Since the discovery of the C9orf72 repeat expansion as the most common genetic cause of frontotemporal dementia (FTD) and amyotrophic lateral sclerosis, it has increasingly been associated with a wider spectrum of phenotypes, including other types of dementia, movement disorders, psychiatric symptoms and slowly progressive FTD. Prompt recognition of patients with C9orf72-associated diseases is essential in light of upcoming clinical trials. The striking clinical heterogeneity associated with C9orf72 repeat expansions remains largely unexplained. In contrast to other repeat expansion disorders, evidence for an effect of repeat length on phenotype is inconclusive. Patients with C9orf72-associated diseases typically have very long repeat expansions, containing hundreds to thousands of GGGGCC-repeats, but smaller expansions might also have clinical significance. The exact threshold at which repeat expansions lead to neurodegeneration is unknown, and discordant cut-offs between laboratories pose a challenge for genetic counselling. Accurate and large-scale measurement of repeat expansions has been severely hindered by technical difficulties in sizing long expansions and by variable repeat lengths across and within tissues. Novel long-read sequencing approaches have produced promising results and open up avenues to further investigate this enthralling repeat expansion, elucidating whether its length, purity, and methylation pattern might modulate clinical features of C9orf72-related diseases.

Skeletal muscle atrophy is a physiological condition that is caused by a myriad of conditions, including immobilization, denervation, spinal cord injury and corticosteroid use and results in decreased muscle size and strength. The molecular genetic events of neurogenic atrophy were analyzed in a previous study using gastrocnemius muscle isolated from mice following 3 days and 14 days of denervation. The gene expression profile in the denervated muscle tissue was analyzed by microarray and compared to control muscle tissue to identify novel genes that are differentially expressed in response to neurogenic atrophy. The microarray data revealed for the first time that F‐box and WD‐40 Domain Protein 5 (Fbxw5) and F‐box Protein 44 are expressed in skeletal muscle and are differentially regulated in response to denervation. Fbxw5 and Fbxo44 are classified as E3 ubiquitin ligases and both have been found to interact with cullin 4 and DNA damage binding protein 1 (DDB1) complexes. Furthermore, Fbxo44 has been found to ubiquitinate and destabilize both BRCA1 and regulator of G protein signaling 2 (RGS2), while Fbxw5 has been shown to destabilize DLC1, a RhoA GTPase‐activating protein, and tuberous sclerosis complex 2 (TSC2), a negative regulator of mTOR. To confirm that Fbxw5 and Fbxo44 are expressed in muscle, quantitative PCR (qPCR) was used to assess the expression levels of these two E3 ligases in both proliferating and differentiated muscle cells and the results demonstrate that expression levels appear to remain relatively constant in proliferating myoblasts and differentiated myotubes. Furthermore, Western blot analysis supported the qPCR data and showed that Fbxw5 and Fbxo44 protein levels remain relatively constant as cultured myoblasts differentiate to myotubes. The discovery that Fbxw5 and Fbxo44 are expressed in skeletal muscle and are differentially regulated in response to neurogenic atrophy helps further our understanding of the molecular genetic events of muscle wasting and may eventually lead to the identification of new therapeutic targets for the treatment and prevention of atrophy.Support or Funding InformationThe work was support by University of North Florida Transformational Learning Opportunity grants and a University of North Florida Foundation Board Grant to D.W.This abstract is from the Experimental Biology 2019 Meeting. There is no full text article associated with this abstract published in The FASEB Journal.