A novel mutation deep within intron 7 of the GBA gene causes Gaucher disease

Abstract

Background: Mutations in the GBA gene that encodes the lysosomal enzyme acid β-glucocerebrosidase cause Gaucher disease (GD), the most common lysosomal storage disorder. Most of the mutations are missense/nonsense, however, a few splicing mutations within or close to conserved consensus donor or acceptor splice sites have also been described. The aim of the study was to identify the mutation(s) in a Cypriot patient with type I GD.

Methods: The genomic DNA of the proband was screened for nine common mutations using Polymerase chain reaction-restriction fragment length polymorphism (PCR-RFLP) analysis. All exons and exon-intron boundaries, and the 5'UTR and 3'UTR regions of the GBA gene, were investigated by Sanger sequencing. RNA analysis was performed using standard procedures, and the abnormal transcript was further cloned into pGEM-T-Easy plasmid vector and sequenced. The relevant intronic region was further sequenced by the Sanger method to identify the genetic variant.

Results: A novel point mutation, g.12599C > A (c.999 + 242C > A), was detected deep in intron 7 of the GBA gene. This type of mutation has been previously described for other diseases but this is the first time, as far as we know, that it is described for GD. This mutation creates a new donor splice site leading to aberrant splicing and resulting in the insertion of the first 239nt of intron 7 as a pseudoexon in the mRNA, creating a premature stop codon.

Conclusion: This study expands the mutation spectrum of GD and highlights the importance of RNA sequencing for the molecular diagnosis of patients bearing mutations in nonexonic regions.

Keywords: GBA; Cypriot; Gaucher disease; deep intronic mutation; glucocerebrosidase.

Figure 1

Family pedigree. Black symbols represent affected persons and symbols with a dot, carriers. The levels of GCase and chitotriosidase activity can be seen underneath each individual. Normal ranges: GCase: 5.4–16.8 nmol mg protein^-1 hr^-1, Chito: 9.5–44 nmol ml^‐1 hr^‐1

Figure 2

(a) cDNA amplicons of patient (P) and healthy control (C) showed an additional transcript at higher molecular weight (arrow) in patient sample. (b) Schematic representation of the gDNA and cDNA sequences of the GBA gene from exon 7 to exon 8 (Ex7‐Ex8), showing the activation of a cryptic splice site (*), 242 bp upstream of exon 7 due to C > A substitution, leading to pseudoexon inclusion. Exonic and intronic sequences are written with capital and small letters, respectively. The GenBank reference sequence of GBA gene is NG_009783.1 (NC_000001.11)

Figure 3

Electropherogram of Sanger sequence analysis of PCR amplicons of Ex7‐Ex8. The patient (II.3) carries a homozygous mutation g.12599C > A (arrow), which is absent from controls. The mutation was also found in the heterozygous state in the patient's mother (I.1), one sister (II.2), and four children (III.1‐III.4), as indicated by a double peak depicting the normal and the mutant sequence. The GenBank reference sequence of GBA gene is NG_009783.1 (NC_000001.11)