Molecular studies on genomic imprinting at APRT locus

对腺嘌呤磷酸核糖转移酶（APRT）基因座的体内体细胞突变的研究已经公开了人类体细胞以令人惊讶的高频率具有突变。大多数突变为杂合性丢失（洛）型。3例APRT缺陷杂合子中未发现洛合性缺失，提示该区域存在基因印迹。通过选择性PCR和反向PCR技术，分析了这3例患者存在抑制洛现象的生殖系突变。该突变基因（APRT^<**>del）具有内含子4的缺失，并连接到未知基因。重组位点的序列测定表明，该缺失至少包含2，000 bp，缺失了一个重要基因（暂命名为LSG：LOH-supporter基因）。因此，当LSG基因在生殖系中作为杂合状态受损时，则另一条染色体上同源区域的洛被抑制。洛缺失抑制机制的新现象的发现有可能应用于各个领域。因此，当肿瘤抑制基因被破坏时，基因治疗破坏同一染色体上邻近的LSG可以成功地抑制另一个完整染色体的洛。这种基因疗法可能有助于治疗癌症发病率高的家庭。此外，目前的基因变化类型可能解释了某些家族中肿瘤发病率低的机制。

Studies on in vivo somatic mutations at the adenine phosphoribosyltransferase (APRT) locus have disclosed that human somatic cells have mutations at surprisingly high frequencies. Most of the mutations are of the loss of heterozygosity (LOH) type. However, in 3 heterozygotes for APRT deficiency, LOH was not observed suggesting the genomic imprinting at this region. Precise analyzes, however, clarified that the genomic imprinting was not observed in this area, but the 3 persons had a unique deletion mutation which suppressed the occurrence of LOH.By the techniques of selective PCR and inverse PCR,the germline mutation suppressing LOH phenomenon was analyzed. This mutation gene (APRT^<**>del) had a deletion from the intron 4 and was ligated to an unknown gene. The determination of the sequence of the recombination site disclosed that the deletion encompassed at least 2,000 bp and an important gene (tentatively named LSG : LOH-supporter gene) was missing. Thus, when a LSG gene is damaged in the germline as a heterozygous state, then the LOH of the homologous area on the other chromosome is suppressed. The discovery of the new phenomenon for the mechanisms of the suppression of LOH is likely to be applied to various areas. Thus, when a tumor suppressing gene is damaged, a gene therapy destroying a nearby LSG on the same chromosome may successfully suppress the LOH of the other intact chromosome. Such a gene therapy may be useful to treat families with high incidence of cancers. Furthermore, the present type of gene changes may explain the mechanisms of low incidence of tumors in some families.