Discontinuities and unsynapsed regions in meiotic chromosomes have a cis effect on meiotic recombination patterns in normal human males

During meiosis, homologous chromosome pairing is essential for subsequent meiotic recombination (cross-over). Discontinuous chromosome regions (gaps) or unsynapsed chromosome regions (splits) in thesynaptonemal complex (SC) indicate anomalies in chromosome synapsis. Recently developed immunofluo-rescence techniques (using antibodies against SC proteins and the crossover-associated MLH1 protein) werecombined with fluorescencein situ hybridization (using centromere-specific DNA probes) to identifybivalents with gaps/splits and to examine the effectof gaps/splits on meiotic recombination patternsduring the pachytene stage of meiotic prophase from three normal human males. Gaps were observedonly in the heterochromatic regions of chromosomes 9 and 1, with 9q gaps accounting for 90% of theseevents. Most splits were also found in chromosomes 9 and 1, with 58% of splits occurring on 9q. Gapsand splits significantly altered the distribution of MLH1 foci on the SC. On gapped SC 9q, the frequency ofMLH1 foci was decreased compared with controls, and single 9q crossovers tended toward a more distaldistribution. Furthermore, the larger the gap the more distal the location of the MLH1 focus closest to theq arm’s telomere. MLH1 foci on split SC 9 had distributions similar to those of gapped SC 9; however,splits did not change the frequencies of MLH1 foci on SC 9. This is the first demonstration that gaps andsplits have an effect on meiotic recombination in humans.INTRODUCTIONMeiotic recombination (crossover) frequency and positioningare under tight genetic control and recombination is acrucial pre-requisite for proper meiotic chromosome segre-gation (1). The first identified molecular correlation betweenhuman non-disjunction and altered placement of recombina-tion events was that an enrichment of single crossovers inthe distal chromosome 21q was associated with an increasedprobability of non-disjunction at meiosis I (2). These typesof crossover configurations are also known to be susceptibleto non-disjunction in the fruit fly and in yeast (3,4). Thus,exploring the factors affecting crossover distribution isneeded in order to understand the basis of the associationbetween recombination and human non-disjunction.The synaptonemal complex (SC), a tripartite structure withtwo parallel lateral elements and a central element heldtogether by transverse filaments (5), mediates homologouschromosome pairing, synapsis and recombination duringmeiotic prophase I (6). A DNA mismatch repair protein,MLH1, has been identified as a marker of meiotic recombina-tion sites (reviewed in 7). The patterns of genetic recombina-tion in males with normal and abnormal spermatogenesis werecharacterized utilizing immunofluorescence methodologies,using antibodies against lateral element proteins (SCP3) andtransverse filament proteins (SCP1) to visualize SCs (8) and