Development of companion diagnostics and molecular target therapy in malignancy : a model of the purine metabolic enzyme deficiency

甲硫腺苷磷酸化酶（MTAP）是一种参与嘌呤和蛋氨酸代谢的酶。对MTAP阴性癌细胞系的遗传分析表明，除一株外，其余所有酶阴性癌细胞系均存在MTAP基因的部分或全部缺失。当这个特殊的细胞系与5 '-脱氮胞苷孵育时，MTAP基因的表达通过RT-PCR证实，这表明启动子超甲基化是恶性肿瘤中MTAP缺陷的机制之一。结果表明，免疫组化和免疫印迹法联合应用对白血病细胞MTAP缺陷的诊断具有重要意义，联合应用免疫组化和免疫印迹法对MTAP缺陷的诊断比单独应用基因检测更准确。由于MTAP缺陷在各种原发性肿瘤中的频率相对较高，因此可以利用正常细胞和癌细胞之间的这种代谢差异来开发选择性化疗。此外，MTAP缺陷作为分子靶点和选择性化疗的组合是伴随诊断的一个很好的例子。

Methylthioadenosine phosphorylase (MTAP) is an enzyme involved in the metabolism of purine and methionine. Genetic analysis of the MTAP-negative cancer cell lines indicated that the all enzyme-negative cell lines but one had the partial or total deletion of MTAP gene. When this exceptional cell line was incubated with 5'-deazacytidine, the MTAP gene expression was confirmed by RT-PCR, indicating that the promoter hypermethylation is one of the mechanisms for MTAP deficiency in malignancy. In addition to IHC and Western blotting with anti-human MTAP monoclonal antibody, FACS analysis was found to be useful for the diagnosis of MTAP deficiency in leukemic cell lines.In conclusions, MTAP deficiency will be diagnosed more precisely with IHC and FACS analysis than with the genetic test alone. Since the frequency of MTAP deficiency in a variety of primary tumors is relatively high, one could exploit this metabolic difference between normal and cancer cells for the development of the selective chemotherapy. Furthermore, the combination of MTAP deficiency as a molecular target and the selective chemotherapy is a good example of companion diagnostics.