PELIZAEUS-MERZBACHER-DISEASE - IDENTIFICATION OF XQ22 PROTEOLIPID-PROTEIN DUPLICATIONS AND CHARACTERIZATION OF BREAKPOINTS BY INTERPHASE FISH

Pelizaeus-Merzbacher disease (PMD) is an X-linked, dysmyelinating diso rder of the CNS. Duplications of the proteolipid protein (PLP) gene ha ve been found in a proportion of patients, suggesting that, in additio n to coding-region or splice-site mutations, overdosage of the gene ca n cause PMD. We show that the duplication can be detected by interphas e FISH, using a PLP probe in five patients and their four asymptomatic carrier mothers. The extent of the duplication was analyzed in each f amily by interphase FISH, with probes from a 1.7-Mb region surrounding the PLP gene between markers DXS83 and DXS94. A large duplication gre ater than or equal to 500 kb was detected, with breakpoints that diffe red, between families, at the proximal end. Distinct separation of the duplicated PLP signals could be seen only on metaphase chromosomes in one family, providing further evidence that different duplication eve nts are involved. Quantitative fluorescent multiplex PCR was used to c onfirm the duplication in patients, by the detection of increased copy number of the PLP gene, Multiallelic markers from the duplicated regi on were analyzed, since the identification of two alleles in an affect ed boy would indicate a duplication. The majority of boys were homozyg ous for all four markers, compared with their mothers, who were hetero zygous for one to three of the markers. These results suggest that int rachromosomal rearrangements may be a common mechanism by which duplic ations arise in PMD. One boy was heterozygous for the PLP marker, indi cating a duplication and suggesting that interchromosomal rearrangemen ts of maternal origin also can be involved. Since duplications are a m ajor cause of PMD, we propose that interphase FISH is a reliable metho d for diagnosis and identification of female carriers.