Identification of mutations in HEXA and HEXB in Sandhoff and Tay-Sachs diseases: a new large deletion caused by Alu elements in HEXA

Hassan Dastsooz,Mohsen Alipour,Majid Fardaei,Fatemeh Dehghanian,Sanaz Mohammadi,Fatemeh Kamgarpour

doi:10.1038/hgv.2018.3

Abstract

GM2 gangliosides are a group of lysosomal lipid storage disorders that are due to mutations in HEXA, HEXB and GM2A. In our study, 10 patients with these diseases were enrolled, and Sanger sequencing was performed for the HEXA and HEXB genes. The results revealed one known splice site mutation (c.346+1G>A, IVS2+1G>A) and three novel mutations (a large deletion involving exons 6–10; one nucleotide deletion, c.622delG [p.D208Ifsx15]; and a missense mutation, c.919G>A [p.E307K]) in HEXA. In HEXB, one known mutation (c.1597C>T [p.R533C]) and one variant of uncertain significance (c.619A>G [p.I207V]) were identified. Five patients had c.1597C>T in HEXB, indicating a common mutation in south Iran. In this study, a unique large deletion in HEXA was identified as a homozygous state. To predict the cause of the large deletion in HEXA, RepeatMasker was used to investigate the Alu elements. In addition, to identify the breakpoint of this deletion, PCR was performed around these elements. Using Repeat masker, different Alu elements were identified across HEXA, mainly in intron 5 and intron 10 adjacent to the deleted exons. PCR around the Alu elements and Sanger sequencing revealed the start point of a large deletion in AluSz6 in the intron 6 and the end of its breakpoint 73 nucleotides downstream of AluJo in intron 10. Our study showed that HEXA is an Alu-rich gene that predisposes individuals to disease-associated large deletions due to these elements.

Highlights

GM2 gangliosides are a group of lysosomal lipid storage disorders that resulted from disease-causing mutations in three autosomal recessive genes, including hexosaminidase A (HEXA), hexosaminidase subunit beta (HEXB) and GM2 ganglioside activator (GM2A)
Beta-hexosaminidase A is made from one alpha subunit produced from the HEXA gene and one beta subunit
Using the repeat masker tool, different Alu elements were found across the HEXA gene (Figure 1), including one Alu at the 5ʹ untranslated region, twelve in intron 1, one in intron 3, four in intron 5 flanking exon 6, one in intron 7, one in intron 10 flanking exon 10 and three Alus at the 3ʹ untranslated region

Summary

Introduction

GM2 gangliosides are a group of lysosomal lipid storage disorders that resulted from disease-causing mutations in three autosomal recessive genes, including hexosaminidase A (HEXA), hexosaminidase subunit beta (HEXB) and GM2 ganglioside activator (GM2A). The HEXB gene produces a subunit of two related enzymes that include beta-hexosaminidase A and beta-hexosaminidase B. Beta-hexosaminidase A is made from one alpha subunit produced from the HEXA gene and one beta subunit (produced from the HEXB gene). Betahexosaminidase B is only produced from two beta subunits encoded from the HEXB gene. Beta-hexosaminidase A and B, which are mainly found in lysosomes, break down sphingolipids, oligosaccharides and glycoproteins, and are critical in the central nervous system as they have very essential roles in breaking down the GM2 ganglioside in this system, preventing the accumulation of this product and its damage to the nervous system.[1,2,3]

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