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Schizophrenia is highly heritable, as shown by family, twin, and adoption studies. For example, for identical twins, if one twin develops schizophrenia, the other twin has about a 50% chance of also developing the disease. The risk of the general population developing the schizophrenia is about 0.3-0.7% worldwide. The search for “schizophrenia genes” has been elusive. Initial linkage studies looked at parts of the genome associated with schizophrenia, and many candidate genes were identified, including APOE, COMT, DAO, DRD1, DRD2, DRD4, DTNBP1, GABRB2, GRIN2B, HP, IL1B, MTHFR, PLXNA2, SLC6A4, TP53, and TPH1. However, some of these have later been questioned. Microdeletions and microduplications have been found to be three times more common in individuals with schizophrenia, compared to controls. Because these deletions and duplications are in genes that are overexpressed in pathways related to brain development, it is possible that the inheritance of multiple rare variants may contribute to the development of schizophrenia. Several genetic disorders feature schizophrenia as a clinical feature. The 22q11.2 Deletion Syndrome comprises many different syndromes, of which one of the most serious is DiGeorge syndrome. Children born with DiGeorge syndrome typically have heart defects, cleft palate, learning difficulties, and immune deficiency. Schizophrenia is a late manifestation, affecting around 30% of individuals. Microdeletions and duplications in chromosome 1, 2, 3, 7, 15 and 16 have also been associated with schizophrenia. In 2014, a genome-wide association study looked at the genomes of over 35,000 patients and 110,00 controls. The study identified 108 SNPs that were associated with schizophrenia, 83 of which had not been previously reported. As expected, many of these loci occurred in genes that are expressed in the brain. For example, the SNPs included a gene that encodes the dopamine D2 receptor, DRD2 (the target of antipsychotic drugs), and many genes involved in glutamine neurotransmitter pathways and synaptic plasticity (e.g., GRM3, GRIN2A, SRR, GRIA1). More surprisingly, however, associations were also enriched among genes expressed in tissues with important immune functions. In 2016, a study based on nearly 65,000 people investigated the association between schizophrenia and variation in the Major Histocompatibility Complex (MHC) locus—a region on chromosome 6 that is important for immune function. The study focused on the C4 gene (complement component 4) that exists as two distinct genes: C4A and C4B, which encode particularly structurally diverse alleles. The study found that the alleles which promoted greater expression of C4A in the brain were associated with a greater risk of schizophrenia. By using mice models, the study showed that C4 is involved in the elimination of synapses during brain maturation. In humans, “synaptic pruning” is most active during late adolescence, which coincides with the typical onset of symptoms of schizophrenia. It is therefore possible that the inheritance of specific C4A alleles could lead to “run away” synaptic pruning, increasing the risk of schizophrenia. Further research may even determine C4 as a potential therapeutic target. [from Medical Genetics Summaries]

Individuals with 22q11.2 deletion syndrome (22q11.2DS) can present with a wide range of features that are highly variable, even within families. The major clinical manifestations of 22q11.2DS include congenital heart disease, particularly conotruncal malformations (ventricular septal defect, tetralogy of Fallot, interrupted aortic arch, and truncus arteriosus), palatal abnormalities (velopharyngeal incompetence, submucosal cleft palate, bifid uvula, and cleft palate), immune deficiency, characteristic facial features, and learning difficulties. Hearing loss can be sensorineural and/or conductive. Laryngotracheoesophageal, gastrointestinal, ophthalmologic, central nervous system, skeletal, and genitourinary anomalies also occur. Psychiatric illness and autoimmune disorders are more common in individuals with 22q11.2DS. [from GeneReviews]

Autosomal dominant Wolfram-like syndrome (WFSL) is characterized by the clinical triad of congenital progressive hearing impairment, diabetes mellitus, and optic atrophy. The hearing impairment, which is usually diagnosed in the first decade of life, is relatively constant and alters mainly low- and middle-frequency ranges (summary by Valero et al., 2008). Wolfram syndrome (WFS1; 222300) is an autosomal recessive allelic disorder characterized by optic atrophy, diabetes mellitus, hearing loss, and diabetes insipidus, and is caused by homozygous or compound heterozygous mutation in the WFS1 gene. An autosomal dominant syndrome involving optic atrophy with or without deafness, ophthalmoplegia, myopathy, ataxia, and neuropathy (125250), is caused by heterozygous mutation in the OPA1 gene (605290). [from OMIM]

Nemaline myopathy is divided into six types. In order of decreasing severity, the types are: severe congenital, Amish, intermediate congenital, typical congenital, childhood-onset, and adult-onset. The types are distinguished by the age when symptoms first appear and the severity of symptoms; however, there is overlap among the various types. The severe congenital type is the most life-threatening. Most individuals with this type do not survive past early childhood due to respiratory failure. The Amish type solely affects the Old Order Amish population of Pennsylvania and is typically fatal in early childhood. The most common type of nemaline myopathy is the typical congenital type, which is characterized by muscle weakness and feeding problems beginning in infancy. Most of these individuals do not have severe breathing problems and can walk unassisted. People with the childhood-onset type usually develop muscle weakness in adolescence. The adult-onset type is the mildest of all the various types. People with this type usually develop muscle weakness between ages 20 and 50.

Nemaline myopathy is a disorder that primarily affects skeletal muscles, which are muscles that the body uses for movement. People with nemaline myopathy have muscle weakness (myopathy) throughout the body, but it is typically most severe in the muscles of the face; neck; trunk; and other muscles close to the center of the body (proximal muscles), such as those of the upper arms and legs. This weakness can worsen over time. Affected individuals may have feeding and swallowing difficulties, foot deformities, abnormal curvature of the spine (scoliosis), and joint deformities (contractures). Most people with nemaline myopathy are able to walk, although some affected children may begin walking later than usual. As the condition progresses, some people may require wheelchair assistance. In severe cases, the muscles used for breathing are affected and life-threatening breathing difficulties can occur. [from MedlinePlus Genetics]

X-linked spinocerebellar ataxia-6 with or without sideroblastic anemia (SCAX6) is an X-linked recessive disorder characterized by delayed motor development apparent in infancy with delayed walking (often by several years) due to ataxia and poor coordination. Additional features may include dysmetria, dysarthria, spasticity of the lower limbs, hyperreflexia, dysdiadochokinesis, strabismus, and nystagmus. The disorder is slowly progressive, and patients often lose ambulation. Brain imaging usually shows cerebellar atrophy. Most affected individuals have mild hypochromic, microcytic sideroblastic anemia, which may be asymptomatic. Laboratory studies show increased free erythrocyte protoporphyrin (FEP) and ringed sideroblasts on bone marrow biopsy. Female carriers do not have neurologic abnormalities, but may have subtle findings on peripheral blood smear (Pagon et al., 1985; D'Hooghe et al., 2012). For a discussion of genetic heterogeneity of X-linked spinocerebellar ataxia (SCAX), see SCAX1 (302500). [from OMIM]

The 1q21.1 recurrent microdeletion itself does not appear to lead to a clinically recognizable syndrome as some persons with the deletion have no obvious clinical findings and others have variable findings that most commonly include microcephaly (50%), mild intellectual disability (30%), mildly dysmorphic facial features, and eye abnormalities (26%). Other findings can include cardiac defects, genitourinary anomalies, skeletal malformations, and seizures (~15%). Psychiatric and behavioral abnormalities can include autism spectrum disorders, attention deficit hyperactivity disorder, autistic features, and sleep disturbances. [from GeneReviews]

Darier-White disease (DAR), also known as keratosis follicularis, is an autosomal dominant skin disorder characterized by warty papules and plaques in seborrheic areas (central trunk, flexures, scalp, and forehead), palmoplantar pits, and distinctive nail abnormalities (Sakuntabhai et al., 1999). Onset is usually before the third decade, and penetrance is complete in adults, although expressivity is variable. Involvement may be severe, with widespread itchy malodorous crusted plaques, painful erosions, blistering, and mucosal lesions. Secondary infection is common. Sun, heat, and sweating exacerbate the symptoms. Darier disease never remits, but oral retinoids may reduce hyperkeratosis. Neuropsychiatric abnormalities, including mild mental retardation and epilepsy, have been described in association with Darier disease in a few families (Burge and Wilkinson, 1992); whether this is an association based on pleiotropism of the mutant gene or reflects coincidence is not clear. Histologic findings are (1) mild nonspecific perivascular infiltration in the dermis; (2) dermal villi protruding into the epidermis; (3) suprabasal detachment of the spinal layer leading to the formation of lacunae containing acantholytic cells; (4) in the more superficial epidermis, dyskeratotic round epidermal cells ('corps ronds'), the most distinctive feature; and (5) in the stratum corneum, 'grains' that resemble parakeratotic cells embedded in a hyperkeratotic horny layer. Electron microscopy reveals loss of desmosomal attachments, perinuclear aggregations of keratin filaments, and cytoplasmic vacuolization. Ultrastructural and immunologic studies suggest the disease results from an abnormality in the desmosome-keratin filament complex leading to a breakdown in cell adhesion. [from OMIM]

A schizophrenia that has material basis in an autosomal dominant mutation of PRODH on chromosome 22q11.21. [from MONDO]

Primrose syndrome is characterized by macrocephaly, hypotonia, developmental delay, intellectual disability with expressive speech delay, behavioral issues, a recognizable facial phenotype, radiographic features, and altered glucose metabolism. Additional features seen in adults: sparse body hair, distal muscle wasting, and contractures. Characteristic craniofacial features include brachycephaly, high anterior hairline, deeply set eyes, ptosis, downslanted palpebral fissures, high palate with torus palatinus, broad jaw, and large ears with small or absent lobes. Radiographic features include calcification of the external ear cartilage, multiple Wormian bones, platybasia, bathrocephaly, slender bones with exaggerated metaphyseal flaring, mild epiphyseal dysplasia, and spondylar dysplasia. Additional features include hearing impairment, ocular anomalies, cryptorchidism, and nonspecific findings on brain MRI. [from GeneReviews]

Phang et al. (2001) noted that prospective studies of HPI probands identified through newborn screening as well as reports of several families have suggested that it is a metabolic disorder not clearly associated with clinical manifestations. Phang et al. (2001) concluded that HPI is a relatively benign condition in most individuals under most circumstances. However, other reports have suggested that some patients have a severe phenotype with neurologic manifestations, including epilepsy and mental retardation (Jacquet et al., 2003). Genetic Heterogeneity of Hyperprolinemia See also hyperprolinemia type II (HYRPRO2; 239510), which is caused by mutation in the gene encoding pyrroline-5-carboxylate dehydrogenase (P5CDH, ALDH4A1; 606811) on chromosome 1p36. [from OMIM]

A schizophrenia that has material basis in an autosomal dominant mutation of SCZD3 on chromosome 6p23. [from MONDO]

X-linked intellectual developmental disorder-9 (XLID9) is characterized by moderately to severely impaired intellectual development. Some patients have also been reported with delayed motor development, seizures, and/or behavioral problems (Hamel et al., 1999; Froyen et al., 2007). [from OMIM]

1q21.1 microduplication is a chromosomal change in which a small amount of genetic material on chromosome 1 is abnormally copied (duplicated). The duplication occurs on the long (q) arm of the chromosome at a location designated q21.1.

Some people with a 1q21.1 microduplication have developmental delay and intellectual disability that is typically mild to moderate. Individuals with this condition can also have features of autism spectrum disorder. These disorders are characterized by impaired communication and socialization skills, as well as delayed development of speech and language. Expressive language skills (vocabulary and the production of speech) tend to be more impaired than receptive language skills (the ability to understand speech) in affected individuals. In childhood, 1q21.1 microduplications may also be associated with an increased risk of attention-deficit/hyperactivity disorder (ADHD) and other behavioral problems. Psychiatric disorders such as schizophrenia or mood disorders such as anxiety or depression occur in some affected individuals, usually during adulthood. Rarely, recurrent seizures (epilepsy) occur in people with a 1q21.1 microduplication.

Some individuals with a 1q21.1 microduplication are born with malformations of the heart, including a particular combination of heart defects known as tetralogy of Fallot. Less commonly, other physical malformations such as the urethra opening on the underside of the penis (hypospadias) in males, inward- and upward-turning feet (clubfeet), or misalignment of the hip joint (hip dysplasia) are present at birth. Individuals with a 1q21.1 microduplication may also have a larger than average head size or taller than average adult stature. Some have slightly unusual facial features such as wide-set eyes or low-set ears. As adults, individuals with a 1q21.1 microduplication may be prone to develop cysts, swollen and knotted (varicose) veins, or carpal tunnel syndrome, which is characterized by numbness, tingling, and weakness in the hands and fingers. However, there is no particular pattern of physical abnormalities that characterizes 1q21.1 microduplications. Signs and symptoms related to the chromosomal change vary even among affected members of the same family. Some people with the duplication have no identified physical, intellectual, or behavioral abnormalities. [from MedlinePlus Genetics]

The 17q12 recurrent deletion syndrome is characterized by variable combinations of the three following findings: structural or functional abnormalities of the kidney and urinary tract, maturity-onset diabetes of the young type 5 (MODY5), and neurodevelopmental or neuropsychiatric disorders (e.g., developmental delay, intellectual disability, autism spectrum disorder, schizophrenia, anxiety, and bipolar disorder). Using a method of data analysis that avoids ascertainment bias, the authors determined that multicystic kidneys and other structural and functional kidney anomalies occur in 85% to 90% of affected individuals, MODY5 in approximately 40%, and some degree of developmental delay or learning disability in approximately 50%. MODY5 is most often diagnosed before age 25 years (range: age 10-50 years). [from GeneReviews]

A schizophrenia that has material basis in a mutation of SHANK3 on chromosome 22q13.33. [from MONDO]

A schizophrenia that has material basis in an autosomal dominant mutation of SCZD1 on chromosome 5q23-q35. [from MONDO]

Mitochondrial myopathy and ataxia (MMYAT) is an autosomal recessive mtDNA depletion disorder characterized by cerebellar ataxia, congenital muscle involvement with histologic findings ranging from myopathic to dystrophic, and pigmentary retinopathy (summary by Donkervoort et al., 2019). [from OMIM]

Neurodevelopmental disorder with speech impairment and dysmorphic facies (NEDSID) is characterized by developmental delay associated with mild to moderately impaired intellectual development or learning difficulties, behavioral or psychiatric abnormalities, and delayed speech and language acquisition. Additional features include dysmorphic facies, distal limb anomalies, gastrointestinal problems or feeding difficulties, and hypotonia. The phenotypic features and severity of the disorder are variable (summary by Kummeling et al., 2021). [from OMIM]

A schizophrenia that has material basis in an autosomal dominant mutation of SCZD10 on chromosome 15q15. [from MONDO]

Autosomal dominant deafness-78 (DFNA78) is characterized by profound congenital bilateral sensorineural hearing loss affecting all frequencies. Some patients may have mild motor delay early in life due to vestibular dysfunction, although the motor skills catch up with age. Affected individuals do not have systemic or other neurologic manifestations (summary by Mutai et al., 2020). [from OMIM]