GENOMICS Vol. 75, Numbers 1–3, July 2001 Copyright © 2001 by Academic Press. All rights of reproduction in any form reserved. 6 Short Communication doi:10.1006/geno.2001.6591, available online at http://www.idealibrary.com on IDEAL Myotubularin and myotubularin-related proteins are dual-specificity phosphatases. Several myotubularin- related proteins have been identified in humans and mice. The members of the myotubularin protein family are highly conserved, from humans to yeast. Mutations in the human myotubularin gene (MTM1) lead to X- linked myotubular myopathy. Here we isolate and localize a novel putative myotubularin-related protein gene (MTMR8) on chromosome 8p22–p23, between the markers D8S550 and D8S265, by exon-trapping experi- ments and RT-PCR. Genomic sequencing revealed that the gene consists of 10 exons and spans approximately 43 kb. The corresponding cDNA is 7081 bp. The open reading frame predicts a protein of 549 amino acids and a calculated molecular mass of 63 kDa. Like myotubu- larin-related protein-5, MTMR8 has no dual-specificity phosphatase domain. It contains a double-helical motif similar to the SET interaction domain, which is thought to have a role in the control of cell proliferation. Myotubularin is a dual-specificity phosphatase (dsPTPase) characterized by its ability to hydrolyze protein phosphotyro- sine as well as phosphothreonine or phosphoserine [1]. It con- tains a protein tyrosine phosphatase consensus sequence (VHCxxGxxRT), and its dephosphorylating abilities on phos- photyrosine and phosphoserine were shown in vitro [2,3]. Although not much is known about the physiological role of this protein, recent studies have revealed that it dephosphory- lates phosphatidylinositol 3-phosphate (PI(3)P), a lipid second messenger, in vitro and in vivo [4,5]. Mutations in the human myotubularin gene (MTM1) cause X-linked myotubular myopathy, a severe recessive congenital muscle disorder (XLMTM, MIM 310400 [6]). It is thought that this disorder results from an arrest in differentiation of the myotubes pre- ceding myofiber formation because the structural features seen in patients with myotubular myopathy resemble fetal myotubes [7–9]. Hence myotubularin seems to be involved in the formation of myofibers and essential for the differentiation of myocytes. Besides the catalytic phosphatase domain, myotubularin has a paired-helical motif, the SET (for the Drosophila melanogaster proteins Su(var)3-9, Enhancer-of-zeste, and Trithorax (trx)) interaction domain (SID). The domain binding to this motif, the SET domain, is found in several pro- teins that play a part in epigenetic mechanisms of gene regu- lation by controlling whether the chromatin state is permissive or restrictive for transcription. Moreover, the SET domain is found in the mammalian homologue of trithorax, the product of the proto-oncogene MLL [2]. This gene is reported to be fre- quently mutated in human leukemias [10–12]. So far, seven different genes encoding human myotubu- larin-related proteins have been reported [3], which map to different chromosomal regions: MTMR1 (MIM 300171, Xq28 [13]), MTMR2 (MIM 603557, 11q22), MTMR3 (MIM 603558, 22q12), MTMR4 (MIM 603559, chr. 17), SBF1 (also known as MTMR5, MIM 603560, 22qter), MTMR6 (MIM 603561, 13q12), and MTMR7 (MIM 603562, 8p22). All the corresponding pro- teins share a common motif, the SET interaction domain. Recently, it has been shown that Charcot-Marie-Tooth disease type 4B (an autosomal recessive demyelinating neuropathy with myelin outfoldings [14]) is caused by mutations in the gene encoding myotubularin-related protein-2 (MTMR2), but the molecular pathogenesis of the disease is not yet known. None of the other myotubularin-related proteins have been associated with disease so far. In contrast to myotubularin, SBF1 (for Set binding factor 1 [2]) lacks a catalytic phos- phatase domain. Overexpression of SBF1 induces oncogenic transformation of NIH 3T3 fibroblasts in vitro [2] and stimu- lates growth of primary B-cell precursors [15]. Moreover, SBF1 is thought to bind to peptides containing phosphotyro- sine residues and act as an anti-phosphatase by preventing substrate dephosphorylation [2]. Therefore, high levels of SBF1 might induce unrestrained proliferation [2]. All in all, the myotubularin protein family seem to be a group of pro- teins with opposing functions. Here we report the isolation and localization of a novel putative myotubularin-related protein gene, MTMR8, between the markers D8S550 and D8S265 on chromosome 8p22–p23. The predicted peptide sequence is highly similar to that of human myotubularin. MTMR8 lacks a catalytic phosphatase domain, but shows Identification and Localization of a New Human Myotubularin-Related Protein Gene, MTMR8, on 8p22–p23 Silke Appel, 1 Kathrin Reichwald, 2 Wolfgang Zimmermann, 2 André Reis, 1,3 André Rosenthal, 2,4 and Hans Christian Hennies 1 * 1 Department of Molecular Genetics and Gene Mapping Center, Max Delbrück Center for Molecular Medicine, Berlin, Germany 2 Genome Sequencing Center, Institute of Molecular Biotechnology, Jena, Germany 3 Institute of Human Genetics, Friedrich Alexander University, Erlangen, Germany 4 Department of Biology, Friedrich Schiller University, Jena, Germany *To whom correspondence and reprint requests should be addressed. Fax: +49-30-94063147. E-mail: hennies@mdc-berlin.de.