Methylation-specific PCR allows for fast diagnosis of X chromosome disomy and reveals skewed inactivation of the X chromosome in men with Klinefelter syndrome.

Klinefelter syndrome (KS) remains the most common, yet often undiagnosed, chromosomal aberration in men. Early diagnosis and treatment can improve the health of patients with KS. The aim of this study was to evaluate the inactivation pattern of supernumerary X chromosomes. The secondary aim was to design a reliable and cost-effective molecular test for detection of X chromosome disomy. Methylation-specific polymerase chain reaction (M-PCR), with primers for familial mental retardation (FMR1) and X chromosome inactive-specific transcript (XIST) genes, was used to detect the presence of X chromosome disomy in men. Seventeen fertile males, 12 females, and 35 males with KS (28 with 47,XXY karyotype, and 7 with 47,XXY/46,XY mosaics) were included in the study. Results of the karyotype were compared with the results of semiquantitative M-PCR. Inactivation of X chromosomes was measured by XIST/FMR-1 methylation ratio. Differences in the methylation patterns of FMR1 and XIST genes between 46,XY men and men with X chromosome disomy allowed for rapid detection of the presence of an additional X chromosome, achieving 100% sensitivity and specificity using M-PCR. The methylated:unmethylated FMR1 amplicon ratio allowed the detection of 1 additional X chromosome per 100 normal XY cells (1% of XX/XY mosaicism). In our series, 50% of 47,XXY men showed skewed inactivation of the X chromosome. Men with KS can have incomplete inactivation of supernumerary X chromosomes. M-PCR is a sensitive, specific, fast, and relatively inexpensive test for the diagnosis of X chromosome disomy.