Mutations of tropomyosin 3 (TPM3) are common and associated with type 1 myofiber hypotrophy in congenital fiber type disproportion

Abstract

Congenital fiber type disproportion (CFTD) is a rare congenital myopathy characterized by hypotonia and generalized muscle weakness. Pathologic diagnosis of CFTD is based on the presence of type 1 fiber hypotrophy of at least 12% in the absence of other notable pathological findings. Mutations of the ACTA1 and SEPN1 genes have been identified in a small percentage of CFTD cases. The muscle tropomyosin 3 gene, TPM3, is mutated in rare cases of nemaline myopathy that typically exhibit type 1 fiber hypotrophy with nemaline rods, and recently mutations in the TPM3 gene were also found to cause CFTD. We screened the TPM3 gene in patients with a clinical diagnosis of CFTD, nemaline myopathy, and with undefined congenital myopathies. Mutations in TPM3 were identified in 6 out of 13 patients with CFTD, as well as in one case of nemaline myopathy. Review of muscle biopsies from patients with diagnoses of CFTD revealed that patients with a TPM3 mutation all displayed marked disproportion of fiber size, without type 1 fiber predominance. Several mutation-negative cases exhibited other abnormalities, such as central nuclei and central cores. These results support the utility of the CFTD diagnosis in directing the course of genetic testing.

(c) 2009 Wiley-Liss, Inc.

Figures

Figure 1

(A): Schematic diagram of TPM3 gene, indicating the 5 ubiquitously expressed exons (white), 5 muscle specific exons (black), and 4 non-muscle exons (gray). Exons are numbered according to Durling et al. (2002). Shown above the gene are previously described mutations associated with nemaline myopathy and/or CFTD (p.Stop286fsX73 was previously reported as p.X285NextX74 (Lehtokari, et al., 2008)). Shown below are the mutations identified in this study. The p.Arg168His mutation was identified in two of our patients, one with CFTD and one with nemaline myopathy. Dominant mutations are defined as any single heterozygous change, whether de novo or dominant in the family. All recessive mutations described thus far have been nonsense mutations or mutations that affected the stop codon, causing production of a larger protein. Previously, two different numbering schemes have been used for numbering the TPM3 gene and protein: accession no. X04201 and no. NM_152263.2. In this report, we employed the current standard numbering scheme, starting from the first ATG for cDNA numbering and with the first methionine for amino acid numbering. Our reference sequence was the National Center for Biotechnology Information (NCBI) accession number NM_152263.2. (B) Western blot analysis for tropomyosins in the muscles of Patients 311-1, 313-1, and 343-1. Molecular weight differences allow the distinction between alpha tropomyosin (αTm) and beta tropomyosin (βTm). Patient 313-1 has an additional band approximately 6 kD larger than than normal α-tropomyosin that reflects the additional 57 amino acids incorporated into TPM3 as a result of the p.268Ser mutation, as previously reported (Wattanasirichaigoon, et al., 2002). MW: Molecular weight marker.

Figure 2

Pathologic features of patients with a diagnosis of CFTD and a TPM3 mutation (Patient 913-1 is shown) included type 1 fiber hypotrophy, in the absence of other pathologic findings, as seen on (A) H and E, (B) ATPase (at pH 9.4) and (C) NADH stains. A biopsy from one case with TPM3 mutation and a diagnosis of nemaline myopathy (Patient 343-1) contained numerous nemaline rods within fibers, as shown on (D) H and E, (E) Gomori trichrome, and (F) toluidine blue staining of ultrathin sections. Bar = 100 μm.