Human centronuclear and myotubular myopathies belong to a genetically heterogeneous nosological group with clinical variability ranging from fatal disorder to mild weakness. The severe X-linked form is attributed to more than 200 different mutations in the myotubularin encoding gene (MTM1). In contrast, there are no reports regarding the molecular etiology or linkage studies on the autosomal forms of the disease. Labrador retrievers affected by spontaneous centronuclear myopathy (cnm) have clinical and histological features of the human disorder and represent the first model of recessive autosomal centronuclear myopathy. We previously mapped the cnm locus to the centromeric region of canine chromosome 2. No gene of the MTM1 family maps to the human homologous chromosomal region. Described herein is a disease-associated insertion within PTPLA exon 2, found in both alleles of all affected Labradors and in a single allele in obligate carriers. The inserted tRNA-derived short interspersed repeat element (SINE) has a striking effect on the maturation of PTPLA mRNA, whereby it can be spliced out, partially exonized or involved in multiple exon-skipping. As a result, the amount of wild-type transcripts falls to 1% in affected muscles. This example therefore recapitulates cumulative SINE-associated transcriptional defects that have been previously described as exclusive consequences of independent mutations. Although the function of PTPLA in metazoa remains unknown, the characterization of a hypomorphic mutation in Labradors with centronuclear myopathy provides new clues about the molecular complexity of skeletal myofiber homeostasis. These results also suggest that impaired PTPLA signaling might be implicated in human myopathies.

Centronuclear myopathies (CNM) are inherited congenital disorders characterized by an excessive number of internalized nuclei. In humans, CNM results from ∼70 mutations in three major genes from the myotubularin, dynamin and amphiphysin families. Analysis of animal models with altered expression of these genes revealed common defects in all forms of CNM, paving the way for unified pathogenic and therapeutic mechanisms. Despite these efforts, some CNM cases remain genetically unresolved. We previously identified an autosomal recessive form of CNM in French Labrador retrievers from an experimental pedigree, and showed that a loss-of-function mutation in the protein tyrosine phosphatase-like A (PTPLA) gene segregated with CNM. Around the world, client-owned Labrador retrievers with a similar clinical presentation and histopathological changes in muscle biopsies have been described. We hypothesized that these Labradors share the same PTPLAcnm mutation. Genotyping of an international panel of 7,426 Labradors led to the identification of PTPLAcnm carriers in 13 countries. Haplotype analysis demonstrated that the PTPLAcnm allele resulted from a single and recent mutational event that may have rapidly disseminated through the extensive use of popular sires. PTPLA-deficient Labradors will help define the integrated role of PTPLA in the existing CNM gene network. They will be valuable complementary large animal models to test innovative therapies in CNM.

Skeletal muscle development, growth and regeneration depend upon the ability of muscle cells to fuse into multinucleated fibers. Surprisingly little is known about the cellular events that underlie fusion during amniote development. Here, we have developed novel molecular tools to characterize muscle cell fusion during chick embryo development. We show that all cell populations arising from somites fuse, but each with unique characteristics. Fusion in the trunk is slow and independent of fiber length. By contrast, the addition of nuclei in limb muscles is three times more rapid than in trunk and is tightly associated with fiber growth. A complex interaction takes place in the trunk, where primary myotome cells from the medial somite border rarely fuse to one another, but readily do so with anterior and posterior border cells. Conversely, resident muscle progenitors actively fuse with one another, but poorly with the primary myotome. In summary, this study unveils an unexpected variety of fusion behaviors in distinct embryonic domains that is likely to reflect a tight molecular control of muscle fusion in vertebrates.