Objective: Molecular diagnosis for subjects with extremely low HDL-C through candidate-gene approaches requires huge effort. Whole exome-sequencing (WES) has already shown approximately ∼30% success in the diagnosis of Mendelian disorders. Moreover, novel in silico prediction software for the pathogenicity of novel missense variants named Combined Annotation Dependent Depletion (CADD) has recently been developed, enabling the objective integration of many diverse annotations into a single measure (C-score) for each variant. Here, we investigated whether WES combined with integrated variant annotation prediction could facilitate the molecular diagnosis of this rare condition.
Methods: WES was performed on 8 individuals including 2 individuals exhibiting extremely low HDL-C (2 mg/dl and 6 mg/dl), 2 unaffected family members, and 4 unrelated individuals as controls. We filtered out the following variants: 1) Benign variants predicted by SnpEff; 2) Minor allele frequency (MAF) > 1%; 3) Segregation unmatched for the recessive form of inheritance; 4) C-score < 10.
Results: Among 305,202 variants found in those individuals, we found 21,708 nonsense, missense, or splice site variants, of which 5192 were rare (MAF ≤ 1% or not reported). Filtering assuming a recessive pattern of inheritance, combined with the use of the C-score, successfully narrowed down the candidates to compound heterozygous mutations in the ABCA1 gene (c.6230C > A or p.P2077H/c.6137G > A or p.S2046N, and c.2842G > A or p.G948R/c.1130C > T or p.P377L).
Conclusions: WES combined with integrated variant annotation prediction successfully identified asymptomatic Tangier disease with novel ABCA1 mutations. This comprehensive approach is useful to determine causative variants, especially in recessive inherited diseases.
Keywords: ABCA1; Exome; Genetics; HDL; Tangier disease.
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