Molecular modelling
In many publications, it has been reported that M-D has been linked to all types of variants including single-exonic deletions, interstitial deletions, indels, in-frame deletions, non-synonymous single-nucleotide missense, splice and the vast majority of indels lead to frameshifts and stops. Le Doux, (2020) has compiled all variants reported and ranked them by scores of deleteriousness, within the gnomAD v2 dataset. The new mutation has not been reported before but the same position has been found c.662G> A causing protein change Gly to Asp (Pell et al., 2014). To understand the effect of the new mutation, a molecular modelling has been conducted. The p.Gly221Val substitution was predicted to be damaging according to six bioinformatics programs including Polyphen, SIFT, Proven, Condel, Mutation Assessor and Mutation Taster (Table 1 ). This mutation was not found in Exome Variant Server (EVS) and Exome Aggregation Consortium (ExAC) databases. Moreover, multi-sequence alignment of the human SGCE protein and its orthologous proteins showed a highly conservation of the Gly221 amino acid among species (Figure 3.A and B ).
A 3D structure of SGCE protein was built to explore the structural impact of the p.Gly221Val mutation. The amino acid interaction analysis has revealed that this missense mutation may disrupt hydrophobic interactions between the amino acid in position 221 and its adjacent residues. The mutated structure lacks one hydrophobic interaction compared to the native structure. The hydrophobic interactions with Lys167 and Tyr223 residues were conserved in the mutated protein, whereas interactions with Ala204, Glu220, Asn168 and Val222 was replaced by interactions with Gly209, Lys219 and Met169 (Figure 4 ). In addition, this substitution is likely to decrease the SGCE protein stability according to the predictions of CUPSAT, mCSM, SDM, DUET and DeepDDG programs (Table 2 ).
This missense mutation is added to other six missense variants reported in (Le Doux.2020) which the CADD_ PHRED scores range from 23.8 to 35 and are predicted to cause disease by MetaLR, MetaSVM, and MutationTaster. REVEL_rankscores ranged from 0.852 (p.Thr36Arg) to 0.997 (p.Tyr115Cys). SGCE variants predicted to be deleterious by in silico analysis may appear in patients undergoing whole-exome or whole-genome sequencing for seemingly unrelated disorders. it has been estimated that approximately 1/348 individuals in the United States population host a SGCE variant with a CADD score ≥ 25 (LeDoux, 2020). This is suggesting that M-D and minor phenotypic variants such as mild isolated myoclonus may be underdiagnosed.