SPEG Interacts with Myotubularin, and Its Deficiency Causes Centronuclear Myopathy with Dilated Cardiomyopathy

Abstract

Centronuclear myopathies (CNMs) are characterized by muscle weakness and increased numbers of central nuclei within myofibers. X-linked myotubular myopathy, the most common severe form of CNM, is caused by mutations in MTM1, encoding myotubularin (MTM1), a lipid phosphatase. To increase our understanding of MTM1 function, we conducted a yeast two-hybrid screen to identify MTM1-interacting proteins. Striated muscle preferentially expressed protein kinase (SPEG), the product of SPEG complex locus (SPEG), was identified as an MTM1-interacting protein, confirmed by immunoprecipitation and immunofluorescence studies. SPEG knockout has been previously associated with severe dilated cardiomyopathy in a mouse model. Using whole-exome sequencing, we identified three unrelated CNM-affected probands, including two with documented dilated cardiomyopathy, carrying homozygous or compound-heterozygous SPEG mutations. SPEG was markedly reduced or absent in two individuals whose muscle was available for immunofluorescence and immunoblot studies. Examination of muscle samples from Speg-knockout mice revealed an increased frequency of central nuclei, as seen in human subjects. SPEG localizes in a double line, flanking desmin over the Z lines, and is apparently in alignment with the terminal cisternae of the sarcoplasmic reticulum. Examination of human and murine MTM1-deficient muscles revealed similar abnormalities in staining patterns for both desmin and SPEG. Our results suggest that mutations in SPEG, encoding SPEG, cause a CNM phenotype as a result of its interaction with MTM1. SPEG is present in cardiac muscle, where it plays a critical role; therefore, individuals with SPEG mutations additionally present with dilated cardiomyopathy.

Current and future therapeutic approaches to the congenital myopathies

“…Extraocular involvement is prominent in all forms except TTN-related CNM. Non-skeletal muscle involvement -for example hepatic peliosis in XLMTM [57], cataracts in DNM2-related CNM [58] and dilated cardiomyopathy in association with SPEG mutations [54] -has been reported with different genetic backgrounds.…”

“…CNM is due to X-linked recessive mutations in MTM1 encoding myotubularin ["X-linked myotubular myopathy (XLMTM)"] [47], autosomal-dominant mutations in DNM2 encoding dynamin2 [48] and the BIN1 gene encoding amphiphysin 2 [49], autosomal-recessive mutations in BIN1 [50], RYR1 [14], and TTN encoding titin [51], and rarer backgrounds [52][53][54]. Whilst mutations in MTM1 and BIN1 are typically associated with a "pure" CNM picture, histopathological appearance in particular in DNM2-, RYR1-and TTN-related forms is more variable and may also feature cores, CFTD and dystrophic features [14,55,56].…”

Management of Cardiac Involvement Associated With Neuromuscular Diseases: A Scientific Statement From the American Heart Association

“…100 In 143 cases of nemaline myopathy, 6 neonates developed transient HF and 1 infant developed LV dysfunction with congenital long-QT syndrome. 101 In another study with 66 patients with CM, no cardiac lesions were noted 102 ; however, hypertrophic, [103][104][105] dilated, [106][107][108][109][110] and LVNC cardiomyopathy phenotypes, 111,112 as well as sudden death, 113 have been described. Recessive mutations in TTN (encoding titin) and MYH7 (encoding myosin heavy chain-7) have been associated with minicore-like disease, with early development of dilated cardiomyopathy, ventricular arrhythmias, and sudden cardiac death.…”

Exploring Mouse Protein Function via Multiple Approaches

“…The intermediate filament keratin proteins, including Keratin 2 (KRT2), bind and interact with signaling molecules, such as CFTR [69], trichoplein [70] and Albatross complexes [71]. SPEG complex locus (SPEG) is a myotubularin (MTM1)-binding protein, and its deficiency has been proven to cause centronuclear myopathy with dilated cardiomyopathy [72]. …”