ALS is a rapidly progressing neurodegenerative disease that is often diagnosed with a delay due to initial non-specific symptoms. Several types of biomarkers were already proposed (miRNAs, mRNAs, proteins, various metabolites) in cerebrospinal fluid, leukocytes, serum, and plasma . None of them is routinely used in the diagnostic at the moment, although some show great potential for further validation. CircRNAs have been already proposed as potential biomarkers for several diseases, including Alzheimer’s disease , multiple sclerosis , major depressive disorder , numerous cancers , and ALS itself .
Here, we further investigated the usefulness of circRNAs as potential biomarkers for ALS. Our framework was based on a gene prioritization approach in order to select the circRNAs with host genes that showed the highest level of conservation and genetic constraints. Because, according to our hypothesis, these conserved genes should be the ones in which any kind of variation should have a more substantial effect on the disease compared to the non-conserved type of genes. Then we performed a linear regression between ALS cases and controls using each circRNA as a predictor variable. With an FDR threshold of 0.1, only six circRNAs passed the filtering and merely one of them remained statistically significant after Bonferroni correction: hsa_circ_0060762 and its host gene CSE1L. Finally, we observed the significant difference in expression levels between patients and healthy controls for both hsa_circ_0060762 and CSE1L. Hsa_circ_0060762 is encoded in CSE1L gene, which is found in the “conserved set” of ALS genes with the highest level of genetic constraints. CSE1L in humans encodes an exporting factor for importin-α. In the spinal cord of mice model of ALS, an altered localization of two proteins of nucleocytoplasmic transport system, importin-α and importin-β, was detected using immunohistochemistry . An abnormal transporter protein distribution was also detected in spinal cords of patients with a sporadic and familial form of ALS . Furthermore, reduced levels of CSE1L were reported in the brains of patients with frontotemporal lobar degeneration (FTLD) , the disease which shares many clinical, pathological and genetic characteristics with ALS, including nuclear trafficking impairment [30,31].
In this study, we report for the first time the reduced expression of has_circ_0060762 and it’s host gene CSE1L in peripheral blood mononuclear cells of patients with ALS. In addition, receiver operating characteristics curve analysis showed some diagnostic potential for CSE1L and hsa_circ_0060762. Hsa_circ_0060762 thus represent a novel potential circRNA biomarker for ALS, together with three other circRNAs that we have previously reported in connection with ALS . Nevertheless, all described potential circRNA biomarkers for ALS need validation and comparison with other neurodegenerative diseases before any conclusion is made on their usefulness as ALS biomarkers. Also, some limitations in this study like samples size and reduced power to detect circRNA with smaller effect have to be considered.
In conclusion, we showed that circRNAs have the potential to be effective blood-based circulating ALS disease biomarkers. However, an extensive validation based on diverse sets of healthy and diseased cases, preferably with a larger number of samples in each group, has to be performed, before we can classify them as useful biomarkers for ALS.