African Ancestry Parkinson’s Disease Linked to Distinct GBA1 and LRRK2 Variants

Redefining the Genetic Map of Parkinson’s Disease

While genome-wide association studies have identified over 90 independent risk variants for Parkinson disease, the vast majority of these data derive from cohorts of European descent [1]. This lack of diversity creates significant gaps in understanding disease etiology in underrepresented groups, particularly in Sub-Saharan Africa, where more than 98% of patients lack a known molecular cause for their condition [2, 3]. As the field shifts toward disease-modifying treatments targeting specific biological pathways, relying on population-specific markers may inadvertently limit the global utility of precision medicine [4, 5]. Understanding how genetic susceptibility varies across different ancestries is essential for ensuring equitable access to future therapeutic innovations [6]. A comprehensive new analysis now provides a detailed characterization of the genetic drivers of Parkinson disease within African and African admixed populations, identifying rare coding variants in the GBA1 gene as the most frequent mutations in this cohort [7].

Absence of Standard European and Asian Risk Markers

The findings highlight a critical divergence between the genetic architecture of Parkinson disease in African ancestry populations and the markers currently utilized in clinical genetic screening. In cohorts of Ashkenazi Jewish and European descent, specific mutations in the GBA1 gene are the most frequently identified risk factors. However, the researchers found that the common disease-associated GBA1 variants p.Asn409Ser, p.Leu483Pro, p.Thr408Met, and p.Glu365Lys were entirely absent in the 710 cases of African and African admixed ancestry. This finding suggests that the diagnostic yield of standard genetic panels, which often prioritize these specific European markers, may be significantly lower when applied to patients of West African descent.

A similar pattern of population-specific variation emerged in the LRRK2 gene, a primary target for emerging kinase inhibitor therapies. The LRRK2 p.Gly2019Ser mutation, highly prevalent in North African Berber and Ashkenazi Jewish populations, and the p.Gly2385Arg variant, common in East Asian populations, did not play a major role in the etiology of Parkinson disease among the West African ancestry study participants. Instead of these established markers, the researchers identified three heterozygous novel missense LRRK2 variants of uncertain significance (genetic alterations where a single nucleotide change results in a different amino acid). Two of these, p.Glu268Ala and p.Arg1538Cys, occurred at higher frequencies in African ancestry reference datasets than in other global populations. For the practicing clinician, this means that a negative result on a standard European-centric genetic screen does not rule out a hereditary component of Parkinson disease in patients with African ancestry, and relying solely on existing commercial panels could lead to false-negative genetic assessments.

Prevalence and Diversity of GBA1 and LRRK2 Mutations

The researchers identified rare coding variants in the GBA1 gene, which encodes the glucocerebrosidase enzyme, as the most frequent genetic alterations in this population. These mutations occurred at a frequency of 4% within the cohort of 710 patients with Parkinson disease. In total, the study identified 18 distinct GBA1 variants among the participants, representing a diverse mutational spectrum that differs markedly from European cohorts. The clinical relevance of these findings is underscored by the fact that 10 of these 18 variants were previously classified as pathogenic or likely pathogenic. However, the genetic landscape in this population remains partially uncharacterized, as four of the identified GBA1 variants were novel and another four were classified as having uncertain clinical significance, a designation indicating that their specific impact on protein function or disease risk has not yet been established.

Alongside the GBA1 findings, the identification of the three heterozygous novel missense LRRK2 variants further illustrates this genetic diversity. Because two of these novel variants, p.Glu268Ala and p.Arg1538Cys, were found at higher frequencies in African ancestry population reference datasets, they may represent unique drivers of leucine-rich repeat kinase 2 pathology within this demographic. These findings highlight the necessity for diagnostic tools that incorporate a broader range of variants to ensure accurate risk assessment, which will be crucial for matching diverse patients to clinical trials for targeted kinase inhibitors and other emerging, genotype-specific therapies.

Genetic Drivers of Early-Onset and Familial Disease

Beyond single nucleotide variations, the researchers explored copy number variants, or large-scale deletions and duplications of DNA segments, to better understand the architecture of early-onset and familial Parkinson disease. Structural variant analyses specifically identified PRKN copy number variants at a frequency of 0.7% within the African and African admixed cases. The PRKN gene, which encodes the parkin protein, is a well-established factor in autosomal recessive juvenile Parkinson disease. The clinical significance of these findings is particularly pronounced in younger patients. Among the PRKN copy number variants detected in early-onset cases, 66% were compound heterozygous or homozygous, meaning that mutations were present on both copies of the gene. This state typically leads to a loss of protein function and a significantly earlier age of symptom onset compared to idiopathic cases.

The study also utilized short tandem repeat analysis, a method used to detect abnormal expansions of repetitive DNA sequences, to investigate genetic anomalies that might mimic or contribute to Parkinsonian phenotypes. This analysis identified ATXN3 CAG repeat expansions within the pathogenic range, defined as having more than 45 repeats (CAGn > 45), in three patients of African ancestry. While expansions in the ATXN3 gene are the primary cause of spinocerebellar ataxia type 3, they can present clinically with Parkinsonian features, potentially leading to diagnostic challenges. For the practicing clinician, these data emphasize that the genetic etiology of Parkinson disease in African populations involves a complex interplay of population-specific coding variants, structural rearrangements in the PRKN gene, and repeat expansions that may overlap with other neurodegenerative syndromes.

Methodology and the Path Toward Inclusive Precision Medicine

To define this genetic landscape, the researchers conducted a comprehensive sequencing characterization of potentially disease-causing, protein-altering, and splicing mutations. This large-scale analysis included 710 Parkinson disease cases and a control group of 11,827 individuals from genetically predicted African or African admixed ancestries. By utilizing such a substantial control cohort, the study provided the statistical rigor necessary to distinguish rare pathogenic variants from common, population-specific polymorphisms. The methodology extended beyond simple variant identification, as the researchers utilized global and local ancestry analyses (statistical techniques used to map the geographic origins of specific chromosomal segments) to explore population-specific genetic effects and their contribution to disease risk.

To further investigate the architecture of early-onset and familial cases, the study analyzed runs of homozygosity, which are specific genomic regions where an individual inherits identical segments of DNA from both parents. This approach is particularly useful for identifying recessive mutations that may be more prevalent in specific ancestral lineages. Through these combined methods, the researchers created a comprehensive genetic catalogue of both known and novel coding and splicing variants linked to Parkinson disease in African ancestry populations. For the practicing clinician, this catalogue serves as a critical reference for interpreting genetic test results in patients of African descent, reinforcing that the absence of standard European risk markers does not preclude a genetic etiology. These findings provide a foundation for the development of targeted therapies and more accurate diagnostic protocols, ensuring that the benefits of precision medicine become accessible to a diverse global patient population.

References

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