Associations of MICB with cervical cancer in north-eastern Thais: identification of major histocompatibility complex class I chain-related gene B motifs influencing natural killer cell activation

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Abstract
The expression of MICB, a member of the major histocompatibility complex class I chain-related gene B family, is induced in response to cellular stress. It is one of the ligands to the NKG2D receptor. MICB is polymorphic, but the distribution of MICB polymorphism in north-eastern Thais and their potential associations with cancer have not yet been elucidated. In this study, polymerase chain reaction–sequence-specific primers were developed to identify 15 MICB alleles and one group of alleles. We performed MICB typing in 100 healthy north-eastern Thai females (NETF) and 99 cervical cancer patients to evaluate the association of MICB polymorphisms and the risk of developing cervical cancer. Eight and nine alleles were detected in the NETF and cervical cancer respectively. MICB*00502 was associated negatively with a corrected P-value of 0·0009, suggesting the existence of a protective allele in cervical cancer. Amino acid substitutions carried by this allele were investigated for their potential involvement in natural killer (NK) cell activation. Although lysine at amino acid position 80 (Lys80) and aspartic acid at position 136 (Asp136) were associated negatively with cervical cancer, only MICB carrying Asp136 could induce NK cell killing more efficiently than MICB-Lys80 when the NK cells were blocked by anti-NKG2D. This result suggested that aspartic acid at position 136 may affect NKG2D binding, leading to different degrees of immune cell activation.