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]

Clinical features of atelosteogenesis type 2 (AO2) include rhizomelic limb shortening with normal-sized skull, hitchhiker thumbs, small chest, protuberant abdomen, cleft palate, and distinctive facial features (midface retrusion, depressed nasal bridge, epicanthus, micrognathia). Other typical findings are ulnar deviation of the fingers, gap between the first and second toes, and clubfoot. AO2 is usually lethal at birth or shortly thereafter due to pulmonary hypoplasia and tracheobronchomalacia. However, it exists in a continuous phenotypic spectrum with diastrophic dysplasia, and long-term survivors have been reported. [from GeneReviews]

No consensus exists on what a stroke-like episode is, but these episodes can be functionally defined as a new neurological deficit, occurring with or without the context of seizures, which last longer than 24 hours. [from HPO]

Keratoconus, the most common corneal dystrophy, is a bilateral, noninflammatory progressive corneal ectasia. Clinically, the cornea becomes progressively thin and conical, resulting in myopia, irregular astigmatism, and corneal scarring. The disease usually arises in the teenage years, eventually stabilizing in the third and fourth decades. The incidence of keratoconus is 1 in 2,000 in the general population; it occurs with no ethnic or gender preponderance, and causes significant visual impairment in young adults. No specific treatment exists except to replace the corneal tissue by surgery (corneal transplantation) when visual acuity can no longer be corrected by contact lenses (summary by Dash et al., 2006). Ihalainen (1986) reviewed various conditions with which keratoconus is at times associated. Keratoconus is frequent in cases of amaurosis congenita of Leber (204000). Genetic Heterogeneity of Keratoconus Also see KTCN2 (608932), mapped to 16q22.3-q23.1; KTCN3 (608586), mapped to 3p14-q13; KTCN4 (609271), mapped to 2p24; KTCN5 (614622), mapped to 5q14.1-q21.3; KTCN6 (614623), mapped to 9q34; KTCN7 (614629), mapped to 13q32; KTCN8 (614628), mapped to 14q24; and KTCN9 (617928), caused by mutation in the TUBA3D gene (617878) on 2q21. [from OMIM]

Meckel-Gruber syndrome is a severe autosomal recessive ciliopathy classically defined by the triad of encephalocele, polydactyly, and renal and biliary ductal dysplasia. Clinical heterogeneity exists even within families (summary by Shaheen et al., 2011). For a general phenotypic description and a discussion of genetic heterogeneity of Meckel syndrome, see MKS1 (249000). [from OMIM]

Normal color vision in humans is trichromatic, being based on 3 classes of cone that are maximally sensitive to light at approximately 420 nm (blue cones; 613522), 530 nm (green cones; 300821), and 560 nm (red cones; 300822). Comparison by neural circuits of light absorption by the 3 classes of cone photoreceptors allows perception of red, yellow, green, and blue colors individually or in various combinations. Dichromatic color vision is severely defective color vision based on the use of only 2 types of photoreceptors, blue plus green (protanopia; see 303900) or blue plus red (deuteranopia). Anomalous trichromacy is trichromatic color vision based on a blue, green, and an anomalous red-like photoreceptor (protanomaly), or a blue, red, and an anomalous green-like photoreceptor (deuteranomaly). The color vision defect is generally mild but may in certain cases be severe. Common variation in red-green color vision exists among both normal and color-deficient individuals (review by Deeb, 2005). [from OMIM]

Although mitochondria and mitochondrial DNA (mtDNA) are typically maternally inherited, evidence exists for rare occurrences of paternal mtDNA transmission (Luo et al., 2018). [from OMIM]

Normal color vision in humans is trichromatic, being based on 3 classes of cone that are maximally sensitive to light at approximately 420 nm (blue cones; 613522), 530 nm (green cones; 300821), and 560 nm (red cones; 300822). Comparison by neural circuits of light absorption by the 3 classes of cone photoreceptors allows perception of red, yellow, green, and blue colors individually or in various combinations. Dichromatic color vision is severely defective color vision based on the use of only 2 types of photoreceptors, blue plus green (protanopia) or blue plus red (deuteranopia; see 303800). Anomalous trichromacy is trichromatic color vision based on a blue, green, and an anomalous red-like photoreceptor (protanomaly), or a blue, red, and an anomalous green-like photoreceptor (deuteranomaly). The color vision defect is generally mild but may in certain cases be severe. Common variation in red-green color vision exists among both normal and color-deficient individuals (review by Deeb, 2005). [from OMIM]

A rare acquired motor neuron disease with characteristics of a slowly progressive unilateral ascending or descending hemiplegia, associated with unilateral or asymmetrical pyramidal signs and no sensory loss. It is a diagnosis of exclusion and controversy exists regarding whether the presence of bulbar symptoms, sphincter disturbances, fasciculations or cognitive manifestations are characteristics of the disease. [from SNOMEDCT_US]

Inflammation of the tonsils that has occurred repeatedly. The definition of recurrent may vary somewhat, but the criteria used recently as a measure of severity were five or more episodes of true tonsillitis per year, symptoms recurring for at least a year, and episodes that are disabling and that prevent normal functioning. In some cases recurrent tonsillitis may be related to immunosusceptibility. Evidence exists for a genetic predisposition for recurrent tonsillitis. [from HPO]

A rare otorhinolaryngological malformation characterized by the presence of a dermoid cyst, located on the dorsum of the nose, which presents a fistula, often extending to the intracranial region. Patients present a firm, slow-growing mass, which contains skin and dermal elements (including hair follicles and sebaceous glands), that do not transilluminate or compress, and may be associated with intermittent or chronic discharge of sebaceous material, soft tissue and skeletal deformity, and local infection. Meningitis, convulsions and cerebral abscess may be observed if intracranial extension exists. [from ORDO]

A form of agenesis of the tricuspid valve in which (although the normal orifice between the right atrium and right ventricle exists) there is no tricuspid valvular tissue. [from HPO]

The obstruction of the right ventricular outflow tract that originates within the body of the right ventricle, that exists at the time of birth; it often occurs in association with other intracardiac anomalies. [from NCI]

Third-degree atrioventricular (AV) block (also referred to as complete heart block) is the complete dissociation of the atria and the ventricles. Third-degree AV block exists when more P waves than QRS complexes exist and no relationship (no conduction) exists between them. [from HPO]

Any ventricular septal defect (VSD) that is small enough to restrict flow across it such that a pressure gradient exists between the two sides of the VSD. [from HPO]

A rare congenital anomaly of the inferior vena cava with complete interruption of the vessel in which no direct continuity exists between the inferior vena cava and the azygos/hemiazygos system. Clinical manifestations depend on the variant drainage patterns or collaterals and include lower extremity deep vein thrombosis, thromboembolic attacks, leg swelling and pain, lower extremity varices, abdominal pain, intraabdominal varices and hematochezia among others. Additional venous abnormalities or cardiac malformations are frequently present. [from SNOMEDCT_US]