SCARB2 mutations as modifiers in Gaucher disease: the wrong enzyme at the wrong place?

Unlike most lysosomal proteins, -glucocerebrosidase (GCase) – the hydrolase defective in Gaucher disease (GD) – is specifically delivered to the lysosome through interaction with the lysosomal integral membrane protein type 2 (LIMP-2). Recently, mutations in the LIMP-2 coding gene, SCARB2, were reported to affect the severity of Gaucher phenotype. To understand the role of variations in SCARB2 in the broad phenotype spectrum observed for patients carrying similar GBA mutations, we have screened the gene in the Portuguese GD patients. After analyzing a total of 91 individuals, that constitutes the whole population of affected individuals referenced in our country, we identified 3 different SCARB2 coding variants. Of those, 2 were known polymorphic variations, with high prevalence in the normal population (p.M159V and p.V396I) and the third was a novel coding variant, p.T398M, present in heterozygozity in one Gaucher patient, a severely affected child, born from healthy unrelated parents of Cape Verdean origin. Initial investigations showed low GCase levels, and the child was referenced for enzyme replacement therapy (ERT), having started treatment. Nevertheless, subsequent therapeutic follow-up with assessment of chitotriosidase levels showed enzyme levels which were disparate from the ones expected for a GD patient under ERT treatment, suggesting lower response to treatment. When analyzed both in silico and in vitro, this variation was predicted to be deleterious for protein function. Preliminary results of Western blot assays in COS7 cells transfected with a minigene carrying the mutation show a decrease of LIMP-2 levels when compared to wild-type protein. Recombinant GCase uptake is known to be dependent from the receptor density (mannose 6-phosphate receptors and LIMP-2). That is, indeed, one the major reasons for the variable organ response to ERT. Taking this into account, it would be expectable that any alteration causing either dysfunction or reduction of LIMP-2 lead to a decrease in the efficacy of ERT in GD patients, as observed in our case. To the best of our knowledge this is the first time that a whole GD population is screened for mutations in this gene. From our results on the Portuguese population it was possible to conclude that SCARB2 mutations neither the only nor the most frequent cause of GD phenotype variability. Nevertheless, with the identification of a novel mutation in one of the patients who present a severe GD phenotype and had a poor response to ERT standard treatment and its subsequent evaluation, our study reinforces previous evidence that SCARB2 mutations do act as modifiers of GD.