The 21st Chromosome and Down Syndrome

The chromosomes are holders of the genes, those bits of DNA that direct the production of a wide array of materials the body needs. This direction by the gene is called the gene's "expression." In trisomy 21, the presence of an extra set of genes leads to overexpression of the involved genes, leading to increased production of certain products. For most genes, their overexpression has little effect due to the body's regulating mechanisms of genes and their products. But the genes that cause Down syndrome appear to be exceptions. Which genes are involved? That's been the question researchers have asked ever since the third 21st chromosome was found. From years of research, one popular theory stated that only a small portion of the 21st chromosome actually needed to be triplicated to get the effects seen in Down syndrome; this was called the Down Syndrome Critical Region. However, this region is not one small isolated spot, but most likely several areas that are not necessarily side by side. The 21st chromosome may actually hold 200 to 250 genes (being the smallest chromosome in the body in terms of total number of genes); but it's estimated that only a small percentage of those may eventually be involved in producing the features of Down syndrome. Right now, the question of which genes do what is highly speculative. However, there are some suspects. Genes that may have input into Down syndrome include: Superoxide Dismutase (SOD1)-- overexpression may cause premature aging and decreased function of the immune system; its role in Senile Dementia of the Alzheimer's type or decreased cognition is still speculative COL6A1 -- overexpression may be the cause of heart defects ETS2 -- overexpression may be the cause of skeletal abnormalities CAF1A -- overexpression may be detrimental to DNA synthesis Cystathione Beta Synthase (CBS) -- overexpression may disrupt metabolism and DNA repair DYRK -- overexpression may be the cause of mental retardation CRYA1 -- overexpression may be the cause of cataracts GART -- overexpression may disrupt DNA synthesis and repair IFNAR -- the gene for expression of Interferon, overexpression may interfere with the immune system as well as other organ systems Other genes that are also suspects include APP, GLUR5, S100B, TAM, PFKL, and a few others. Again, it is important to note that no gene has yet been fully linked to any feature associated with Down syndrome.