Expanding the genotypic and phenotypic spectrum of Egyptian children with maple syrup urine disease

Abstract

Maple Syrup Urine Disease (MSUD, OMIM# 248600) is an autosomal recessive inborn error of metabolism characterized by elevated branched chain amino acids (BCAA) leucine/isoleucine and valine in blood of affected children. The phenotypic and genotypic spectrum of MSUD is largely unreported in Egypt. We recruited ten patients (4 males/6 females, 2weeks-12years) from nine unrelated families with clinical and biochemical evidence of MSUD. We performed Sanger sequencing for the three most-commonly responsible genes: BCKDHA, BCKDHB and DBT and conducted exome sequencing for unresolved cases. Through Sanger sequencing, we detected eight homozygous pathogenic/likely pathogenic variants (four in BCKDHB, three in BCKDHA and one in DBT gene) in eight different families. The proband of family VI, who had no significant genetic findings by Sanger, had a peculiar phenotype and atypical radiological findings. His exome sequencing revealed a previously reported homozygous likely pathogenic variant in the RARS2 gene (NM_020320.5:c.1026G > A;p.(Met342Ile)) causing the mitochondrial-encephalopathy disorder pontocerebellar hypoplasia, type 6 (OMIM# 611523). Furthermore, the copy-number-variant analysis of the exome data revealed a biallelic duplication affecting exons 2-6 of the BCKDHB gene (GRCh38: Chr.6-g.80127496:80171441dup) evaluated as variant of uncertain significance but expected to cause a breakpoint and may disrupt gene function, which can explain the markedly elevated BCAA levels in the patient's blood. In conclusion, we expanded the genotypic and phenotypic spectrum of the disease and showed that aggressive intervention with specific treatment in the first few days of life resulted in normal development even in a developing country setting. Inclusion of MSUD in the national newborn screening program in Egypt is highly recommended.

Keywords: Egypt; Exome sequencing; Maple-syrup-urine disease; Sanger sequencing; Novel variant.

Fig. 1

Family pedigrees and genetic variants in affected maple syrup urine disease (MSUD) patients. (A) Family pedigrees of all recruited children. (B) Detected genetic variants in BCKDHA, BCKDHB and DBT genes. All detected variants fully segregated in all tested family members (homozygous in the MSUD patients and heterozygous in their parents) consistent with the autosomal recessive inheritance nature of the disease. NM_number represents each gene transcript. H homozygous, h heterozygous, A adenine, C cytosine, G guanine, T thymine.

Fig. 2

All reported variants in this study and the literature in Egyptian MSUD patients with their familial frequencies. The number of families affected by each variant is represented by the number of circles adjacent to each variant based on the variants reported by Khalifa et al., 2020; Dahpy et al., 2021 and this study. The symbol @ represents variants reported in our study and # represents variants reported only in Egyptian patients and absent in the literature or international MSUD databases so far.