Characterization of two HEXB gene mutations in Argentinean patients with Sandhoff disease

β-hexosaminidase A ($\text{β-N-}\text{acetyl-}\text{d}\text{-hexosaminidase}$, EC 3.2.1.52) is a lysosomal hydrolase composed of an α- and a β-subunit. It is responsible for the degradation of G_M2 ganglioside. Mutations in the HEXB gene encoded β-subunit cause a form of G_M2 gangliosidosis known as Sandhoff disease. Although this is a rare disease population, several geographically isolated groups have a high carrier frequency. Most notably, a 1 in 16–29 carrier frequency has been reported for an Argentinean population living in an area contained within a 375-km radius from Córdoba. Analysis of the genomic DNA of two patients from this region revealed that one was homozygous for a G to A substitution at the 5′ donor splice site of intron 2. This mutation completely abolishes normal mRNA splicing. The other patient was a compound of the intron 2 G → A susbtitution and a second allele due to a 4-bp deletion in exon 7. The β-subunit mRNA of this allele is unstable, presumably as a result of an early stop codon introduced by the deletion. Two novel PCR-based assays were developed to detect these mutations. We suggest that one of these assays could be modified and used as a rapid procedure for 5′ donor splice site defects in other genes. These results provide a further example of the genetic heterogeneity that can exist even in a small geographically isolated population.