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]

Alzheimer disease is the most common form of progressive dementia in the elderly. It is a neurodegenerative disorder characterized by the neuropathologic findings of intracellular neurofibrillary tangles (NFT) and extracellular amyloid plaques that accumulate in vulnerable brain regions (Sennvik et al., 2000). Terry and Davies (1980) pointed out that the 'presenile' form, with onset before age 65, is identical to the most common form of late-onset or 'senile' dementia, and suggested the term 'senile dementia of the Alzheimer type' (SDAT). Haines (1991) reviewed the genetics of AD. Selkoe (1996) reviewed the pathophysiology, chromosomal loci, and pathogenetic mechanisms of Alzheimer disease. Theuns and Van Broeckhoven (2000) reviewed the transcriptional regulation of the genes involved in Alzheimer disease. Genetic Heterogeneity of Alzheimer Disease Alzheimer disease is a genetically heterogeneous disorder. See also AD2 (104310), associated with the APOE*4 allele (107741) on chromosome 19; AD3 (607822), caused by mutation in the presenilin-1 gene (PSEN1; 104311) on 14q; and AD4 (606889), caused by mutation in the PSEN2 gene (600759) on 1q31. There is evidence for additional AD loci on other chromosomes; see AD5 (602096) on 12p11; AD6 (605526) on 10q24; AD7 (606187) on 10p13; AD8 (607116) on 20p; AD9 (608907), associated with variation in the ABCA7 gene (605414) on 19p13; AD10 (609636) on 7q36; AD11 (609790) on 9q22; AD12 (611073) on 8p12-q22; AD13 (611152) on 1q21; AD14 (611154) on 1q25; AD15 (604154) on 3q22-q24; AD16 (300756) on Xq21.3; AD17 (615080) on 6p21.2; and AD18 (615590), associated with variation in the ADAM10 gene (602192) on 15q21. Evidence also suggests that mitochondrial DNA polymorphisms may be risk factors in Alzheimer disease (502500). Finally, there have been associations between AD and various polymorphisms in other genes, including alpha-2-macroglobulin (A2M; 103950.0005), low density lipoprotein-related protein-1 (LRP1; 107770), the transferrin gene (TF; 190000), the hemochromatosis gene (HFE; 613609), the NOS3 gene (163729), the vascular endothelial growth factor gene (VEGF; 192240), the ABCA2 gene (600047), and the TNF gene (191160) (see MOLECULAR GENETICS). [from OMIM]

MN antigens reside on GYPA, one of the most abundant red-cell glycoproteins. The M and N antigens are 2 autosomal codominant antigens encoded by the first 5 amino acids of GYPA and include 3 O-linked glycans as part of the epitope. M and N differ at amino acids 1 and 5, where M is ser-ser-thr-thr-gly, and N is leu-ser-thr-thr-glu. M is the ancestral GYPA allele and is common in all human populations and Old World apes. GYPA, glycophorin B (GYPB; 617923), and glycophorin E (GYPE; 138590) are closely linked on chromosome 4q31. The N terminus of GYPB is essentially identical to that of GYPA except that it always expresses the N antigen, denoted 'N' or N-prime. Antigens of the Ss blood group (111740) reside on GYPB, and recombination and gene conversion between GYPA, GYPB, and GYPE lead to hybrid glycophorin molecules and generation of low-incidence antigens. Thus, the MN and Ss blood groups are together referred to as the MNSs or MNS blood group system. The U antigen refers to a short extracellular sequence in GYPB located near the membrane. Recombination results in 3 glycophorin-null phenotypes: En(a-) cells lack GYPA due to recombination between GYPA and GYPB; GYPB-negative (S-s-U-) cells lack GYPB due to recombination in GYPB; and M(k) cells (M-N-S-s-U-) lack both GYPA and GYPB due to recombination between GYPA and GYPE. Individuals with glycophorin-null phenotypes have decreased sialic acid content and increased resistance to malarial infection (see 611162). GYPA and GYPB are not essential for red-cell development or survival, and GYPA- and GYPB-null phenotypes are not associated with anemia or altered red-cell function (review by Cooling, 2015). [from OMIM]

Ss blood group antigens reside on the red-cell glycoprotein GYPB. The S and s antigens result from a polymorphism at amino acid 29 of GYPB, where S has met29 and s has thr29. The U antigen refers to a short extracellular sequence in GYPB located near the membrane. GYPB, glycophorin A (GYPA; 617922), and glycophorin E (GYPE; 138590) are closely linked on chromosome 4q31. Antigens of the MN blood group (111300) reside on GYPA. The M and N antigens differ at amino acids 1 and 5 of GYPA, where M is ser-ser-thr-thr-gly, and N is leu-ser-thr-thr-glu. The N terminus of GYPB is essentially identical to that of GYPA except that it always expresses the N antigen, denoted 'N' or N-prime. Recombination and gene conversion between GYPA, GYPB, and GYPE lead to hybrid glycophorin molecules and generation of low-incidence antigens. Thus, the MN and Ss blood groups are together referred to as the MNSs blood group system (see 111300). Recombination results in 3 glycophorin-null phenotypes: En(a-) cells lack GYPA due to recombination between GYPA and GYPB; GYPB-negative (S-s-U-) cells lack GYPB due to recombination in GYPB; and M(k) cells (M-N-S-s-U-) lack both GYPA and GYPB due to recombination between GYPA and GYPE. Individuals with glycophorin-null phenotypes have decreased sialic acid content and increased resistance to malarial infection (see 611162). GYPA and GYPB are not essential for red-cell development or survival, and GYPA- and GYPB-null phenotypes are not associated with anemia or altered red-cell function (review by Cooling, 2015). [from OMIM]