatic at-risk individuals)

Predictive testing for at-risk relatives is possible once the C9orf72 G₄C₂ repeat expansion has been identified in an affected family member.
Potential consequences of such testing (including but not limited to socioeconomic changes and the need for long-term follow up and evaluation arrangements for individuals with a positive test result) as well as the capabilities and limitations of predictive testing should be discussed in the context of formal genetic counseling prior to testing.

Predictive testing in minors (i.e., testing of asymptomatic at-risk individuals younger than age 18 years)

For asymptomatic minors at risk for adult-onset conditions for which early treatment would have no beneficial effect on disease morbidity and mortality, predictive genetic testing is considered inappropriate, primarily because it negates the autonomy of the child with no compelling benefit. Further, concern exists regarding the potential unhealthy adverse effects that such information may have on family dynamics, the risk of discrimination and stigmatization in the future, and the anxiety that such information may cause.
For more information, see the National Society of Genetic Counselors position statement on genetic testing of minors for adult-onset conditions and the American Academy of Pediatrics and American College of Medical Genetics and Genomics policy statement: ethical and policy issues in genetic testing and screening of children.

In a family with an established diagnosis of C9orf72-FTD/ALS, it is appropriate to consider testing of symptomatic individuals regardless of age.

Family planning

The optimal time for determination of genetic risk and discussion of the availability of prenatal/preimplantation genetic testing is before pregnancy.
It is appropriate to offer genetic counseling (including discussion of potential risks to offspring and reproductive options) to young adults who are affected or at risk.

Prenatal Testing and Preimplantation Genetic Testing

Once a C9orf72 G₄C₂ repeat expansion has been identified in an affected family member, prenatal and preimplantation genetic testing for the presence of the C9orf72 G₄C₂ repeat expansion are possible. (Note: The presence of a C9orf72 G₄C₂ repeat expansion cannot predict the disease course in any given individual.)

Differences in perspective may exist among medical professionals and within families regarding the use of prenatal testing. While most centers would consider use of prenatal testing to be a personal decision, discussion of these issues may be helpful. For more information, see the National Society of Genetic Counselors position statement on prenatal testing in adult-onset conditions.

Resources

GeneReviews staff has selected the following disease-specific and/or umbrella support organizations and/or registries for the benefit of individuals with this disorder and their families. GeneReviews is not responsible for the information provided by other organizations. For information on selection criteria, click here.

ALS Association
Phone: 800-782-4747
Email: [email protected]
www.alsa.org
Amyotrophic Lateral Sclerosis Society of Canada
Canada
Phone: 800-267-4257 (toll-free); 416-497-2267
Email: [email protected]
www.als.ca
Association for Frontotemporal Degeneration (AFTD)
Phone: 866-507-7222
Email: [email protected]
www.theaftd.org
International Alliance of ALS/MND Associations
1333 Race Street
PO Box 40777
Philadelphia PA 19107
Phone: +1 215 568-2462
Fax: +1 215 543-3366
Email: [email protected]
http://www.alsmndalliance.org
Les Turner ALS Foundation (Amyotrophic Lateral Sclerosis)
5550 West Touhy Avenue
Suite 302
Skokie IL 60077-3254
Phone: 888-257-1107 (toll-free); 847-679-3311
Fax: 847-679-9109
Email: [email protected]
www.lesturnerals.org
Rare Dementia Support
United Kingdom
Email: [email protected]
www.raredementiasupport.org
ARTFL-LEFFTDS Longitudinal Frontotemporal Lobar Degeneration - Registry
www.allftd.org

Molecular Genetics

Information in the Molecular Genetics and OMIM tables may differ from that elsewhere in the GeneReview: tables may contain more recent information. —ED.

Table A.

C9orf72-Related Amyotrophic Lateral Sclerosis and/or Frontotemporal Dementia: Genes and Databases

Table B.

OMIM Entries for C9orf72-Related Amyotrophic Lateral Sclerosis and/or Frontotemporal Dementia (View All in OMIM)

Molecular Pathogenesis

The G₄C₂ hexanucleotide repeat expansion is the only known pathogenic variant in C9orf72 frontotemporal dementia and/or amyotrophic lateral sclerosis (C9orf72-FTD/ALS). There is currently no evidence that variants that alter the C9orf72 protein sequence are pathogenic.

Mechanism of disease causation. Although the molecular basis of C9orf72-FTD/ALS has been under intense investigation, the exact disease mechanism is not yet fully understood. Mounting evidence indicates the involvement of multiple disease mechanisms in a multiple-hit model of both gain-of-function and loss-of-function mechanisms:

Gain-of-function mechanisms
- RNA toxicity caused by sequestration of RNA-binding proteins and normal C9orf72 transcripts by RNA species containing the pathogenic G₄C₂ (GGGGCC) hexanucleotide repeat expansion into nuclear RNA foci, thereby interfering with their physiologic functions [Mori et al 2013b]
- G₄C₂ repeat-associated, non-ATG bidirectional translation of the expanded G₄C₂ hexanucleotide repeat sequences into diverse aggregation-prone dipeptide repeat (DPR) proteins (poly-GA, poly-GP, poly-GR, poly-PA, and poly-PR), leading to DPR-positive inclusion pathology [Mori et al 2013a, Mori et al 2013c]
Haploinsufficiency due to loss of expression of C9orf72 from the G₄C₂ expanded allele and reduced C9orf72 protein levels [Gijselinck et al 2012, van der Zee et al 2013, Braems et al 2020]

C9orf72 technical considerations (see Table 7). Accurate sizing of the G₄C₂ repeat expansion has proven cumbersome, complicating direct observation of expansion of the repeat and anticipation in large numbers of parent-offspring pairs. This is due to the 100% GC content, large size, somatic instability, and repetitive nature of its flanking sequences. Another barrier to accurately determining pathogenic repeat size is the difference in repeat length as measured in different tissues. As such, analysis on blood-derived DNA may not be a correct representation of the G₄C₂ repeat length in neuronal tissue. Reports have shown marked intraindividual differences in the repeat length between tissues, as well as a smaller variation in length within the same tissue [van Blitterswijk et al 2013, Nordin et al 2015, Gijselinck et al 2016]. Repeat length may also fluctuate during the lifetime of an individual, giving a significant correlation between repeat length and age at sample collection [Fournier et al 2019, Jackson et al 2020].

Table 7.

C9orf72 Technical Considerations

Methods to characterize C9orf72 G₄C₂ repeats. Due to the technical challenges of detecting and sizing C9orf72 G₄C₂ repeat expansions, multiple methods may be needed to rule out or detect G₄C₂ repeat expansion (see Table 8). Repeats in the normal range (2-24) may be detected by traditional PCR. However, detection of apparent homozygosity for a normal G₄C₂ repeat does not rule out the presence of an expanded G₄C₂ repeat; thus, testing by RP-PCR or Southern blotting is required. In addition, somatic and germline instability of expanded repeats must be considered.

Table 8.

Methods to Characterize C9orf72 G₄C₂ Repeats

Table 9.

Notable C9orf72 Variants

Chapter Notes

Author Notes

The Neurodegenerative Brain Diseases group investigates the molecular mechanisms underlying neurodegenerative dementias and related disorders. We identify novel key proteins in neurodegeneration as potential targets for early diagnosis, risk prediction, and drug and biomarker development. Concurrently, we study the post-genomic consequences of disease-related genetic defects to improve our knowledge of the molecular mechanisms underlying these brain diseases and accelerate the development of more effective treatments. The expertise of the group is in genetics, genomics and functional genomics of Alzheimer disease, frontotemporal lobar degeneration, dementia with Lewy bodies, and Parkinson disease.

VIB Center for Molecular Neurology

Acknowledgments

The research is in part funded by the Flemish Government initiated Methusalem Excellence program and the Flanders Impulse Program on Networks for Dementia, Belgium.

Author History

Marc Cruts, PhD; University of Antwerp (2015-2020)
Sebastiaan Engelborghs, MD, PhD (2015-present)
Helena Gossye, MD (2020-present)
Christine Van Broeckhoven, PhD, DSc (2015-present)
Julie van der Zee, PhD (2015-present)

Revision History

17 December 2020 (bp) Comprehensive update posted live
8 January 2015 (me) Review posted live
28 July 2014 (mc) Original submission

References

Published Guidelines / Consensus Statements

Committee on Bioethics, Committee on Genetics, and American College of Medical Genetics and Genomics Social, Ethical, Legal Issues Committee. Ethical and policy issues in genetic testing and screening of children. Available online. 2013. Accessed 12-29-22.
National Society of Genetic Counselors. Position statement on genetic testing of minors for adult-onset conditions. Available online. 2018. Accessed 12-29-22.

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Publication Details

Author Information and Affiliations

Helena Gossye, MD

VIB Center for Molecular Neurology, VIB;
Neurogenetics Lab, Institute Born-Bunge;
Department of Biomedical Sciences, University of Antwerp;
Department of Neurology and Memory Clinic, Hospital Network Antwerp
Antwerp, Belgium

Department of Neurology, University Hospital Antwerp
Edegem, Belgium

Department of Neurology, UZ Brussel and Center for Neurosciences
Vrije Universiteit Brussel
Brussels, Belgium

Email: [email protected]

Sebastiaan Engelborghs, MD, PhD

Department of Neurology, UZ Brussel and Center for Neurosciences
Vrije Universiteit Brussel
Brussels, Belgium

BIODEM Lab, Institute Born-Bunge;
Department of Biomedical Sciences, University of Antwerp
Antwerp, Belgium

Email: [email protected]

Christine Van Broeckhoven, PhD, DSc

VIB Center for Molecular Neurology, VIB;
Neurogenetics Lab, Institute Born-Bunge;
Department of Biomedical Sciences, University of Antwerp
Antwerp, Belgium

Email: [email protected]

Julie van der Zee, PhD

VIB Center for Molecular Neurology, VIB;
Neurogenetics Lab, Institute Born-Bunge;
Department of Biomedical Sciences, University of Antwerp
Antwerp, Belgium

Email: [email protected]

Publication History

Initial Posting: January 8, 2015; Last Update: December 17, 2020.

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NLM Citation

Gossye H, Engelborghs S, Van Broeckhoven C, et al. C9orf72 Frontotemporal Dementia and/or Amyotrophic Lateral Sclerosis. 2015 Jan 8 [Updated 2020 Dec 17]. In: Adam MP, Feldman J, Mirzaa GM, et al., editors. GeneReviews® [Internet]. Seattle (WA): University of Washington, Seattle; 1993-2024.

Diagnosis	Frequency	Comments (Frequency)
FTD	34.8%	Behavioral variant FTD (31.4%) Nonfluent/agrammatic variant PPA (1.8%) Semantic variant PPA (0.9%) Other tauopathy: corticobasal degeneration, progressive supranuclear palsy, other PPA (0.7%)
ALS	19.3%
FTD-ALS	11.0%
Atypical presentations mimicking other kinds of neurodegenerative brain diseases	35.0%	Alzheimer disease, Parkinson disease, Huntington disease, & dementia w/Lewy bodies are common. Also incl vascular dementia & dementia not otherwise specified Atypical parkinsonian syndromes ¹

Feature		Frequency			Comment
Feature		Nearly all	Common ¹	Infrequent	Comment
Cognitive deterioration
Executive dysfunction		●			Issues w/planning, problem solving, organizing
Memory impairment			●		Amnesia, mostly recent memory
Language impairment			●		Deficits in speech production or comprehension
Apraxia			●		Impaired execution of learned motor tasks
Dyscalculia			●		Diminished mathematical reasoning
Behavioral & psychological manifestations of dementia
Disinhibition			●		Impulsivity, socially unacceptable behavior, risk taking
Apathy			●		Indifference, lack of interest
Delusions/hallucinations			●		Often bizarre delusions, mostly visual hallucinations
Psychosis			●		Psychosis, often as initial symptom
Anxiety			●		Generalized stress & apprehension
Repetitive, compulsive behavior			●		Often complex, ritualistic behaviors mimicking OCD
Preference for sweet food				●	↑ craving for sweet foods
Motor symptoms
Upper MND			●		Weakness, spasticity, altered muscle tone
Lower MND			●		Weakness, fasciculations, atrophy
Bulbar involvement	Dysarthria			●	Motor language deficit
Bulbar involvement	Dysphagia			●	Problems swallowing food &/or liquids
Parkinsonism			●		Extrapyramidal findings such as resting tremor, rigidity, akinesia

System/Concern	Evaluation	Comment
Neurologic	Complete neurologic exam	Assess: UMN involvement: spasticity, Babinski signs, hyperreflexia; LMN involvement: weakness, amyotrophy, fasciculations; EMG.
Cognitive function	Neuropsychological exam	Evaluate extent & profile of cognitive disturbance.
Musculoskeletal/ ADL	Orthopedics / physical medicine & rehab / PT eval	To incl assessment of: Muscle tone, joint range of motion, posture, mobility, strength, coordination & endurance, pain, bedsores Need for adaptive devices Footwear needs PT needs Need for assistive walking devices (e.g., canes, walker, walker w/wheels, walker w/seat, wheelchairs)
	OT	Assess: Fine motor function, e.g., hands, feet, face, fingers, & toes; Home adaptations for ADL & safety.
	Eval of driving safety	In case of cognitive impairment & impaired judgment, evaluate driving safety.
Psychiatric illness	History of psychiatric illness ¹	Attention to possible alcohol or drug abuse Referral for psychiatric eval as needed
Dysarthria	For those w/dysarthria: speech/language eval	Referral for speech therapy as needed
Dysphagia	For those w/frequent choking or severe dysphagia, assess: Nutritional status; Aspiration risk.	Consider involving a gastroenterology/nutrition/feeding team, incl formal swallowing eval.
Respiratory function	By pulmonologist	Assess respiratory function & need for respiratory support.
Genetic counseling	By genetics professionals ²	To inform affected persons & their families re nature, MOI, & implications of C9orf72-FTD/ALS spectrum to facilitate medical & personal decision making
Family support & resources	Assess need for: Community or online resources such as Parent to Parent; Social work involvement for parental support; Home nursing referral.	Early discussion of advanced care planning The affected person's perspective & burden must be considered in clinical decision making. The presence of cognitive impairment may raise ethical concerns.

Manifestation/Concern	Treatment	Considerations/Other
UMN & LMN involvement / ADL	Physical medicine & rehab / PT & OT Riluzol Edaravone	Ankle-foot braces, walkers, wheelchairs, hospital beds, toileting equipment, lifts to improve functionality Note: Edaravone is not approved worldwide.
Spasticity	Baclofen, tizanidine, cannabinoids, & muscle stretching
Muscle cramps	Magnesium supplements, quinine sulfate, gabapentin, or carbamazepine
Parkinsonism	PT, levodopa trial	Because of psychiatric levodopa side effects, use only when functional impairment is significant.
Cognitive function	Cognitive rehab
Psychiatric/ behavioral manifestations	Environmental, behavioral, & physical interventions	To minimize occurrence & consequences of undesired behaviors
	Counseling	For those w/affective disorders or to support affected person &/or caretakers.
	SSRIs	For those w/affective disorders or disinhibition & challenging behaviors, the 1st-line approach is pharmacologic therapy.
	Venlafaxine	Used when apathy is prominent
	Atypical antipsychotics	For severe manifestations (agitation, aggressiveness, psychosis) refractory to SSRIs Often a temporizing measure until affected person becomes more apathetic Note: Risk of iatrogenic extrapyramidal syndrome
Pseudobulbar affect	Dextromethorphan/quinidine
Dysarthria	Speech/language therapy	Use of augmentative communication devices
Dysphagia	Continuous eval & therapy	Safe swallowing techniques, diet modifications, gastrostomy tube
Sialorrhea	Anticholinergic medications, salivary gland botulinum toxin injections, or radiotherapy	Note: Anticholinergic medication can affect cognition.
Respiratory function	Assisted ventilation	Noninvasive at first, proceeding to tracheostomy if necessary
Bladder dysfunction	Anticholinergics & intravesical botulinum toxin	Note: Anticholinergic medication can affect cognition.
Family/caregiver support & resources	Psychosocial support & education via caregiver & patient support groups	To ↓ stress & burden on caregivers

System/Concern	Evaluation	Frequency
System/Concern	Evaluation	ALS	FTD
Neurologic	Neurologic exam for new manifestations &/or response to medications	Every 2-3 mos	Undefined; depends on disease progression & presenting symptoms
Mobility/ADL	Physical medicine & rehab / PT/OT
Cognitive function	Rapid screening tools, incl tests of verbal fluency
Psychiatric/behavioral manifestations	Medical history, neurologic exam
Pseudobulbar affect	Medical history		Not applicable
Dysarthria	Eval by speech therapist	Every 3-6 mos	Undefined; depends on disease progression & presenting symptoms
Dysphagia	Medical history	Every 2-3 mos	Not applicable
Sialorrhea	Medical history
Respiratory function	Medical history, clinical exam, addl testing (e.g., forced vital capacity, vital capacity)
Bladder function	Medical history
Family/caregiver support/resources	Medical history; assess need for addl support.		Undefined; depends on disease progression & presenting symptoms

105550	FRONTOTEMPORAL DEMENTIA AND/OR AMYOTROPHIC LATERAL SCLEROSIS 1; FTDALS1
614260	CHROMOSOME 9 OPEN READING FRAME 72; C9ORF72

Technical Issue	Comment [Reference]
Sequence of repeat	While it is plausible that repeat expansion producing thousands of repeat units could lead to imperfect replication & interruption of the pure G₄C₂ repeat sequence [Gao et al 2017], this has not been demonstrated to date owing to the difficulty of sequencing the C9orf72 G₄C₂ repeat.
Methods to detect expanded allele (See Table 8 .)	Repeat-primed PCR (RP-PCR) & fragment length analysis [Gijselinck et al 2012], Southern blotting following pulse-field gel electrophoresis [Akimoto et al 2014]. Note: The Southern blotting methods for this repeat expansion require biomaterials, expertise, & equipment not commonly available in clinical labs.
Somatic instability	Alleles w/abnormal number of G₄C₂ repeats may display somatic instability of the repeat, appearing as "smeared" expanded alleles w/multiple distinct expansion alleles on PCR & Southern blot analyses [Gijselinck et al 2016].
Germline instability	There is evidence that contraction of the repeat is primarily seen in paternal transmissions [Jackson et al 2020].

Interpretation of G₄C₂ Repeat Number	Expected Results by Method
Interpretation of G₄C₂ Repeat Number	Conventional PCR	Repeat-primed PCR ¹	Expanded repeat analysis ²
Normal: 2-24 ³	Detected ⁴	See footnote 1.	Expansions can be detected & repeat size can be approximated. ^5, 6
Intermediate: 25-60 ³	Detected ⁴	Expansions may be detected but repeat size cannot be determined. ^7, ⁸
Pathogenic: 61->4,000	Detected up to ~80 repeats ^4, 9	Expansions are detected but repeat size cannot be determined. ⁸

Reference Sequences	DNA Nucleotide Change	Predicted Protein Change	Repeat Range
NM_145005.6	c.-45+258_-45+263delGGGGCC[2_24]	--	Normal
	c.-45+258_-45+263delGGGGCC[25_60]	--	Of uncertain significance
	c.-45+258_-45+263delGGGGCC[61_>4000]	--	Pathogenic w/age-dependent reduced penetrance