Di George Syndrome - Genetics

Genetics

Most people with 22q11.2 deletion syndrome are missing about 3 million base pairs on one copy of chromosome 22 in every cell of their body. This region contains about 45 genes, but some of these genes have not been well characterized. A small percentage of affected individuals have shorter deletions in the same region.

Researchers have not yet identified the genes that contribute to the features of 22q11.2 deletion syndrome. They have determined that the loss of one particular gene on chromosome 22, TBX1, is probably responsible for some of the syndrome's characteristic signs (such as heart defects). Carrying only one copy of this gene does not appear to cause learning disabilities, however. Additional genes in the deleted region are likely to contribute to the signs and symptoms of 22q11.2 deletion syndrome and genes outside the 22q11.2 region may also play a role.

The 22q11.2 deletion syndrome can be inherited, but this is the case in the minority of newly diagnosed individuals. Only 5–10% have inherited the 22q11.2 deletion from a parent, whereas about 90–95% of cases have a de novo (new to the family) deletion of 22q11.2. This is because the 22q11.2 region has a structure that makes it highly prone to rearrangements during sperm or egg formation. The deletion is almost equally likely to occur when an egg is formed as when a sperm is formed. The deletion follows standard Mendelian inheritance, so an individual carrying the deletion 22q11.2 has a 50% (one in two) chance of passing it to their offspring. Prenatal testing, such as amniocentesis, is available for pregnancies determined to be at risk. Also, pregnancies with findings of congenital heart disease and/or palate anomalies detected by ultrasound examination may be offered prenatal testing for 22q11.2 deletion syndrome. Because most of the signs of this cluster of defects can also be inherited as autosomal recessive or x-linked traits, only genetic testing of both parents can determine with any certainty the likelihood of these anomalies occurring in any subsequent children.