Mutations in the HLA class II genes leading to loss of expression of HLA-DR and HLA-DQ in diffuse large B-cell lymphoma

Loss of expression of human leukocyte antigen (HLA) class II molecules on tumor cells affects the onset and modulation of the immune response through lack of activation of CD4+ T lymphocytes. Previously, we showed that the frequent loss of expression of HLA class II in diffuse large B-cell lymphoma (DLBCL) of the testis and the central nervous system (CNS) is mainly due to homozygous deletions in the HLA region on chromosome band 6p21.3. A minority of cases showed hemizygous deletions or mitotic recombination, implying that mutation of the remaining copy of the class II genes might be involved. Here, we studied three DLBCLs with loss of HLA-DQ expression for mutations in the DQB1 and DQA1 genes and three tumors with loss of HLA-DR expression for mutations in the DRB1 and DRA genes. In one case, a point mutation in exon 2 of the DQB1 gene, leading to the formation of a stop codon, was detected at position 47. In a second case, a stop codon was found at position 11 due to a deletion of 19 bp in exon 1 of the DRA gene. No mutations were found in the promoter sequences of the DRA, DQA1 and DQB1 genes. We conclude that both homozygous deletions and hemizygous deletions or mitotic recombination with mutations of the remaining allele may lead to loss of expression of the HLA class II genes, which is comparable to the mechanisms affecting HLA class I expression in solid cancers.