Trisomy 21 or Down syndrome

• 47, XY, +21 or 47, XX, +21 is seen in approximately 1/800 live births making it the most common. Was first described in 1866 by John Langdon Down.
  
• In 1959 it was discovered that it was caused by the presence of an extra chromosome 21.
• There are some significant medical problems associated with this condition.
  
• These include obstruction of the duodenum or atresia of the esophagus, duodenum, or anus.
  
• Respiratory infections are quite common and the risk of developing leukemia is 15 to 20 times higher in Down syndrome patients.
  
• In addition approximately 40% are born with structural heart defects including, atrioventricular canal, a defect which allows the blood to flow from the left to the right side and then the pulmonary vasculature producing pulmonary hypertension. Ventricular septal defects are also common in individuals with Down syndrome.
  
• Approximately 95% of Down syndrome cases are caused by non-disjunction the remainder is caused by chromosome translocation. In 90 to 95% of the cases the non-disjunction was found to be maternally derived. Mosaicism is seen in 1-3 % of all trisomy 21 cases. These individuals have some normal cells and some trisomy 21 cells. This often results in milder clinical symptoms. This is likely caused by a trisomy conception followed by specific loss of the extra chromosome from some cells.
  
• The part of chromosome 21 that is responsible for this phenotype has been narrowed down 21q22.
  
• There are a number of genes in this region of the chromosome that may be directly involved in this phenotype including DYRK kinase as well as the APP gene.
  
• The APP gene has been found to be mutated in some patients Alzheimers like dementia.

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