Objective: To gain insights into the pathophysiology by which the G-allele of rs3115534 confers risk for PD.

Background: A recent paper defined the G-allele of rs3115534 in GBA1 as a risk factor for PD. The variant is largely confined to subjects of Sub-Saharan ancestry (‘African allele’). It differs from classical GBA1 risk factors in being intronic (position c.1225-34 in intron 8). The mechanism by which it confers PD risk is thus not clear.

Method: Dried blood spot samples from two resources in CENTOGENE’s BioDatabank were used: (A) the ROstock PArkinson’s Disease study (ROPAD; ClinicalTrials.gov NCT03866603), an observational clinical study having genetically analyzed >18K PD patients, and (B) our routine diagnostic exomes/genomes. RNA, proteins, and metabolites were extracted for RT-PCR-based analysis of the GBA1 gene, for fluorometry-based measurement of glucocerebrosidase (GCase) activity, and for mass spectrometry-based determination of the sphingolipid Lyso-Gb1, respectively.

Results: Overall GBA1 expression did not differ between heterozygotes for the G-allele (G/T) and homozygotes for the T-allele (T/T). Intron 8 sequence could be amplified from cDNA in both cohorts, but pertinent transcripts were much more abundant for G/T than for T/T genotypes. The intron 8 sequence was determined to derive from usage of an alternative splice acceptor in intron 8 as well as from complete intron 8 retention. It was also shown to almost exclusively come from the G-allele in G/T heterozygotes. Consistent with these observations, in silico analyses suggested that the G-allele disrupts the canonical intron 8 branchpoint. GCase activity was slightly decreased in G/T heterozygotes and in G/G homozygotes. The levels were as for the ‘mild’ classical GBA1 risk factors p.E365K and p.T408M, but higher than for the ‘more severe’ variants p.N409S and p.L483P. Surprisingly, Lyso-Gb1 was unaltered (GT) or even decreased (G/G), and this contrasted with what was observed for all classical GBA1 risk factors.

Conclusion: The G-allele of rs3115534 triggers partial GBA1 missplicing by disrupting an intronic branchpoint. Further studies are required to explain the biochemical consequences (reduced GCase activity, normal to low levels of Lyso-Gb1). Neither a loss-of-function nor a gain-of-function mutational mechanism can currently be excluded.

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MDS Abstracts - https://www.mdsabstracts.org/abstract/the-functional-consequences-of-the-african-gba1-allele-of-rs3115534-in-parkinsons-disease/