Mental Retardation
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There are numerous causes of mental retardation.  I add articles as I come across them.

Angiotensin II Receptor-2 Male Mental Retardation: Known as AGTR2, the gene defect causes 1.5% unexplained retardation in males. It is located on the X chromosome.  More than 100 of the approximately 1,200 genes on the X chromosome are involved in brain development and function. The AGTR1 gene plays a part in blood vessel development, blood pressure regulation, water and electrolyte balance and hormone secretion. The researchers suspect that AGTR2 is linked to blood vessel function or development and is likely to be important for brain formation or function. Science 6/28/02 Duane Alexander, Clemson Univ.

Bardet-Biedl Disorder Cilia Gene Defect: A relatively rare condition called Bardet-Biedl syndrome (BBS) is characterized by obesity, learning disabilities and eye and kidney problems.  Researchers have discovered a gene involved in the disease. The gene's protein, BBS8, is found only at the base of cilia. Sept. 21, 2003, Nature.

Battan Disease Cause Found: Juvenile Batten Disease leads to the seizures and mental impairment.  Immune attack inactivates an enzyme called glutamic acid decarboxylase 65 (GAD65) that normally converts the neurotransmitter glutamate into another, gamma-aminobutyric acid or GABA. The loss of the enzyme's activity leads to an excess of glutamate in the brain. Symptoms appear between the ages of 5 and 10 and are fatal. It is due to mutation of CLN3 gene. U Rochester, David Pearce, June 1, 2002, Human Molecular Genetics. It is the most common of a group of disorders called neuronal ceroid lipofuscinoses, or NCLs. NCLs are characterized by a buildup of pigments called lipofuscins in the body's cells.

Rett Syndrome MECP2 Gene Causes Some Schizophrenia and Autism: The gene MECP2 is mutated in Rett Syndrome girls, a cause in mental retardation in 1/10,000 births. Earlier research suggested that the MeCP2 protein was responsible for making sure that genes the cell has marked with a molecular tag, called a methyl group, are silenced. The MeCP2 protein latches on to these methyl groups and prevents them from being translated into protein.  Cases of both classic autism and schizophrenia are caused by mutations in this gene. Neuron 7/18/02

SMS is a chromosome microdeletion syndrome that is characterized by a very distinct series of physical, developmental and behavioral features, including varying levels of mental retardation, cranio-facial abnormalities, sleep disturbances and self-injurious behaviors. It is caused by deletion of retionic acid induced 1 gene deletion on one of the two chromosomes and occurs in 1:25,000 births. Sarah Elsea, Mich State U, Nature Genetics 3/24/03.

Williams-Beuren Syndrome Mental Retardation: WBS is a complex developmental disorder with multisystemic manifestations including supravalvular aortic stenosis (SVAS), a so-called elfin face, a hoarse voice, and mental retardation, facial features, esotopia dental, malocclusion, hernias, join limitation, WBS personality and short stature. Most WBS patients have a >1 Mb deletion on one of their chromosomes 7 in q11 but except for elastin, whose haploinsufficiency causes the cardiovascular malformations. Other missing genes characterization of WBSCR16, WBSCR17, WBSCR18, WBSCR20A, WBSCR20B, WBSCR20C, WBSCR21, WBSCR22, and WBSCR23, nine novel genes contained in the WBS commonly deleted region or its flanking sequences. They encode an RCC1-like G-exchanging factor, an N-acetylgalactosaminyltransferase, a DNAJ-like chaperone, NOL1/NOP2/sun domain-containing proteins, a methyltransferase, or proteins with no known homologies.