GRAMD1B Gene's Role in Multiple Sclerosis Susceptibility

Multiple Sclerosis (MS) is a complex inflammatory disease of the central nervous system (CNS), where both genetic and environmental factors play significant roles. While genome-wide association studies (GWAS) have identified common genetic variants contributing to MS, the impact of rare variants remains largely unexplored. This study by Esposito et al. (2022) sheds light on the role of the GRAMD1B gene in MS susceptibility through whole-genome sequencing (WGS) in a consanguineous Italian family with multiple MS cases.

The study involved WGS of four MS-affected and four healthy relatives from a consanguineous family. The primary goal was to identify rare genetic variants that segregate with the disease. Samples were genotyped using the HumanOmniexpress BeadChip Kit, and linkage analysis was performed to prioritize variants for further investigation.

Biological samples (DNA, RNA, serum, plasma, and peripheral blood mononuclear cells) were collected from 14 family members. Genomic DNA was extracted, quantified, and assessed for quality. Whole-genome genotyping was done, and stringent quality control measures were applied to ensure the reliability of the data.

Linkage and Variant Analysis Linkage analysis revealed a significant region on chromosome 11, with a LOD score of 2.194, indicating a strong genetic linkage. Variants within this region were filtered based on functionality, frequency, evolutionary conservation, and predicted pathogenicity. A novel missense variant (c.1801T > C; p.S601P) in the GRAMD1B gene was identified, which was shared among the affected individuals but absent in the healthy relatives.

Key Findings
Identification of GRAMD1B Variant
The c.1801T > C (p.S601P) variant in GRAMD1B was predicted to be deleterious and highly conserved across species. This variant was not present in public databases such as dbSNP, the 1000 Genomes Project, or the gnomAD Database. Further validation by Sanger sequencing confirmed the variant's presence in all affected family members.

GRAMD1B Expression and Function
GRAMD1B, a gene with previously unknown CNS functions, was found to be expressed in various CNS cell types, including astrocytes, microglia, and neurons, as well as in peripheral immune cells. Notably, GRAMD1B expression was downregulated in vessel-associated astrocytes in active MS lesions and in peripheral monocytes under inflammatory conditions, suggesting its role in modulating inflammatory responses.

Targeted Sequencing and Burden Tests
Targeted resequencing of GRAMD1B in 91 unrelated familial MS cases identified additional rare missense and splice-site variants. Although these variants did not reach statistical significance in association tests, an excess of rare GRAMD1B alleles was observed in MS patients compared to healthy controls, hinting at its potential involvement in MS susceptibility.

Functional Studies and Replication
Functional assays demonstrated that GRAMD1B expression could be induced by interferon-beta (IFNβ), a common MS treatment. This finding was corroborated by in vitro experiments showing significant upregulation of GRAMD1B in PBMCs upon IFNβ stimulation.

In a replication study involving a Canadian cohort, two additional rare functional variants in GRAMD1B were identified, further supporting the gene's association with MS.

Conclusion
This study provides compelling evidence that rare variants in the GRAMD1B gene contribute to MS susceptibility. The identification of the c.1801T > C (p.S601P) variant and its functional implications in modulating inflammatory responses highlight the importance of rare genetic variants in complex diseases like MS. Future research should focus on elucidating the precise biological mechanisms through which GRAMD1B influences MS pathogenesis and exploring its potential as a therapeutic target.

References
Esposito, F., Osiceanu, A. M., Sorosina, M., Ottoboni, L., Bollman, B., Santoro, S., ... & Martinelli Boneschi, F. (2022). A Whole-Genome Sequencing Study Implicates GRAMD1B in Multiple Sclerosis Susceptibility. Genes, 13(12), 2392.