The molecular basis of Gaucher Disease in black South Africans

Abstract

Gaucher Disease (GD) is the most common recessively inherited lysosomal storage disorder. The clinical symptoms result from non-functional β- glucocerebrosidase (βGC), an important enzyme in the metabolism of glycosphingolipids. Gaucher disease is caused by mutations in the GBA gene, located on chromosome 1q21, with over 300 reported pathogenic mutations. Age of onset and clinical presentation vary significantly and affected individuals are categorised according to the absence (type 1) or presence (type 2 and type 3) of neurological pathology. The prevalence and the GBA mutation profile in black South African GD patients were previously unknown. In this study, a mutation screen was undertaken in 21 black South African GD patients by DNA sequencing. All patients were found to be compound heterozygotes but one disease allele remained unidentified (1/42). Mutation p.T36del, a 3bp in-frame deletion mutation in exon three of the GBA gene constituted the majority of pathogenic alleles (17/42). The second most common disease allele was RecNci I (8/42), a complex allele present in exon 10 of the GBA gene. In addition, the following three missense mutations were found to account for 5% (2/42 alleles) each: W184R, L444P and R120W. A novel deletion mutation, c.413delC, was detected in 2 individuals (2/42 alleles) and the remaining eight disease alleles were missense mutations, each only detected once (8/42). Two of these eight (D405V and W357C) were novel mutations which are predicted to be damaging to βGC by computational methods. A carrier frequency of 1 in 95 for p.T36del (3/286 individuals) and 1 in 33 for RecNci I (2/66 individuals) was found in unaffected black South Africans. The allele frequencies for five single nucleotide polymorphisms (SNP) (rs9628662, rs2242577, rs2361543, rs932972 and rs11264375) were determined in 115 black South African control individuals by pyrosequencing and haplotypes were computationally inferred. A consistent association of one haplotype (ATATG) was revealed in 17 GD patients with the p.T36del allele, supporting a single origin hypothesis for this mutation. Gaucher disease is treated with enzyme replacement therapy and chitotriosidase activity is one of the biomarkers used to monitor treatment efficiency. The investigation of 212 subjects from the general SA black population revealed a low chitotriosidase deficiency rate of 0.94% (2/212) which validates the effective use of this enzyme as a biomarker. A 4bp deletion across the exon / intron 10 boundary (E/I-10_delGAgt) of the chitotriosidase encoding gene CHIT1 was identified as the molecular cause of this uncommon non-pathogenic phenomenon in South African Blacks. In conclusion, the study presented here is the first investigation into the molecular cause of GD on the African continent and reveals a unique mutation profile in black GD patients. Thus, a diagnostic test and a genetic counselling service can now be offered to the black South African population.