PhD Scientific Days 2024

Poster Session E - Molecular Medicine 2.

Identification of incompletely penetrant COL4A3 variants in Alport syndrome

Text of the abstract

Introduction: Autosomal recessive disorders are typically completely penetrant (CP), i.e. patients with biallelic pathogenic variants are always affected. However, some variants do not cause detectable symptoms in all cases and thus show incomplete penetrance (IP).
Alport syndrome (AS) is a hereditary glomerulopathy resulting from the disruption of collagen α345(IV) defect caused by COL4A3, COL4A4 or COL4A5 variants. No incomplete penetrance has ever been described in COL4A3-associated Alport syndrome. We recently developed a population-genetic algorithm to identify IP variants.
Aims: To identify IP COL4A3 variants and to examine the α345(IV) heterotrimerization and secretion of the IP collagen variants in vitro.
Methods: A cohort of 194 European patients with biallelic COL4A3 variants was established from the medical literature and analyzed by the population-genetic algorithm. Vectors containing the three different α(IV) chains (COL4A4-3FLAG, COL4A3(SmBiT) and COL4A5(LgBiT)) were kindly provided by the group of Prof. Hirofumi (Kumamoto University, Japan). Plasmids encoding the IP COL4A3 variants are created by side-directed mutagenesis. To quantify the trimerization of each split nanoluciferase-fused α3(IV) and α5(IV) collagen together with Flag-tagged α4(IV) will be transfected into HEK293 cells, and intracellular and secreted heterotrimer will be detected by luminescence as developed recently (Omachi et al., 2018).
Results: Seven frequent variants were identified as incompletely penetrant (L1474P, R1661C, G695R, C1665Y, G818R, P1461L and I1330T). They were all enriched in the patient population (p < 0.026), supporting their pathogenicity, but to a lesser extent to the LOF variants, reflecting their incomplete penetrance (1-26%, p < 0.025). These variants are more frequently trans-associated to missense than LOF variants, indicating the role of a dominant negative effect in their pathogenicity.
Conclusion: We identified seven incompletely penetrant COL4A3 variants. Their knowledge is important for appropriate genetic counseling. The functional characterization of their effect on heterotrimerization and secretion is under investigation.
Funding: NKFIH/OTKA K135798, 2023-1.2.1-ERA_NET-2023-00013 and TKP2021-EGA-24