MedGen

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]

Immunodeficiency-83 (IMD83) is characterized by increased susceptibility to severe viral infections, including herpes simplex virus (HSV), varicella zoster virus (VZV), influenza A virus (IAV), hantavirus, and possibly respiratory syncytial virus (RSV). The age at onset varies widely from infancy to adulthood, and there is incomplete penetrance. The susceptibility to encephalitis or pneumonitis appears to result from impaired TLR3-dependent interferon production by nonhematopoietic cells that reside within the central nervous system (CNS) or lung epithelial cells (review by Zhang et al., 2013; summary by Mork et al., 2015; Sironi et al., 2017, Lim et al., 2019, Partanen et al., 2020). For a general phenotypic description of herpes simplex encephalitis (HSE) and a discussion of genetic heterogeneity of acute infection-induced encephalopathy, see 610551. [from OMIM]

Retinitis pigmentosa-83 (RP83) is characterized by onset of night blindness in the first decade of life, with decreased central vision in the second decade of life in association with retinal degeneration. The retinal dystrophy is associated with cataract, and macular edema has also been reported in some patients (Holtan et al., 2019). [from OMIM]

Developmental and epileptic encephalopathy-83 (DEE83) is a severe autosomal recessive neurodevelopmental disorder characterized by onset of frequent seizures in the first days to months of life that are usually refractory to medical treatment and are associated with significant EEG abnormalities. Affected individuals have profoundly impaired development, with no motor or language skill acquisition, poor or absent visual tracking, and poor oromotor function necessitating tube feeding. Many patients die in the first years of life (summary by Perenthaler et al., 2020). For a general phenotypic description and a discussion of genetic heterogeneity of DEE, see 308350. [from OMIM]

Spermatogenic failure-83 (SPGF83) is characterized by male infertility due to asthenozoospermia. Patient sperm are immotile, and exhibit an asymmetric fibrous sheath of the flagella (Wu et al., 2023). Patients with reduced sperm motility due to morphologic abnormalities of the flagella (asthenoteratozoospermia) and patients with reduced sperm counts as well as flagellar defects and reduced or absent motility (oligoasthenoteratozoospermia) have been observed (Sha et al., 2022; Zhang et al., 2024). For a general phenotypic description and discussion of genetic heterogeneity of spermatogenic failure, see SPGF1 (258150). [from OMIM]

Autosomal recessive spastic paraplegia-83 (SPG83) is a neurologic disorder characterized by progressive lower limb spasticity resulting in gait instability. Patients develop symptoms in the second decade, consistent with juvenile onset. Some patients may have myalgia or mild dysarthria, but the phenotype is considered to be a pure type of SPG with no additional neurologic abnormalities (summary by Husain et al., 2020). For a discussion of genetic heterogeneity of autosomal recessive spastic paraplegia, see SPG5A (270800). [from OMIM]

Autosomal dominant deafness-83 (DFNA83) is characterized by the onset of progressive sensorineural hearing loss at an average age of 24 years. A notable finding is a normal distortion product otoacoustic emissions (DPOAE) test, implicating dysfunction of spiral ganglia neurons rather than outer hair cells as a disease mechanism (Cui et al., 2020). [from OMIM]

An autosomal recessive nonsyndromic deafness that has material basis in variation in the chromosome region 9p23-p21.2. [from MONDO]

Acquired partial lipodystrophy (APLD) is characterized clinically by the gradual onset of bilaterally symmetrical loss of subcutaneous fat from the face, neck, upper extremities, thorax, and abdomen, in the 'cephalocaudal' sequence, sparing the lower extremities. A large group of patients (83%) with acquired partial lipodystrophy have low serum levels of complement component C3 due to the presence of C3 nephritic factor, an IgG antibody that causes continuous activation of the alternative complement pathway and consumption of serum C3. About 22% of patients with this acquired complement defect develop membranoproliferative glomerulonephritis. Some individuals may also show an increased risk of infection (Misra et al., 2004). Acquired partial lipodystrophy is not inherited in a classic mendelian pattern; it rather represents a phenotype with a complex etiology. Affected individuals may have genetic susceptibility factors that require the additional presence of environmental factors or acquired disorders to be expressed (summary by Hegele et al., 2006). Most cases are sporadic, family history is negative, and females are more often affected than males (ratio, 4:1) (summary by Misra et al., 2004). See 608709 for a subtype of APLD not associated with low complement C3 or renal disease. [from OMIM]