Genetics of the nemaline myopathies and the myotubular myopathies

Triggering typical nemaline myopathy with compound heterozygous nebulin mutations reveals myofilament structural changes as pathomechanism

Nebulin is a giant protein that winds around the actin filaments in the skeletal muscle sarcomere. Compound-heterozygous mutations in the nebulin gene (NEB) cause typical nemaline myopathy (NM), a muscle disorder characterized by muscle weakness with limited treatment options. We created a mouse model with a missense mutation p.Ser6366Ile and a deletion of NEB exon 55, the Compound-Het model that resembles typical NM. We show that Compound-Het mice are growth-retarded and have muscle weakness. Muscles have a reduced myofibrillar fractional-area and sarcomeres are disorganized, contain rod bodies, and have longer thin filaments. In contrast to nebulin-based severe NM where haplo-insufficiency is the disease driver, Compound-Het mice express normal amounts of nebulin. X-ray diffraction revealed that the actin filament is twisted with a larger radius, that tropomyosin and troponin behavior is altered, and that the myofilament spacing is increased. The unique disease mechanism of nebulin-based typical NM reveals novel therapeutic targets.

Nebulin-based nemaline myopathy is a heterogenous disease with unclear pathological mechanisms. Here, the authors generate a mouse model that mimics the most common genetic cause of the disease and demonstrate that muscle weakness in this model is associated with twisted actin filaments and altered tropomyosin and troponin behaviour.

Distinct underlying mechanisms of limb and respiratory muscle fiber weaknesses in nemaline myopathy

Nemaline myopathy is the most common congenital myopathy and is caused by mutations in various genes such as ACTA1 (encoding skeletal α-actin). It is associated with limb and respiratory muscle weakness. Despite increasing clinical and scientific interest, the molecular and cellular events leading to such weakness remain unknown, which prevents the development of specific therapeutic interventions. To unravel the potential mechanisms involved, we dissected lower limb and diaphragm muscles from a knock-in mouse model of severe nemaline myopathy expressing the ACTA1 His40Tyr actin mutation found in human patients. We then studied a broad range of structural and functional characteristics assessing single-myofiber contraction, protein expression, and electron microscopy. One of the major findings in the diaphragm was the presence of numerous noncontractile areas (including disrupted sarcomeric structures and nemaline bodies). This greatly reduced the number of functional sarcomeres, decreased the force generation capacity at the muscle fiber level, and likely would contribute to respiratory weakness. In limb muscle, by contrast, there were fewer noncontractile areas and they did not seem to have a major role in the pathogenesis of weakness. These divergent muscle-specific results provide new important insights into the pathophysiology of severe nemaline myopathy and crucial information for future development of therapeutic strategies.

Congenital myopathies at their molecular dawning

The introduction and application of molecular techniques have commenced to influence and alter the nosology of congenital myopathies. Long-known entities such as nemaline myopathies, core diseases, and desmin-related myopathies have now been found to be caused by unequivocal mutations. Several of these mutations and their genes have been identified by analyzing aggregates of proteins within muscle fibers as a morphological hallmark as in desminopathy and actinopathy, the latter a subtype among the nemaline myopathies. Immunohistochemistry has played a crucial role in recognizing this new group of protein aggregate myopathies within the spectrum of congenital myopathies. It is to be expected that other congenital myopathies marked by inclusion bodies may turn out to be such protein aggregate myopathies, depending on analysis of individual proteins within these protein aggregates and their association with putative gene mutations.

PTEN and myotubularin: novel phosphoinositide phosphatases

Protein tyrosine phosphatases (PTPs) are a diverse group of enzymes that contain a highly conserved active site motif, Cys-x5-Arg (Cx5R). The PTP superfamily enzymes, which include tyrosine-specific, dual specificity, low-molecular-weight, and Cdc25 phosphatases, are key mediators of a wide variety of cellular processes, including growth, metabolism, differentiation, motility, and programmed cell death. The PTEN/MMAC1/TEP1 gene was originally identified as a candidate tumor suppressor gene located on human chromosome 10q23; it encodes a protein with sequence similarity to PTPs and tensin. Recent studies have demonstrated that PTEN plays an essential role in regulating signaling pathways involved in cell growth and apoptosis, and mutations in the PTEN gene are now known to cause tumorigenesis in a number of human tissues. In addition, germ line mutations in the PTEN gene also play a major role in the development of Cowden and Bannayan-Zonana syndromes, in which patients often suffer from increased risk of breast and thyroid cancers. Biochemical studies of the PTEN phosphatase have revealed a molecular mechanism by which tumorigenesis may be caused in individuals with PTEN mutations. Unlike most members of the PTP superfamily, PTEN utilizes the phosphoinositide second messenger, phosphatidylinositol 3,4,5-trisphosphate (PIP3), as its physiologic substrate. This inositol lipid is an important regulator of cell growth and survival signaling through the Ser/Thr protein kinases PDK1 and Akt. By specifically dephosphorylating the D3 position of PIP3, the PTEN tumor suppressor functions as a negative regulator of signaling processes downstream of this lipid second messenger. Mutations that impair PTEN function result in a marked increase in cellular levels of PIP3 and constitutive activation of Akt survival signaling pathways, leading to inhibition of apoptosis, hyperplasia, and tumor formation. Certain structural features of PTEN contribute to its specificity for PIP3, as well as its role(s) in regulating cellular proliferation and apoptosis. Recently, myotubularin, a second PTP superfamily enzyme associated with human disease, has also been shown to utilize a phosphoinositide as its physiologic substrate.

Nebulin expression in patients with nemaline myopathy

Nemaline myopathy is a structural congenital myopathy which may show both autosomal dominant and autosomal recessive inheritance patterns. Mutations in three different genes have been identified as the cause of nemaline myopathy: the gene for slow alpha-tropomyosin 3 (TPM3) at 1q22-23, the nebulin gene (NEB) at 2q21.1-q22, and the actin gene (ACTA1) at 1q42. The typical autosomal recessive form appears to be the most common one and is caused by mutations in the nebulin gene. We have studied the pattern of nebulin labeling, in patients with the typical congenital form (ten patients), the severe congenital form (two patients) or the mild, childhood-onset form (one patient), using antibodies against three different domains of nebulin. A qualitative and quantitative nebulin analysis in muscle tissue showed the presence of nebulin in myofibers from all patients. Some differences relating to the rod structure were observed. The majority of the largest subsarcolemmal rods were not labeled with the N2 nebulin antibody (I-band epitope) and showed an indistinct pattern with the two antibodies directed to the Z-band portion of nebulin (epitopes M176-181 and serine-rich domain). Diffuse rods were not revealed using the three antibodies. A discordant pattern of nebulin N2 epitope labeling was found in two affected sisters with a mutation in the nebulin gene, suggesting that modifications in nebulin distribution inside the rods might occur with the progression of the disease. Western blot analysis showed no direct correlation with immunofluorescence data. In nine patients, the band had a molecular weight comparable to the normal control, while in one patient, it was detected with a higher molecular weight. Our results suggest that presence/absence of specific nebulin Z-band epitopes in rod structures is variable and could depend on the degree of rod organization.

Mutations in the nebulin gene associated with autosomal recessive nemaline myopathy

The congenital nemaline myopathies are rare hereditary muscle disorders characterized by the presence in the muscle fibers of nemaline bodies consisting of proteins derived from the Z disc and thin filament. In a single large Australian family with an autosomal dominant form of nemaline myopathy, the disease is caused by a mutation in the alpha-tropomyosin gene TPM3. The typical form of nemaline myopathy is inherited as an autosomal recessive trait, the locus of which we previously assigned to chromosome 2q21.2-q22. We show here that mutations in the nebulin gene located within this region are associated with the disease. The nebulin protein is a giant protein found in the thin filaments of striated muscle. A variety of nebulin isoforms are thought to contribute to the molecular diversity of Z discs. We have studied the 3' end of the 20. 8-kb cDNA encoding the Z disc part of the 800-kDa protein and describe six disease-associated mutations in patients from five families of different ethnic origins. In two families with consanguineous parents, the patients were homozygous for point mutations. In one family with nonconsanguineous parents, the affected siblings were compound heterozygotes for two different mutations, and in two further families with one detected mutation each, haplotypes are compatible with compound heterozygosity. Immunofluorescence studies with antibodies specific to the C-terminal region of nebulin indicate that the mutations may cause protein truncation possibly associated with loss of fiber-type diversity, which may be relevant to disease pathogenesis.