22q11.2 deletion syndrome

DiGeorge syndrome was once thought to be a distinct syndrome, but is now recognized to fall within the disorder spectrum known as 22q11.2 deletion syndrome. Symptoms of what was formerly known as DiGeorge syndrome were variable and the underlying cause (deletions of 22q11.2) is also responsible for related/overlapping syndromes. Terms such as 'complete' and 'partial' DiGeorge syndrome have been used in reference to individual cases which had all the characteristic signs and symptoms (e.g., hypoparathyroidism, absent thymus, and congenital heart disease) verses those with only some of them.^[1]

Most people with 22q11.2 deletion syndrome are missing a sequence of about 3 million DNA building blocks (base pairs) on one copy of chromosome 22 in each cell. This region contains 30 to 40 genes, many of which have not been well characterized. A small percentage of affected individuals have shorter deletions in the same region.^[2] We are unaware of any cases of homozygous 22q11.2 deletions. However, in addition to the loss of genes in the 22q11.2 region (e.g., TBX1 and COMT), the highly variable signs and symptoms of 22q11.2 deletion syndrome are in part due to changes outside the deleted region or on the remaining (non-deleted) copy of chromosome 22 (e.g., SNAP29).^[2][3][4]