Informativeness of linkage analysis for genetic diagnosis of haemophilia A in India

The objective of this study was to assess the frequency of factor VIII (FVIII) gene intron 1 and intron 22 inversions and the informativeness of polymorphic markers for the genetic diagnosis of patients with haemophilia A (HA). Fifty unrelated patients with HA were first assessed for the intron 1 and intron 22 inversion mutations. Inversion-negative families were then screened for the bi-allelic intragenic markers - intron 7 G-->A polymorphism, HindIII site in intron 19 and XbaI site in intron 22 and the multiallelic dinucleotide CA repeat alleles in introns 13 and 22. The extragenic, multiallelic VNTR DXS52 (st14) was also analysed. Intron 22 inversion mutation was found in 38% (n = 19) of all patients and 46% of those with severe HA. Intron 1 inversion was found in one (2%) patient. Of the 30 inversion-negative families, XbaI site polymorphism was the single most informative marker (70%, n = 21/30) followed by HindIII (60%, n = 18/30), intron 13 CA repeats (56.66%, n = 17/30), intron 22 CA repeats (50%, n = 15/30), DXS52 VNTR (23.33%, n = 7/30) and intron 7 G-->A polymorphism (6.66%, n = 2/30). The combined use of these markers was informative in 92% (n = 46/50) of HA families. Based on the informativeness of these markers a comprehensive algorithm has been proposed for genetic diagnosis of HA in India.