A new assay for the analysis of X-chromosome inactivation in carriers with an X-linked disease.

Symptoms of X-linked recessive diseases are usually observed in males, but also observed in some female carriers because of nonrandom X inactivation in which the mutated X chromosome is active and the normal X chromosomes is inactive. Therefore, it is important to investigate the patterns of X-chromosome inactivation (XCI) for clinical assessment of carriers with an X-linked disease. We have recently developed a new assay for XCI studies based on a methylation-specific polymerase chain reaction (PCR) technique. The assay involves the chemical modification of DNA with sodium bisulfite and subsequent PCR amplification. The assay is more rapid than conventional cytogenetic assays and more accurate than the current PCR-based assay for XCI studies. Because the new assay produces not only the pattern of inactive X chromosomes but also the pattern of active X chromosomes, their combination turns out to be a more reliable XCI pattern-diminishing PCR artifact. In this review, I will discuss the basics of this new assay, and its clinical applications to various X-linked diseases, including a potential application for Rett syndrome research.