Linked marker analysis of spontaneous HLA variants of somatic cells

Fourteen independent, spontaneously arising HLA-A1 and -B27 variant clones were isolated from the pseudodiploid B lymphoid line T5-1 by selection using A1 and B27 alloantiserum, respectively, and complement. T5-1 is heterozygous for the tightly linked loci HLA-DR, -B, and -A and probably for -C as well. Following recloning, each of the variants was tested for the HLA specificities of T5-1. None of these spontaneous variants had a genetic lesion which extended to the nearest flanking HLA marker, less than 1 cM distant. On the other hand, a variant isolated from mutagenized cells had a lesion which extended completely through one HLA region in cis, suggesting that haploid expression in the HLA region is compatible with viability, that there are no recessive lethals in the opposite HLA region, and that spontaneously arising variants with extended lesions could have been recovered had they occurred. From these results, we conclude that extended genetic lesions of 0.8 cM or longer contribute less than 10% to the overall rate of spontaneous variation for HLA alleles, which we previously estimated at about 5 x 10(-7) per cell per generation.