α-1，3-N-乙酰半乳糖胺等位基因421T〉C突变导致A抗原弱表达

Identification of a novel T421C mutation of α-1,3-N-acetylgalactosaminyltransferase allele responsible for an A variant

目的探讨1例ABO血型系统A变异型的分子遗传学背景。方法采用血清学方法鉴定ABO血型，测定血浆α-1，3-N-乙酰半乳糖胺转移酶的活性，用PCR扩增AB0基因的7个外显子及启动子序列，测序分析第7外显子单倍体。结果先证者红细胞ABO血型为A弱表现型，AB0基因全编码区测序发现8处核苷酸杂合位点，分别为第6外显子的261delG缺失和297A／G、第7外显子的421c／T、467C／T、646T／A、681G／A、771C／T以及829G／A杂合变异。单链测序分析提示，其中1条为002等位基因，另一个等位基因除421T〉C和467C〉T突变外，其他变异位点与A101等位基因一致。421T〉C突变可导致A糖基转移酶第141位丝氨酸替换为脯氨酸（Serl41Pro）。结论产生弱A表型的原因可能是由于A糖基转移酶基因421T〉C突变引起编码的氨基酸改变，使α-1，3-N乙酰半乳糖胺转移酶活性减弱，导致A抗原弱表达。

Objective To investigate the molecular basis of an individual featuring weak A phenotype of ABO blood group system. Methods Serologic investigations, serum transferases activity assay and absorption elution test were carried out to identify the ABO blood group. The 7 exons and flanking introns of ABO glycosyltransferase gene were amplified with polymerase chain reaction （PCR）. The products were sequenced bidirectinally following enzyme digestion. Haplotypes of exons 6 and 7 of the ABO gene were analyzed. Results A weak A antigen was identified on red blood cells of the proband. Eight heterozygous sites in exons 6 and 7 （261delG 297A/G, 421C/T, 467C/T, 646T/A, 681G/A, 771C/T, 829G/A） of the ABO gene were identified. Based on haplotype analysis, one allele was determined as 002, while a novel mutation 421T〉C was identified in another allele, which resulted in the amino acid change Ser141Pro of the A glycosyltransferase. Conclusion Above results suggested that amino acid substitutions resulted from a novel mutation 421T〉C of the ABO gene may decrease the enzymatic activity and result in the weak A phenotype.