Characterizing the frequency of non-allelic homologous recombination (NAHR) on the human X chromosome (Xq28) using digital inverted PCR

Little is currently known about the rates at which non-allelic homologous recombination (NAHR) occurs. However, most current research suggests that NAHR is rare. Previous work by Small, et al (1998), examined an inversion polymorphism on the long arm of the X-chromosome, involving two genes (FLNA and EMD), and determined the frequency of the two gene arrangements in a group of European individuals. Here we quantify the rate at which the causal NAHR, in inverted repeats flanking the FLNA and EMD genes, occurs in meiosis using digital PCR of sperm samples, with male cheek cells as controls. NAHR was documented in all samples, including the cheek cell samples at a mean recombination rate of 1.8%, indicating that NAHR occurs much more frequently than initially believed, and appears to be occurring in mitosis. The increase in NAHR frequency in spermatogenesis is not significant leaving in question NAHR occurrence in meiosis. This study reveals a more accurate way to quantitate NAHR, serving as an important first step in better understanding various NAHR-associated diseases. Author SummaryWe sought to more accurately quantitate and characterize NAHR at a site at the end of the long arm of the X chromosome that contains a set of inverted repeats flanking two genes, filamin and emerin. We determined that NAHR is happening far more frequently than previously thought, and in this case unequally, depending on the direction of the inversion. We speculate on the possibility of local adaptation playing a role in this. These high-resolution results were obtained by modifying a previously published assay which can be easily adapted to other inversions. This could be especially helpful in studying those NAHR inversions related to disease.