Buard J, Shone A C, Jeffreys A J
Department of Genetics, University of Leicester, United Kingdom.
Am J Hum Genet. 2000 Aug;67(2):333-44. doi: 10.1086/303015. Epub 2000 Jun 26.
Unequal crossover has long been suspected to play a role in the germline-specific instability of tandem-repeat DNA, but little information exists on the dynamics and processes of unequal exchange. We have therefore characterized new length alleles associated with flanking-marker exchange at the highly unstable human minisatellite CEB1, which mutates in the male germline by a complex process often resulting in the gene conversion-like transfer of repeats between alleles. DNA flanking CEB1 is rich in single-nucleotide polymorphisms (SNPs) and shows extensive haplotype diversity, consistent with elevated recombinational activity near the minisatellite. These SNPs were used to recover mutant CEB1 molecules associated with flanking-marker exchange, directly from sperm DNA. Mutants with both proximal and distal flanking-marker exchange were shown to contribute significantly to CEB1 turnover and suggest that the 5' end of the array is very active in meiotic unequal crossover. Coconversions involving the interallelic transfer of repeats plus immediate flanking DNA were also common, were also polarized at the 5' end of CEB1, and appeared to define a conversion gradient extending from the repeat array into adjacent DNA. Whereas many mutants associated with complete exchange resulted in simple recombinant-repeat arrays that show reciprocity, coconversions were highly gain-biased and were, on average, more complex, with allele rearrangements similar to those seen in the bulk of sperm mutants. This suggests distinct recombination-processing pathways producing, on the one hand, simple crossovers in CEB1 and, on the other hand, complex conversions that sometimes extend into flanking DNA.
长期以来，人们一直怀疑不等交换在串联重复DNA的种系特异性不稳定性中起作用，但关于不等交换的动力学和过程的信息却很少。因此，我们鉴定了与高度不稳定的人类小卫星CEB1侧翼标记交换相关的新长度等位基因，该小卫星在雄性生殖系中通过一个复杂的过程发生突变，该过程常常导致等位基因之间重复序列的类似基因转换的转移。CEB1侧翼的DNA富含单核苷酸多态性（SNP），并表现出广泛的单倍型多样性，这与小卫星附近重组活性的升高相一致。这些SNP被用于直接从精子DNA中回收与侧翼标记交换相关的突变型CEB1分子。具有近端和远端侧翼标记交换的突变体被证明对CEB1的更新有显著贡献，并表明阵列的5'端在减数分裂不等交换中非常活跃。涉及重复序列加上紧邻侧翼DNA的等位基因间转移的共转换也很常见，在CEB1的5'端也呈极化分布，并且似乎定义了一个从重复序列阵列延伸到相邻DNA的转换梯度。虽然许多与完全交换相关的突变体产生了显示相互性的简单重组重复序列阵列，但共转换具有高度的增益偏向性，平均而言更复杂，其等位基因重排比大多数精子突变体中的更为相似。这表明存在不同的重组加工途径，一方面在CEB1中产生简单的交叉，另一方面产生有时延伸到侧翼DNA的复杂转换。
