Kingzette M, Spieker-Polet H, Yam P C, Zhai S K, Knight K L
Department of Microbiology and Immunology, Loyola University of Chicago, 2160 South First Avenue, Maywood, IL 60153, USA.
Proc Natl Acad Sci U S A. 1998 Sep 29;95(20):11840-5. doi: 10.1073/pnas.95.20.11840.
Somatic DNA rearrangements in B lymphocytes, including V(D)J gene rearrangements and isotype switching, generally occur in cis, i. e., intrachromosomally. We showed previously, however, that 3 to 7% of IgA heavy chains have the VH and Calpha regions encoded in trans. To determine whether the trans-association of VH and Calpha occurred by trans-chromosomal recombination, by trans-splicing, or by trans-chromosomal gene conversion, we generated and analyzed eight IgA-secreting rabbit hybridomas with trans-associated VH and Calpha heavy chains. By ELISA and by nucleotide sequence analysis we found that the VH and Calpha regions were encoded by genes that were in trans in the germline. We cloned the rearranged VDJ-Calpha gene from a fosmid library of one hybridoma and found that the expressed VH and Calpha genes were juxtaposed. Moreover, the juxtaposed VH and Calpha genes originated from different IgH alleles. From the same hybridoma, we also identified a fosmid clone with the other expected product of a trans-chromosomal recombination. The recombination breakpoint occurred within the Smicro/Salpha region, indicating that the trans-association of VH and Calpha genes occurred by trans-chromosomal recombination during isotype switching. We conclude that trans-chromosomal recombination occurs at an unexpectedly high frequency (7%) within the IgH locus of B lymphocytes in normal animals, which may explain the high incidence of B-cell tumors that arise from oncogene translocation into the IgH locus.
B淋巴细胞中的体细胞DNA重排，包括V(D)J基因重排和同种型转换，通常发生在顺式，即染色体内。然而，我们之前发现，3%至7%的IgA重链的VH和Cα区域是反式编码的。为了确定VH和Cα的反式关联是通过染色体间重组、反式剪接还是染色体间基因转换发生的，我们生成并分析了8个分泌IgA的兔杂交瘤，其VH和Cα重链存在反式关联。通过ELISA和核苷酸序列分析，我们发现VH和Cα区域由种系中反式的基因编码。我们从一个杂交瘤的fosmid文库中克隆了重排的VDJ-Cα基因，发现表达的VH和Cα基因是并列的。此外，并列的VH和Cα基因来自不同的IgH等位基因。从同一个杂交瘤中，我们还鉴定出一个fosmid克隆，其带有染色体间重组的另一个预期产物。重组断点发生在Sμ/Sα区域内，表明VH和Cα基因的反式关联是在同种型转换过程中通过染色体间重组发生的。我们得出结论，在正常动物的B淋巴细胞的IgH基因座内，染色体间重组以出乎意料的高频率（7%）发生，这可能解释了因癌基因易位到IgH基因座而产生的B细胞肿瘤的高发病率。
