PLP1 duplication at the breakpoint regions of an apparently balanced t(X;22) translocation causes Pelizaeus-Merzbacher disease in a girl

PLP1 (proteolipid protein1 gene) mutations cause Pelizaeus-Merzbacher disease (PMD), characterized by hypomyelination of the central nervous system, and affecting almost exclusively males. We report on a girl with classical PMD who carries an apparently balanced translocation t(X;22)(q22;q13). By applying array-based comparative genomic hybridization (a-CGH), we detected duplications at 22q13 and Xq22, encompassing 487-546 kb and 543-611 kb, respectively. The additional copies were mapped by fluorescent in situ hybridization to the breakpoint regions, on the derivative X chromosome (22q13 duplicated segment) and on the derivative 22 chromosome (Xq22 duplicated segment). One of the 14 duplicated X-chromosome genes was PLP1.The normal X chromosome was the inactive one in the majority of peripheral blood leukocytes, a pattern of inactivation that makes cells functionally balanced for the translocated segments. However, a copy of the PLP1 gene on the derivative chromosome 22, in addition to those on the X and der(X) chromosomes, resulted in two active copies of the gene, irrespective of the X-inactivation pattern, thus causing PMD. This t(X;22) is the first constitutional human apparently balanced translocation with duplications from both involved chromosomes detected at the breakpoint regions.