Fine Mapping of IGAD1 in IgA Deficiency and Common Variable Immunodeficiency: Identification and Characterization of Haplotypes Shared by Affected Members of 101 Multiple-Case Families

To limit the region containing a mutation predisposing to selective IgA deficiency (IgAD) and common variable immunodeficiency (CVID), 554 informative members of 101 multiple-case families were haplotyped at the IGAD1 candidate locus in the MHC. Microsatellite markers were placed onto the physical map of IGAD1 to establish their order and permit rapid haplotype analyses. Linkage analysis of this extended family set provided additional support for a strong susceptibility locus at IGAD1 with a maximum multipoint nonparametric linkage score in excess of 3. Although the transmission of maternal IGAD1 haplotypes from unaffected heterozygous parents to the affected offspring was in excess, this was not apparent in multiple-case families with a predominance of affected mothers, suggesting that this parental bias is influenced by the affection status of transmitting parents and supporting a maternal effect in disease susceptibility. Of 110 haplotypes shared by 258 affected family members, a single haplotype (H1) was found in 44 pairs of affected relatives, accounting for the majority of the IGAD1 contribution to the development of IgAD/CVID in our families. The H1 allelic variability was higher in the telomeric part of the class III region than in the distal part of the class II region in both single- and multiple-case families. Incomplete H1 haplotypes had most variant alleles in the telomeric part of the analyzed region in homozygous IgAD/CVID patients, whereas this was not observed in unaffected homozygotes. These data suggest that a telomeric part of the class II region or centromeric part of the class III region is the most likely location of IGAD1 . This study was supported by the Swedish and British Medical Research Councils, the Swedish Strategic Research Foundation, the Primary Immunodeficiency Association of the United Kingdom MSMT VS96097, the Karolinska Institute, and federal funds from the National Cancer Institute, National Institutes of Health, under Contract NO1-CO-56000. The content of this publication does not necessarily reflect the views or policies of the Department of Health and Human Services, nor does mention of trade names, commercial products, or organizations imply endorsement by the U.S. Government. Address correspondence and reprint requests to Dr. Igor Vořechovský, Department of Biosciences at NOVUM, Karolinska Institute, CBT Halsovagen 7, SE-14157 Huddinge, Sweden. E-mail address: igvo@smtp.biosci.ki.se Abbreviations used in this paper: IgAD, selective IgA deficiency; CVID, common variable immunodeficiency; TDT, transmission disequilibrium test; NPL, nonparametric linkage; PCR-SSP, PCR using sequence-specific oligonucleotide primers.