868 Functional genotype-phenotype associations in recessive dystrophic epidermolysis bullosa

Recessive dystrophic epidermolysis bullosa (RDEB) is often caused by heterozygous biallelic mutations in COL7A1. Previous genotype-phenotype association studies only elucidated that biallelic COL7A1 premature termination codon (PTC) mutations result in more severe clinical disease due to truncation of COL7A1. Our aim was to reconceptualize the functional impact of mutations on protein status and re-classify mutations as follows: "PTC/PTC, PTC/splice" as severe, "PTC/missense, splice/splice" as moderate and "splice/missense, missense/missense" as mild. We enrolled 83 subjects in a cross-sectional study (mean age 24.1 years, range 6-73 years) with 104 pathogenic variants. Of these, 9.6% (n=10/104) were re-classified based on updated literature or in silico predictions. Severe mutations (PTC/PTC, PTC/splice) were associated with more severe RDEB subtypes compared to moderate mutations (OR: 8.0, 95% CI [2.5, 25.9], p<0.01), resulted in higher Investigator Global Assessment scores in a dose dependent manner (11.1% vs. 31.8% vs. 76.9% for mild vs. moderate vs. severe mutations, p<0.001), and had 40.8-fold greater odds of squamous cell carcinoma compared to mild mutations (p=0.02) and 5.7-fold greater odds of death compared to moderate mutations (p=0.05). Mutation severity was correlated with anemia (OR severe vs. moderate mutations: 6.7, 95% CI [1.0, 42.9], p=0.05; OR severe vs. mild mutations: 13.5, 95% CI [1.2, 153.2], p=0.04), ocular manifestations (OR severe vs. mild mutations: 15.2, 95% CI [1.6, 141.3], p=0.02) and need for gastrostomy tube (22.2% vs. 27.3% vs. 48.1% for mild vs. moderate vs. severe mutations p=0.03). In summary, we propose a novel functional genotype classification strategy that incorporates splice site mutations, a concept that has not been explored in the literature before. This enhances our understanding of the cumulative effect of gene mutation dosage on phenotypic severity and helps with prognostication pending further validation in a larger, more diverse cohort.