Real-time quantitative PCR as a routine method for screening large rearrangements in Rett syndrome: Report of one case of MECP2 deletion and one case of MECP2 duplication

Abstract

Mutations in the X-linked methyl-CpG-binding protein 2 gene (MECP2) are found in 70-80% of cases of classical Rett syndrome (RTT) and in about 50% of cases of preserved speech variant (PSV). This high percentage of MECP2 mutations, especially in classical RTT cases, suggests that another major RTT locus is unlikely. Missed mutations may be due to the limited sensitivity of the methodology used for mutation scanning and/or the presence of intronic mutations. In a double-copy gene, such as MECP2 in females, current methodologies (e.g., DGGE, SSCP, DHPLC, direct sequencing) are prone to miss gross rearrangements. Three previous reports during 2001-2003 have shown the presence of large deletions in a fraction of MECP2-negative classical RTT patients. We developed a reliable, single tube, quantitative PCR assay for rapid determination of MECP2 gene dosage. This method involves a multiplex reaction using a FAM labeled TaqMan probe with a TAMRA quencher derived from MECP2 exon 4 and two primers derived from the same exon and RNAaseP as an internal reference. The copy number of the MECP2 gene was determined by the comparative threshold cycle method (ddCt). Each sample was run in quadruplicate. We validated this assay through the analysis of 30 healthy controls (15 female and 15 male) and we then applied this method to eight classical RTT and six PSV patients, all negative for MECP2 mutations. We identified gross rearrangements in two patients: a deletion in a classical RTT patient and a duplication in a PSV patient. Our results confirm that a fraction of MECP2-negative RTT cases have MECP2 gross rearrangements and we propose real-time quantitative PCR as a simple and reliable method for routine screening of MECP2 in addition to DHPLC analysis.