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1.

Schizophrenia

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]

MedGen UID:
48574
Concept ID:
C0036341
Mental or Behavioral Dysfunction
2.

Major depressive disorder

Marked depression appearing in the involution period and characterized by hallucinations, delusions, paranoia, and agitation. [from MeSH]

MedGen UID:
266123
Concept ID:
C1269683
Mental or Behavioral Dysfunction
3.

Alcohol dependence

For most adults, moderate alcohol use is probably not harmful. However, about 18 million adult Americans have an alcohol use disorder. This means that their drinking causes distress and harm. It includes alcoholism and alcohol abuse. Alcoholism, or alcohol dependence, is a disease that causes. -Craving - a strong need to drink. -Loss of control - not being able to stop drinking once you've started. -Physical dependence - withdrawal symptoms. -Tolerance - the need to drink more alcohol to feel the same effect. With alcohol abuse, you are not physically dependent, but you still have a serious problem. The drinking may cause problems at home, work, or school. It may cause you to put yourself in dangerous situations, or lead to legal or social problems. Another common problem is binge drinking. It is drinking about five or more drinks in two hours for men. For women, it is about four or more drinks in two hours. Too much alcohol is dangerous. Heavy drinking can increase the risk of certain cancers. It can cause damage to the liver, brain, and other organs. Drinking during pregnancy can harm your baby. Alcohol also increases the risk of death from car crashes, injuries, homicide, and suicide. If you want to stop drinking, there is help. Start by talking to your health care provider. Treatment may include medicines, counseling, and support groups. NIH: National Institute on Alcohol Abuse and Alcoholism.  [from MedlinePlus]

MedGen UID:
1801
Concept ID:
C0001973
Mental or Behavioral Dysfunction
4.

Obsessive-compulsive disorder

Obsessive-compulsive disorder (OCD) is characterized by recurring obsessions and/or compulsions and has been estimated to affect nearly 5 million people in the United States (Karno et al., 1988). Evidence for a strong genetic component in OCD comes from twin studies, family genetics studies, and segregation analyses, as reviewed by Alsobrook et al. (2002). Zhang et al. (2002) suggested that hoarding is likely to be an evolutionarily conserved trait that, in times of adversity, was associated with increased survival and reproductive fitness. However, extreme forms of this trait are associated with marked disability and poor response to treatment (Black et al., 1998; Mataix-Cols et al., 1999). [from OMIM]

MedGen UID:
320254
Concept ID:
C1834037
Disease or Syndrome
5.

Anorexia nervosa 1

Eating disorders are characterized by severe disturbances in eating behavior that typically have onset during late adolescence and early adulthood. Three major types are recognized: anorexia nervosa (AN), bulimia nervosa (BN; 607499), and eating disorder not otherwise specified (EDNOS). AN is characterized by obsessive fear of weight gain, severely restricted eating, and low body weight. In women, AN has the highest mortality among the psychiatric disorders (Sullivan, 1995). AN is divided into 2 clinical subtypes, restricting anorexia nervosa (RAN) and binge-eating/purging anorexia nervosa (BPAN). BN can occur at any body weight and is characterized by binge-eating and compensatory weight-loss behaviors. Family studies have indicated an increased prevalence of eating disorders in relatives of probands with AN (Lilenfeld et al., 1998), and twin studies (Holland et al., 1984; Wade et al., 2000) have estimated concordance rates for monozygotic twins with AN to be 52 to 56%, whereas concordance rates for dizygotic twins with AN have been estimated to be 5 to 11% (Grice et al., 2002). [from OMIM]

MedGen UID:
376139
Concept ID:
C1847492
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