Finette B A, Poseno T, Albertini R J
Department of Pediatrics, University of Vermont, Burlington 05401, USA.
Cancer Res. 1996 Mar 15;56(6):1405-12.
Somatic mutations in the hypoxanthine-guanine phosphoribosyltransferase (hprt) gene are rare occurrences in T-lymphocytes of normal individuals. Lacking pathogenic significance, these events can serve as reporters for assessing environmental genotoxicity. The present molecular analyses of hprt mutations arising spontaneously in normal children show that 30-35% of the genomic hprt changes in children under 5 years of age have approximately 20 Kb deletions encompassing exons 2 and 3. The frequency of these specific changes are dramatically decreased in older children. Sequence analysis of these deletion breakpoint and joining regions reveal the molecular hallmarks of V(D)J recombinase-mediated recombination events. This early childhood hprt mutational spectrum is quite distinct from the adult background spectrum but similar to that reported previously for newborns, as determined in lymphocytes from placental cord blood. The present study also demonstrates that definition of sequences in the hprt deletion joining regions that are analogous to the N-nucleotide insertion hypervariable regions of rearranged T-cell receptor genes allows the same identification of in vivo clonality of mutants as does analysis of the T-cell receptor gene rearrangements themselves. These methods reveal an in vivo clonal amplification of a V(D)J recombinase-mediated hprt mutant clone in one child in the present study. This newly found age-frequency distribution of V(D)J recombinase-mediated hprt mutations correlates with the age-frequency distribution of childhood acute lymphocytic leukemia. A significant number of these malignancies, including acute T-cell leukemia, are also characterized by V(D)J recombinase-mediated recombinations but in critical regions of the genome. hprt, therefore, captures a pathogenic mutagenic mechanism as a harmless mistake which, when it occurs in other genetic regions, may result in malignancy.
次黄嘌呤 - 鸟嘌呤磷酸核糖转移酶（hprt）基因的体细胞突变在正常个体的T淋巴细胞中很少见。这些事件缺乏致病意义，但可作为评估环境遗传毒性的报告指标。目前对正常儿童自发产生的hprt突变进行的分子分析表明，5岁以下儿童基因组中hprt变化的30 - 35%有大约20 Kb的缺失，涵盖外显子2和3。在年龄较大的儿童中，这些特定变化的频率显著降低。对这些缺失断点和连接区域的序列分析揭示了V(D)J重组酶介导的重组事件的分子特征。正如在胎盘脐带血淋巴细胞中所确定的那样，这种幼儿期hprt突变谱与成人背景谱截然不同，但与先前报道的新生儿谱相似。本研究还表明，hprt缺失连接区域中与重排的T细胞受体基因的N - 核苷酸插入高变区类似的序列定义，能够像对T细胞受体基因重排本身进行分析一样，鉴定突变体的体内克隆性。这些方法在本研究中揭示了一名儿童体内一个由V(D)J重组酶介导的hprt突变克隆的体内克隆扩增。新发现的V(D)J重组酶介导的hprt突变的年龄 - 频率分布与儿童急性淋巴细胞白血病的年龄 - 频率分布相关。包括急性T细胞白血病在内的大量此类恶性肿瘤也以V(D)J重组酶介导的重组为特征，但发生在基因组的关键区域。因此，hprt捕捉到了一种致病的诱变机制，它作为一个无害的错误，当发生在其他遗传区域时，可能导致恶性肿瘤。
