Complex genotypes in family with metachromatic leukodystrophy: Effect of trans and cis mutations distribution on the phenotype variability

Metachromatic leukodystrophy (MLD) is a severe metabolic disorder caused by the deficient activity of arylsulfatase A due to ARSA gene mutations. According to the age of onset, MLD is classified into three forms: infantile, juvenile, and adult. In our study, we aimed to perform a genetic analysis for two siblings with juvenile MLD for a better characterization of the molecular mechanisms behind the disease. A consanguineous family including two MLD patients (PII.1 and PII.2) was enrolled in our study. The diagnosis was made based on the clinical and neuroimaging investigations. The sequencing of ARSA gene was performed followed by in silico analysis. Besides, the cis/trans distribution of the variants was verified through a PCR-RFLP. The ARSA gene sequencing revealed three known variants, two exonic c.1055A > G and c.1178C > G and an intronic one (c.1524 + 95A > G) in the 3′UTR region. All variants were present at heterozygous state in the two siblings and their mother. The assessment of the cis/trans distribution showed the presence of these variants in cis within the mother, while PII.2 and PII.2 present the c.1055A > G/c.1524 + 95A > G and the c.1178C > G in trans. Additionally, PII.1 harbored a de novo novel missense variant c.1119G > T, whose pathogenicity was supported by our predictive results. Our genetic findings, supported by a clinical examination, confirmed the affection of the mother by the adult MLD. Our results proved the implication of the variable distribution of the found variants in the age of MLD onset. Besides, we described a variable severity between the two siblings due to the de novo pathogenic variant. In conclusion, we identified a complex genotype of ARSA variants within two MLD siblings with a variable severity due to a de novo variant present in one of them. Our results allowed the establishment of an adult MLD diagnosis and highlighted the importance of an assessment of the trans/cis distribution in the cases of complex genotypes.