Unravelling the genetic background of individuals with a clinical Familial Hypercholesterolemia phenotype.

Familial Hypercholesterolemia (FH) is a common genetic disorder of lipid metabolism caused by pathogenic/likely pathogenic variants in LDLR, APOB and PCSK9 genes. Variants in FH-phenocopy genes (LDLRAP1, APOE, LIPA, ABCG5, ABCG8), polygenic hypercholesterolemia and hyper-Lp(a) can also mimic a clinical FH phenotype. We aim to present a new diagnostic tool to unravel the genetic background of clinical FH phenotype. Biochemical and genetic study was performed in 1005 individuals with clinical diagnosis of FH, referred to the Portuguese FH Study. A NGS panel, covering 8 genes and 8-SNPs to determine LDL-c polygenic risk score (PRS) and LPA genetic score, was validated, and used in this study. FH was genetically confirmed in 417 index-cases: 408 heterozygotes and 9 homozygotes. Cascade screening increased the identification to 1000 FH individuals, including 11 homozygotes. FH-negative individuals (phenotype-positive, genotype-negative) have Lp(a)>50mg/dL (30%), high PRS (16%), other monogenic lipid metabolism disorders (1%) and heterozygous pathogenic variants in FH-phenocopy genes (2%). Heterozygous variants of uncertain significance were identified in primary genes (12%) and phenocopy genes (7%). Overall, 42% of our cohort was genetically confirmed with FH. In the remaining individuals, other causes for high LDL-c were identified in 68%. Hyper-Lp(a) or polygenic hypercholesterolemia may be the cause of the clinical FH phenotype in almost half of FH-negative individuals. A small part has pathogenic variants in ABCG5/ABCG8 in heterozygosity that can cause hypercholesterolemia and should be further investigated. This extended NGS panel identifies individuals with FH and FH-phenocopies, allowing to personalize each person's treatment according to the affected pathway.