DHPLC analysis of unrelated CMT patients in the Myotubularin related 2 gene,MTMR2, responsible for Charcot-Marie-Tooth disease type 4B. A. Bolino¹, L. Lonie¹, M. Zimmer¹, C.F. Boerkoel², A.P. Monaco¹, J.R. Lupski². 1) Wellcome Trust Ctr, Univ Oxford, Oxford, England; 2) Department of Molecular and Human Genetics, Baylor College of Medicine, Houston, TX.
   Charcot-Marie-Tooth type 4B (CMT4B) represents an autosomal recessive demyelinating neuropathy characterised by infantile onset with progressive symmetric distal and proximal weakness starting in the lower extremities, cranial nerve involvement in most of the cases, and focally folded myelin sheaths in the peripheral nerve. We recently demonstrated that CMT4B is caused by mutations in the gene encoding myotubularin related protein 2, a dual specificity phosphatase,MTMR2, on chromosome 11q22. Five different loss of function mutations, all of them in homozygosity, were demonstrated in unrelated CMT4B patients. MTMR2 is the fifth gene identified as responsible for a CMT disorder and the first gene encoding a phosphatase to be associated with a peripheral neuropathy. To estimate the frequency of mutations in MTMR2, we undertook a screening of 183 unrelated CMT patients, using the Denaturing High Performance Liquid Chromatography (DHPLC). These patients have been found to be negative for mutations occurring in the known genes responsible for CMT. After analysing the 15 coding exons of the MTMR2 gene in a first set of 95 patients, we found a missense mutation in exon 15 (1805C->G, Ala602Gly) in a patient with a Congenital Hypomyelinating Neuropathy, CHN. This change was not detected on 392 normal chromosomes analysed. In addition, we demonstrated two high frequency polymorphisms, 8C->A, Thr3Lys (allele frequencies 61% and 39%) and 1131T->C, Thr377Thr (allele frequencies 64% and 36%), in exons 1 and 10, respectively. Three rare polymorphisms in exon 4, 298G->A, Ala100Thr (2/392 chromosomes, 0.5%); in exon 11, 1233 G->A, Thr411Thr (2/190 chrs, 1%); and in exon 13, 1504G->C, Glu502Gln (8/392 chrs, 2%) were also detected. The finding of a heterozygous missense mutation in a patient with CHN is suggesting that MTMR2 can act through different biological mechanisms in causing CMT disorders.