Importance: Ichthyoses are clinically and genetically heterogeneous disorders characterized by scaly skin. Despite decades of investigation identifying pathogenic variants in more than 50 genes, clear genotype-phenotype associations have been difficult to establish. Objective: To expand the genotypic and phenotypic spectra of ichthyosis and delineate genotype-phenotype associations. Design, Setting, and Participants: This cohort study recruited an international group of individuals with ichthyosis and describes characteristic and distinguishing features of common genotypes, including genotype-phenotype associations, during a 10-year period from June 2011 to July 2021. Participants of all ages, races, and ethnicities were included and were enrolled worldwide from referral centers and patient advocacy groups. A questionnaire to assess clinical manifestations was completed by those with a genetic diagnosis. Main Outcomes and Measures: Genetic analysis of saliva or blood DNA, a phenotyping questionnaire, and standardized clinical photographs. Descriptive statistics, such as frequency counts, were used to describe the cases in the cohort. Fisher exact tests identified significant genotype-phenotype associations. Results: Results were reported for 1000 unrelated individuals enrolled from around the world (mean [SD] age, 50.0 [34.0] years; 524 [52.4%] were female, 427 [42.7%] were male, and 49 [4.9%] were not classified); 75% were from the US, 12% from Latin America, 4% from Canada, 3% from Europe, 3% from Asia, 2% from Africa, 1% from the Middle East, and 1% from Australia and New Zealand. A total of 266 novel disease-associated variants in 32 genes were identified among 869 kindreds. Of these, 241 (91%) pathogenic variants were found through multiplex amplicon sequencing and 25 (9%) through exome sequencing. Among the 869 participants with a genetic diagnosis, 304 participants (35%) completed the phenotyping questionnaire. Analysis of clinical manifestations in these 304 individuals revealed that pruritus, hypohydrosis, skin pain, eye problems, skin odor, and skin infections were the most prevalent self-reported features. Genotype-phenotype association analysis revealed that the presence of a collodion membrane at birth (odds ratio [OR], 6.7; 95% CI, 3.0-16.7; P <.001), skin odor (OR, 2.8; 95% CI, 1.1-6.8; P =.02), hearing problems (OR, 2.9; 95% CI, 1.6-5.5; P <.001), eye problems (OR, 3.0; 95% CI, 1.5-6.0; P <.001), and alopecia (OR, 4.6; 95% CI, 2.4-9.0; P <.001) were significantly associated with TGM1 variants compared with other ichthyosis genotypes studied. Skin pain (OR, 6.8; 95% CI, 1.6-61.2; P =.002), odor (OR, 5.7; 95% CI, 2.0-19.7; P <.001), and infections (OR, 3.1; 95% CI, 1.4-7.7; P =.03) were significantly associated with KRT10 pathogenic variants compared with disease-associated variants in other genes that cause ichthyosis. Pathogenic variants were identified in 869 (86.9%) participants. Most of the remaining individuals had unique phenotypes, enabling further genetic discovery. Conclusions and Relevance: This cohort study expands the genotypic and phenotypic spectrum of ichthyosis, establishing associations between clinical manifestations and genotypes. Collectively, the findings may help improve clinical assessment, assist with developing customized management plans, and improve clinical course prognostication.
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