G.P.151: Loss of FHL1 function impairs motility and causes myopathy in vivo

Abstract

Mutations of the X-chromosomal <i>four and a half LIM domain 1</i> (FHL1) lead to inclusion body myopathies such as reducing body myopathy or Emery-Dreifuss muscular dystrophy, termed as FHL1-opathies. Family screens identified association of different missense mutations of FHL1 to skeletal muscle myopathy either isolated or linked with cardiomyopathy. Until now it is unknown whether dominant negative effects (accumulation of cytotoxic mutated FHL1 protein) or loss of FHL1 function (reduced FHL1 protein levels) underlie the pathogenesis of these FHL1-opathies. Therefore, we exploited the model organism zebrafish to evaluate the role of FHL1 for pathogenesis of FHL1-opathies in vivo. By Morpholino-modified oligonucleotide mediated gene knockdown of the two zebrafish FHL1 orthologs, <i>fhl1a</i> and <i>fhl1b</i>, we examined loss of FHL1 function in vivo. In both morphant embryos we find impaired motility and skeletal muscle function consistent with FHL1-opathy. To evaluate whether three common missense FHL1-mutations identified in FHL1-opathy affected patients either act dominant-negative or lead to loss of FHL1 function in vivo, we injected mutated human FHL1 mRNA in both wild-type and fhl1-depleted zebrafish embryos. Hence, injection of mutated human FHL1 mRNA into wild-type embryos, does not impact on skeletal muscle function, suggesting that the investigated mutations do not act dominant-negative in vivo. Interestingly, in contrast to wild-type FHL1 mRNA, expression of the mutated human FHL1 in fhl1-depleted embryos was not capable of reconstituting skeletal muscle function, indicating that the common human FHL1 mutations lead to a loss of FHL1 function thereby causing the FHL1-opathy. In summary, we demonstrate here, that loss of FHL1 function leads to impaired motility and myopathy in the zebrafish in vivo model. Furthermore, our functional studies reveal that three common human FHL1-mutations do not act dominant-negative in vivo but rather lead to a loss of FHL1 function.