Molecular diversity of Glanzmann thrombasthenia in southern India:: New insights into mRNA splicing and structure-function correlations of αIIbβ3 integrin (ITGA2B, ITGB3)

The molecular basis of Glanzmann thrombastlienia (GT) was studied in 40 families from southern India. Of 23 identified mutations (13 in the alpha IIb (ITGA2B) gene and 10 in the beta 3 (ITGB3) gene), 20 were novel and three were described previously. Three mutations in the beta 3 gene-p.Leu143Trp (Leu117Trp), p.Tyr307Stop (Tyr281Stop), and p.Arg119GIn (Arg93GIn)-were detected in 12, three, and two families, respectively, with definite founder effects observed for the first two mutations. Alternative splicing was predicted in silico for the normal variant and a missense variant of the beta 3 gene, and for 10/11 frameshift or nonsense mutations in alpha IIb or beta 3. The prediction was confirmed experimentally for a c.2898_2902dupCCCCT Mutation in exon 28 of the alpha IIb gene that induced exon skipping. Seven Out of nine missense mutations substituted highly conserved amino acids buried in the proteins' cores, predicting structural abnormalities. Among these, a 03 substitution, p.Cys39Gly (Cys13GIy) was found to cause intracellular degradation of the beta 3 subunit, in contrast to previous findings that mutations at Cys435, the partner of Cys13 in a disulfide bond, cause constitutive activation of alpha IIb beta 3. The two patients with a beta 3 Arg93GIn mutation had normal clot retraction, consistent with a recent finding that this substitution is associated with normal surface expression of alpha IIb beta 3. In conclusion, this study demonstrates that a variety of mutations account for GT in southern Indian patients, provides new insights into mRNA splicing, and highlights the role of specific amino acids in structure-function correlations of alpha IIb beta 3.