Glycosyl phosphatidylinositol-linked blood group antigens and paroxysmal nocturnal hemoglobinuria.
Human erythrocyte cell surface molecules that are attached to the cell membrane by glycosyl-phosphatidylinositol (GPI) anchors include the complement regulatory proteins decay accelerating factor (DAF, CD55) and membrane inhibitor of reactive lysis (MIRL, CD59), as well as the proteins that bear the Cartwright, Dombrock, and JMH blood group antigens. The acquired hematopoietic stem cell disorder paroxysmal nocturnal hemoglobinuria (PNH) results from the absence or marked deficiency in expression of GPI-anchored proteins in affected hematopoietic cells. PNH usually if not always results from a somatic mutation of an X-linked gene called PIG-A; the product of the PIG-A gene is a glycosyl transferase necessary for construction of the GPI anchor. DAF is a ubiquitously expressed protein present in many tissues, including gastrointestinal epithelia, corneal epithelia, and serosa of urinary and reproductive organs. DAF is a 70 kD glycoprotein containing complement regulatory short consensus repeats (SCRs); its gene is located in the regulation of complement activation (RCA) gene cluster on chromosome 1 and is about 40 kb in size. The Cromer blood group antigens, which reside on DAF, include 10 currently defined antigens, of which seven are of high incidence. The molecular basis of the Cr (a-) phenotype has been determined to be a single base pair substitution in DAF SCR4 (G-->C, leading to an ala193 to pro amino acid substitution). The Tc alpha antigen appears to be determined by the amino acid sequence of SCR1, with the Tc (a-b+) phenotype arising from a base pair substitution of G55-->T, leading to an arg18 to leu amino acid substitution. The null phenotype for Cromer antigens occurs when DAF is completely absent; only one example has been completely studied on the molecular level. That individual is homozygous for a point mutation in SCR1 (G314-->A) that creates a stop codon (TGA) in place of one normally encoding trp53 (TGG) and thus prevents further translation of the mRNA. The Dr(a-) phenotype expresses reduced quantities of DAF (approximately 40% of normal levels), as well as a polymorphism of DAF. Lack of the Dr alpha antigen has been proved to result from a single point mutation in SCR3 (C-->T in codon 165) that leads to a single amino acid substitution (ser-->leu). The Cartwright (Yt) antigens reside on acetylcholinesterase (AChE). In erythroid cells, a small exon that encodes the signal for attachment of the GPI anchor is retained in a tissue-specific process.(ABSTRACT TRUNCATED AT 400 WORDS)
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