Platelet Hexosaminidase A Enzyme Assay Effectively Detects Carriers Missed by Targeted DNA Mutation Analysis

Abstract

Biochemical testing of hexosaminidase A (HexA) enzyme activity has been available for decades and has the ability to detect almost all Tay-Sachs disease (TSD) carriers, irrespective of ethnic background. This is increasingly important, as the gene pool of those who identify as Ashkenazi Jewish is diversifying. Here we describe the analysis of a cohort of 4,325 individuals arising from large carrier screening programs and tested by the serum and/or platelet HexA enzyme assays and by targeted DNA mutation analysis. Our results continue to support the platelet assay as a highly effective method for TSD carrier screening, with a low inconclusive rate and the ability to detect possible disease-causing mutation carriers that would have been missed by targeted DNA mutation analysis. Sequence analysis performed on one such platelet assay carrier, who had one non-Ashkenazi Jewish parent, identified the amino acid change Thr259Ala (A775G). Based on crystallographic modeling, this change is predicted to be deleterious, as threonine 259 is positioned proximal to the HexA alpha subunit active site and helps to stabilize key residues therein. Accordingly, if individuals are screened for TSD in broad-based programs by targeted molecular testing alone, they must be made aware that there is a more sensitive and inexpensive test available that can identify additional carriers. Alternatively, the enzyme assays can be offered as a first tier test, especially when screening individuals of mixed or non-Jewish ancestry.

Next‐generation DNA sequencing of HEXA: a step in the right direction for carrier screening

“…It is proposed that a mutation of Thr to Ala at this position would remove important stabilizing hydrogen bonds and thus disrupt the overall catalytic domain structure (Nakagawa et al. ). However, definite evidence of the enzymatic effects of substitutions at this residue awaits in vitro functional studies.…”

“…Recent speculation based on crystallographic modeling of p.Thr259Ala indicates that this mutation is likely to be detrimental due to its key location in the catalytic core domain. It is proposed that a mutation of Thr to Ala at this position would remove important stabilizing hydrogen bonds and thus disrupt the overall catalytic domain structure (Nakagawa et al 2012 ). However, definite evidence of the enzymatic effects of substitutions at this residue awaits in vitro functional studies.…”

Tay-Sachs Carrier Screening by Enzyme and Molecular Analyses in the New York City Minority Population

“…However, we did not identify any publications to date that have systematically evaluated the performance of the Hex A enzymatic assay in leukocytes, serum, or platelets in individuals of non-AJ descent. Hex A enzyme activity reference ranges have been established in the AJ population (Petersen et al , 1983 ; Nakagawa et al , 2012 ; Strom et al , 2013 ), and these values have seemingly been applied to assess risk in all ethnic groups without further validation. In addition, recent data support the likelihood that average levels of Hex A activity are ∼5% lower in the African American population, which would explain higher rates of positive and inconclusive enzyme results (Neitzel et al , 2015 ).…”

Carrier Screening: Past, Present, and Future

“…Subsequently, three mutations identified in the HEXA gene were reported to account for 92%–98% of all TSD carriers in the AJ population, whereas 100+ disease-causing mutations have been identified in non-Jewish carriers [ 8 ]. Because three mutations account for the majority of carriers within the AJ population, molecular screening is highly sensitive within this population, though a strategy that uses only molecular screening without HexA activity assay would miss up to 10% of carriers within the Jewish population [ 9 ]. Widespread implementation of TSD carrier screening has significantly decreased the incidence of disease in the AJ population.…”