Heterogeneous rearrangements of the PLP genomic region in Pelizaeus-Merzbacher Disease: genotype-phenotype correlation in 41 patients. O. Boespflug-Tanguy¹, G. Giraud¹, C. Mimault¹, V. Isabelle¹, D. Pham Dinh², European Network on Brain Dysmyelinating Diseases¹. 1) Inserm U384, Faculte de Medecine, Clermont-Ferrand, France; 2) UMR CNRS 7624, Universite Pierre et Marie Curie, Paris, France.

   Pelizaeus-Merzbacher disease (PMD) is an X-linked developmental defect of brain myelination due to proteolipoprotein (PLP) gene mutations. Using a quantitative fluorescent multiplex PCR technique in 104 PMD affected families, we shown that duplication of the entire PLP gene is the most frequent mutation accounting for 50% of all PMD affected families and 64% of all PLP identified mutations. Furthermore, we found a large PLP gene deletion in a spastic paraplegia 2 family. Clinical evaluation of the 84 PLP-duplicated patients revealed a large range of disease severity with patients able to walk with support in the mildest form to patients with no motor achievment in the most severe form. An intrafamilial homogeneity was observed in the 14 familial forms analysed. The extend of the genomic duplication was further determined in 41 families by using a multiplex PCR with 10 pairs of primers chosen along the Xq22 region encompassing the PLP locus and by pulse field gel electrophoresis. Size of the duplications were greatly variable (40kb<to>1000kb) and not correlated to the clinical severity of the disaese. The two patients with the most severe form had PCR ratio compatible with a PLP triplication. For two patients the telomeric end of the duplication was localised inside the 20 kb end of the cosmid cV698D2 containing the PLP gene. Southern blot and sequencing analysis of the two breakpoint regions have demonstrated distinct rearrangements. Finally in PLP-related diseases, severity is correlated to the number of PLP copies and not to the size of the duplication. Existence of both duplication and deletion events associated with a large male mutation imbalance for duplications (C. Mimault, in press) suggest a mechanism of unequal sister chromatid exchange, as in the demyelinating neuropathies related to PMP22 mutations (CMT1 and HNPP). However, the large diversity found in the break points of PLP duplications suggests a more complex mechanism leading to the great genomic instability of the xq22 region.