A form of hereditary spastic paraplegia which usually presents in late adolescence or early adulthood as a pure phenotype of lower limb spasticity with hyperreflexia and extensor plantar responses, as well as mild bladder disturbances and pes cavus. Rarely, it can present as a complex phenotype with additional manifestations including epilepsy, variable peripheral neuropathy and/or memory impairment. Caused by mutations in the NIPA1 gene (15q11.2) encoding the magnesium transporter NIPA1.

PLP1 disorders of central nervous system myelin formation include a range of phenotypes from Pelizaeus-Merzbacher disease (PMD) to spastic paraplegia 2 (SPG2). PMD typically manifests in infancy or early childhood with nystagmus, hypotonia, and cognitive impairment; the findings progress to severe spasticity and ataxia. Life span is shortened. SPG2 manifests as spastic paraparesis with or without CNS involvement and usually normal life span. Intrafamilial variation of phenotypes can be observed, but the signs are usually fairly consistent within families. Heterozygous females may manifest mild-to-moderate signs of the disease.

Spastic paraplegia 7 (SPG7) is characterized by insidiously progressive bilateral leg weakness and spasticity. Most affected individuals have decreased vibration sense and cerebellar signs. Onset is mostly in adulthood, although symptoms may start as early as age 11 years and as late as age 72 years. Additional features including ataxia (gait and limbs), spastic dysarthria, dysphagia, pale optic disks, ataxia, nystagmus, strabismus, ptosis, hearing loss, motor and sensory neuropathy, amyotrophy, scoliosis, pes cavus, and urinary sphincter disturbances may be observed.

Spastic paraplegia 11 (SPG11) is characterized by progressive spasticity and weakness of the lower limbs frequently associated with the following: mild intellectual disability with learning difficulties in childhood and/or progressive cognitive decline; peripheral neuropathy; pseudobulbar involvement; and increased reflexes in the upper limbs. Less frequent findings include: cerebellar signs (ataxia, nystagmus, saccadic pursuit); retinal degeneration; pes cavus; scoliosis; and parkinsonism with characteristic brain MRI features that include thinning of the corpus callosum. Onset occurs mainly during infancy or adolescence (range: age 1-31 years) and in rare cases as late as age 60 years. Most affected individuals become wheelchair bound one or two decades after disease onset.

Amyotrophic lateral sclerosis is a neurodegenerative disorder characterized by the death of motor neurons in the brain, brainstem, and spinal cord, resulting in fatal paralysis. ALS usually begins with asymmetric involvement of the muscles in middle adult life. Approximately 10% of ALS cases are familial (Siddique and Deng, 1996). ALS is sometimes referred to as 'Lou Gehrig disease' after the famous American baseball player who was diagnosed with the disorder. Rowland and Shneider (2001) and Kunst (2004) provided extensive reviews of ALS. Some forms of ALS occur with frontotemporal dementia (FTD); see 105500. Ranganathan et al. (2020) provided a detailed review of the genes involved in different forms of ALS with FTD, noting that common disease pathways involve disturbances in RNA processing, autophagy, the ubiquitin proteasome system, the unfolded protein response, and intracellular trafficking. The current understanding of ALS and FTD is that some forms of these disorders represent a spectrum of disease with converging mechanisms of neurodegeneration. Familial ALS is distinct from a form of ALS with dementia reported in cases on Guam (105500) (Espinosa et al., 1962; Husquinet and Franck, 1980), in which the histology is different and dementia and parkinsonism complicate the clinical picture. Genetic Heterogeneity of Amyotrophic Lateral Sclerosis ALS is a genetically heterogeneous disorder, with several causative genes and mapped loci. ALS6 (608030) is caused by mutation in the FUS gene (137070) on chromosome 16p11; ALS8 (608627) is caused by mutation in the VAPB gene (605704) on chromosome 13; ALS9 (611895) is caused by mutation in the ANG gene (105850) on chromosome 14q11; ALS10 (612069) is caused by mutation in the TARDBP gene (605078) on 1p36; ALS11 (612577) is caused by mutation in the FIG4 gene (609390) on chromosome 6q21; ALS12 (613435) is caused by mutation in the OPTN gene (602432) on chromosome 10p13; ALS15 (300857) is caused by mutation in the UBQLN2 gene (300264) on chromosome Xp11; ALS18 (614808) is caused by mutation in the PFN1 gene (176610) on chromosome 17p13; ALS19 (615515) is caused by mutation in the ERBB4 gene (600543) on chromosome 2q34; ALS20 (615426) is caused by mutation in the HNRNPA1 gene (164017) on chromosome 12q13; ALS21 (606070) is caused by mutation in the MATR3 gene (164015) on chromosome 5q31; ALS22 (616208) is caused by mutation in the TUBA4A gene (191110) on chromosome 2q35; ALS23 (617839) is caused by mutation in the ANXA11 gene (602572) on chromosome 10q23; ALS26 (619133) is caused by mutation in the TIA1 gene (603518) on chromosome 2p13; ALS27 (620285) is caused by mutation in the SPTLC1 gene (605712) on chromosome 9q22; and ALS28 (620452) is caused by mutation in the LRP12 gene (618299) on chromosome 8q22. Loci associated with ALS have been found on chromosomes 18q21 (ALS3; 606640) and 20p13 (ALS7; 608031). Intermediate-length polyglutamine repeat expansions in the ATXN2 gene (601517) contribute to susceptibility to ALS (ALS13; 183090). Susceptibility to ALS24 (617892) is conferred by mutation in the NEK1 gene (604588) on chromosome 4q33, and susceptibility to ALS25 (617921) is conferred by mutation in the KIF5A gene (602821) on chromosome 12q13. Susceptibility to ALS has been associated with mutations in other genes, including deletions or insertions in the gene encoding the heavy neurofilament subunit (NEFH; 162230); deletions in the gene encoding peripherin (PRPH; 170710); and mutations in the dynactin gene (DCTN1; 601143). Some forms of ALS show juvenile onset. See juvenile-onset ALS2 (205100), caused by mutation in the alsin (606352) gene on 2q33; ALS4 (602433), caused by mutation in the senataxin gene (SETX; 608465) on 9q34; ALS5 (602099), caused by mutation in the SPG11 gene (610844) on 15q21; and ALS16 (614373), caused by mutation in the SIGMAR1 gene (601978) on 9p13.

Hereditary spastic paraplegia 8 (SPG8) is a slowly progressive pure spastic paraplegia of the lower limbs (i.e., pyramidal signs including hyperreflexia, spasticity, and occasionally clonus without other neurologic findings). Some affected individuals have urinary urgency that usually becomes apparent at the same time as the spasticity. Onset is between ages ten and 59 years. Affected individuals often become wheelchair dependent. While intra- and interfamilial phenotypic variability is high, SPG8 is typically more severe than other types of hereditary spastic paraplegia.

Spastic paraplegia 4 (SPG4; also known as SPAST-HSP) is characterized by insidiously progressive bilateral lower-limb gait spasticity. More than 50% of affected individuals have some weakness in the legs and impaired vibration sense at the ankles. Sphincter disturbances are very common. Onset is insidious, mostly in young adulthood, although symptoms may start as early as age one year and as late as age 76 years. Intrafamilial variation is considerable.

Spastic paraplegia 3A (SPG3A; also known as ATL1-HSP) is characterized by progressive bilateral and mostly symmetric spasticity and weakness of the legs. Compared to other forms of autosomal dominant hereditary spastic paraplegia (HSP), in which diminished vibration sense (caused by degeneration of the corticospinal tracts and dorsal columns) and urinary bladder hyperactivity are present in all affected individuals, these findings occur in a minority of individuals with SPG3A. The average age of onset is four years. More than 80% of reported individuals manifest spastic gait before the end of the first decade of life. Most persons with early-onset ATL1-HSP have a "pure" ("uncomplicated") HSP; however, complicated HSP with axonal motor neuropathy and/or distal amyotrophy with lower motor neuron involvement (Silver syndrome phenotype) has been observed. The rate of progression in ATL1-HSP is slow, and wheelchair dependency or need for a walking aid (cane, walker, or wheelchair) is relatively rare.