Mutation hot spots in MICA/MICB extracellular domains

Abstract

MICA and MICB are non-classical MHC molecules that indicate cellular stress. They act as ligands for NKG2D receptors found on NK cells, thereby triggering a cytotoxic response against damaged, infected, or transformed cells. The production of soluble forms of MICA/MICB occurs via the cleavage of their extracellular domains (ECDs). The expression of MICA/MICB molecules in tumor sections or the levels of their soluble forms in blood have potential as diagnostic tools for cancer. They can predict important clinical outcomes for cancer patients, such as overall and recurrence-free survival. However, their extensive molecular polymorphism complicates the development of monoclonal antibodies (mAbs) for diagnostic use. Therefore, the diagnostic value of mAb-based assays may vary depending on the frequencies of allelic variants in local human populations. We examined the ECD amino acid sequences of more than 280 MICA and 50 MICB allelic variants. Additionally, we identified 172 and 58 single nucleotide polymorphisms (SNPs) located in the coding regions of the respective genes and resulting in amino acid replacements. The most frequent amino acid replacements (> 10%) in the ECD occur at 11 and 4 sites of MICA and MICB, respectively. We found that the frequencies of SNPs in the identified hot spots strongly correlate with each other in different human populations, despite the diversity of allelic variant frequencies. The functional role of only one site is known. The replacement of valine with methionine at position 152 enhances the affinity of MICA to NKG2D receptor. As the hot spots are dispersed throughout the entire ECD sequences, they may play a role other than modulating affinity with the NKG2D receptor interaction. We recommend that Ag sets used to validate anti-MICA/MICB mAbs meet two criteria. First, they should include both MICA and MICB alleles, as these genes have highly similar sequences. Second, the alleles should cover the variability observed in the identified hot spots.