👤 Ahmed Bouhouche

Background: Parkinson's disease (PD), although widely heterogeneous and manifesting with numerous motor and non-motor symptoms, presents clinically as a single entity worldwide. Its genetic causes are also heterogeneous and include highly penetrant variants in a single gene representing rare monogenic forms, and rare or common variants conferring a relative disease risk representing more frequent multigenic forms. Most of these variants have been discovered in patients of European ancestry. Since the genetic basis of PD can vary significantly between populations due to differences in allele frequencies, little is known about the genetics of PD in other populations, particularly from Africa. Morocco, located in a region of North Africa, constitutes a subcontinent known for a weak external genetic influence and for a local genetic continuity for millennia, which makes it a region of interest to study the genetic causes of PD. Summary: This review aimed to summarize published research data on the genetic profile of PD patients from the Moroccan population to describe its genetic architecture. Unlike in Western countries, PD in Morocco is predominantly a Mendelian disease reaching up to 50%, due to the high prevalence of the LRRK2 G2019S dominant variant and to relatively less frequent PRKN and PINK1 recessive variants due to the high rate of consanguinity. Additionally, rare high-risk variants in LRRK2, VPS13C, MAPT, and POLG, in oligo- or polygenic ways, may contribute to increasing the genetic risk of the disease. Key Messages: We, therefore, show that the genetic architecture of PD in Morocco, a country in the subcontinent of North Africa, was different from that of sub-Saharan Africa and the rest of the world. This will help improve diagnostic accuracy, subdivide the clinical variability of the disease into groups of common genetic and biological causes for a better therapeutic management strategy, and test molecules from ongoing clinical trials.

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Parkinson's disease (PD) is a neurodegenerative movement disorder which can be either familial or sporadic. While it is well known that monogenic mutations are not a very common cause of PD, GWAS studies have shown that an additional fraction of the PD heritability could be explained by rare or common variants. To identify the rare variants that could influence the risk of PD in the Moroccan population, a cohort of 94 sporadic PD patients negative for the LRRK2 G2019S mutation was subjected to NGS gene panel sequencing, and gene dosage using the MLPA method. Mean age of onset at enrollment was 51.7 ± 11.51 years, and 60% of patients were men. We identified 70 rare variants under 0.5% of frequency in 16 of the 20 genes analyzed, of which 7 were novel. Biallelic disease-causing variants in genes with recessive inheritance were found in 5 PD cases (5.31%), whereas 13 patients (13.8%) carried likely pathogenic variants in genes with dominant inheritance. Moreover, 8 patients (8.5%) carried a single variant in MAPT or POLG, whereas co-occurrence of rare variants involving more than one gene was observed in 28 patients (30%). PD patients with variants in recessive genes had a younger mean age at onset than patients with dominant ones (33.40 (12.77) vs. 53.15 (6.63), p < 0.001), while their clinical features were similar. However, patients with rare variants in the risk factor genes or in more than one gene tended to have less resting tremor (p < 0.04), but more dystonia (p < 0.006) and dementia (p < 0.002) than those without any rare variants in known PD-associated genes. Our results showed a significant enrichment of rare variants particularly in LRRK2, VPS13C, POLG, and MAPT and underline their impact on the risk of sporadic form of the disease. Show less