LETTER TO JMG Variability of sexual phenotype in 46,XX(SRY+) patients: the influence of spreading X inactivation versus position effects

D uring male meiosis the X and Y chromosomes pair along much of their length, with a single obligatory recombination event usually occurring in the pseudoautosomal region (PAR) at the tip of Xp and Yp, thus maintaining identity of the sex chromosome PARs. Occasionally illegitimate crossover occurs outside the PAR, resulting in the transfer of Y specific sequences onto the X chromosome. Such translocations between distal Xp and Yp occur relatively frequently, resulting in the generation of 46,XX individuals, the majority of whom display an overtly male phenotype due to transfer of the SRY gene onto the short arm of the paternal X. However, a small number of Yp translocations are associated with hermaphroditism, defined as the presence of both testicular and ovarian tissue in the same individual. While the size of the translocated Yp fragment in XX males is variable, a recombination hotspot defined by the X/Y homologous genes PRKX and PRKY accounts for approximately one third of cases. 11 A common inversion polymorphism in proximal Yp flanks this recombination hotspot. In XX individuals one of the two X chromosomes is inactivated in early embryonic development as a mechanism of dosage compensation for sex linked genes. This results in the conversion of the inactive X to a heterochromatic state and the transcriptional silencing of most of the genes upon it. Studies of X;autosome translocations have demonstrated that the X inactivation signal is also capable of spreading into cis linked chromatin in a variable manner, and it has been proposed that spreading of X inactivation into the translocated Yp segment carrying the SRY gene could account for the incomplete masculinisation which is occasionally observed in individuals with X/Y translocations. 7 8 While evidence to support this view comes from studies of the Sxr mouse, 21 only a single murine study has examined the spreading of X inactivation into Y chromatin, and no such studies have been performed in humans. It is also becoming clear that chromosomal rearrangements can in themselves result in a disruption of normal gene expression, a phenomenon known as ‘‘position effect’’. Studies of an increasing number of human diseases, including aniridia, campomelic dysplasia, and X linked deafness, have shown that rearrangements, some located up to 900 kb from the affected gene, can result in transcriptional repression, perhaps by removing essential regulatory elements or by alterations of local chromatin structure. Here we describe detailed studies of 15 individuals with segments of Yp translocated onto the distal short arm of the X chromosome. In each case we have mapped X and Y breakpoints, determined X inactivation ratios, analysed the expression and methylation status of translocated Y chromosome genes, and performed phenotype/genotype correlations. Our analysis finds little evidence for spreading of X inactivation into Yp chromatin, and instead suggests that position effects resulting from chromosomal rearrangement account for the occurrence of incomplete masculinisation in some individuals with X/Y translocation.