Both the rate and spectrum of loss of heterozygosity differ between human lymphoblastoid cells derived from various donors

Loss of heterozygosity (LOH) contributes significantly to the inactivation of tumor suppressor genes and may involve a variety of mechanisms. Studying loss of HLA-AZ alleles in human lymphoblastoid cell lines, we previously showed that mitotic recombination and chromosome loss with concomitant duplication of the non-selected chromosome were the most frequent mechanisms of LOH. In the present study we used the HLA system to determine the rate and spectrum of LOH mutations in the EBV transformed lymphoblastoid cell Line R83-4915. Spontaneous loss of HLA-A2 in R83-4915 occurred with a rate of 7.9 X 10(-7) which was 5 to 10-times lower compared to the previously observed rate of loss of HLA-A2 in other lymphoblastoid cell lines. Among the HLA-A2 mutants, 27% did not show LOH of additional chromosome 6 markers. Molecular analysis showed that neither large deletion nor gene conversion was the cause for their mutant phenotype. The remaining mutants showed LOH, which was caused by mitotic recombination (40%) and chromosome loss (33%). However, the chromosome loss observed in mutants of R83-4915 was not accompanied by the duplication of the remaining chromosome. Instead 3 out of 5 mutants became polyploid suggesting that different mechanisms exist to compensate for chromosome loss. In conclusion, the rate and types of LOH that can be observed in cell lines obtained from various donors may depend on the genetic make-up or the transformation status of these cells (C) 1999 Elsevier Science B.V. All rights reserved.