Thymus and parathyroids, hypoplasia of

Also known as the DiGeorge syndrome (DGS), this disorder is characterized by (1) low blood calcium levels (hypocalcemia) due to underdevelopment (hypoplasia) of the parathyroid glands needed to control calcium; (2) underdevelopment (hypoplasia) of the thymus, an organ behind the breastbone in which lymphocytes mature and multiply; and (3) defects of the outflow tracts from the heart. Most cases of DGS are due to a microdeletion in chromosome band 22q11.2. A small number of cases have defects in other chromosomes, notably 10p13. Named after the American pediatric endocrinologist Angelo DiGeorge. Another name for DGS is the third and fourth pharyngeal pouch syndrome (since the faulty structures in DGS are embryologically derived from the third and fourth pharyngeal pouches). According to a report in the October 14th, 1999 issue of The New England Journal of Medicine, transplantation of thymus tissue can restore normal immune function in infants with complete DiGeorge syndrome. Thymus tissue discarded from infants undergoing heart surgery (the thymus becomes disposable with time) was cultured and transplanted into 5 patients with profound immunodeficiency caused by DiGeorge syndrome. Four of the 5 patients then developed T-cell proliferative responses to mitogens (agents that trigger cell division). Early thymus transplantation may thus be useful for patients with DiGeorge syndrome who lack a T-cell system. The thymus transplant promotes successful immune reconstitution. Sources: M. L. Markert and others. Transplantation of Thymus Tissue in Complete DiGeorge Syndrome. N Engl J Med 1999;341:1180-1189. (The original article in the October 14, 1999 issue of The New England Journal of Medicine.) I. L. Weissman and J. A. Shizuru. Immune Reconstitution. N Engl J Med 1999;341:1227-1229. (The accompanying editorial in the October 14, 1999 issue of The New England Journal of Medicine.)