RASopathies - what they reveal about RAS/MAPK signaling in skeletal muscle development

Abstract

RASopathies are rare developmental genetic syndromes caused by germline pathogenic variants in genes that encode components of the RAS/mitogen-activated protein kinase (MAPK) signal transduction pathway. Although the incidence of each RASopathy syndrome is rare, collectively, they represent one of the largest groups of multiple congenital anomaly syndromes and have severe developmental consequences. Here, we review our understanding of how RAS/MAPK dysregulation in RASopathies impacts skeletal muscle development and the importance of RAS/MAPK pathway regulation for embryonic myogenesis. We also discuss the complex interactions of this pathway with other intracellular signaling pathways in the regulation of skeletal muscle development and growth, and the opportunities that RASopathy animal models provide for exploring the use of pathway inhibitors, typically used for cancer treatment, to correct the unique skeletal myopathy caused by the dysregulation of this pathway.

Keywords: Cardio-facio-cutaneous syndrome; Costello syndrome; Myopathy; Neurofibromatosis type 1; RAS pathway; RASopathy; Rare disorder; Skeletal myogenesis; Treatment.

Fig. 1.

RASopathies and the affected components of the RAS/MAPK pathway. The highlighted RASopathies are caused by germline variants in genes that encode components or regulators of the RAS/MAPK signal transduction pathway. These syndromes and their causative genes include neurofibromatosis type 1 (NF1, beige), capillary malformation-arteriovenous malformation syndrome (CM-AVM, blue), Costello syndrome (CS, red), central conducting lymphatic anomalies (CCLAs, green), Legius syndrome (light blue), cardio-facio-cutaneous syndrome (CFC, light green), Noonan syndrome (NS, yellow), Noonan syndrome with multiple lentigines (NSML, light yellow) and SYNGAP1 syndrome (pink). Inactivated RAS is stimulated by numerous environmental stimuli, here depicted by stimulation through the receptor tyrosine kinase EPHB4, which is one of the causes of CM-AVM. There are numerous RAS family members (HRAS, KRAS, MRAS and NRAS) that, once activated, go on to activate RAF family members, i.e. ARAF, BRAF and/or CRAF. These RAFs, in turn, phosphorylate and activate MEK1 and/or MEK2, which then can phosphorylate and activate ERK1 and/or ERK2. Phosphorylated ERKs (pERK1 and pERK2) have numerous nuclear and cytosolic substrates. All causative RASopathy pathogenic variants, whether they cause loss of function or protein activation, result in increased phosphorylation of ERK1 and/or ERK2, the final effectors of the RAS/MAPK pathway. Shown here are all the gene products that have been identified in the literature to cause a RASopathy phenotype, with some pathway components requiring more investigation to identify their pathogenetic and biochemical mechanisms of action. KRAS, depicted in light green and yellow, causes both CFC and NS phenotypes. Specific mutations in the protein tyrosine phosphatase SHP2, the protein product of the gene PTPN11, and CRAF (both in light yellow) cause NSML, but specific variants may also cause NS (components in yellow).

Fig. 2.

Reduced muscle mass in RASopathy individuals. (A) Adolescent male with cardio-facio-cutaneous syndrome (CFC) pictured from behind, who has thin upper arms and uneven scapulae. (B) Another adolescent male with CFC who is pictured from the front and shows overall reduced musculature and an anterior chest deformity. (C) An adult male in his 30s with Costello syndrome (CS). This image demonstrates the paucity of muscle mass on his extremities and shows that reduced musculature persists into adulthood. (D) Another adult male with CS in his 20s showing reduced muscle mass of the upper and lower arm.

Fig. 3.

RAS/MAPK signaling effects on the stages of skeletal myogenesis. RAS/MAPK pathway signaling stimulates proliferation of muscle precursor cells (MPCs) and myoblasts, and inhibits terminal differentiation of myoblasts and the formation of myotubes. MPCs are irreversibly determined to become proliferating myoblasts. Decreased signaling of the RAS/MAPK pathway induces myoblasts to differentiate, forming post-mitotic muscle cells known as myocytes. Myocytes eventually fuse together to form multinucleated immature muscle fibers, known as myotubes. Shown here are the temporal protein expression patterns of transcription factors that coordinate myogenesis. These transcription factors include PAX3 and PAX7, MyoD, MYF5, myogenin (MYOG), MRF4 and myocyte enhancer factor 2 (MEF2). Current research using mouse models for neurofibromatosis type 1 (NF1), Costello syndrome (CS) and cardio-facio-cutaneous syndrome (CFC) have demonstrated inhibition of myoblast differentiation in vitro, as well as a decrease in the number of MyoD- and Myog-expressing cells, which are markers for differentiation during embryonic skeletal muscle formation. This is indicated by a pink line and a downwards arrow (↓) (Kossler et al., 2011; Maeda et al., 2021; Tidyman et al., 2021). Concomitant with this, RASopathy mouse models have demonstrated an increase in the number of cells expressing the PAX7 protein, which is a marker for muscle progenitor cells and myoblast proliferation, as indicated by a pink line and an upwards arrow (↑).