Introduction: In Slovenia, the majority of individuals with familial hypercholesterolemia (FH) caused by genetic variants remain undiagnosed and untreated. Therefore, it is important to identify possible genetic causes and establish a strategy for early detection of familial hypercholesterolemia (FH) and other conditions that manifest with increased blood cholesterol levels. Hypercholesterolemia may result from a variant in a gene associated with FH (APOB, LDLR, or PCSK9), other rare dyslipidemia, or may have a polygenic cause. This doctoral thesis aimed to explain the genetic background of hypercholesterolemia in children and adolescents from our population.
Methods: We included four different groups of participants in our study. The first group was a cohort for evaluating the screening program for FH and investigating the association between total cholesterol (TC) and body mass index (BMI), consisting of 3,782 children. The second group comprised children and adolescents (n=1,598) who underwent genetic testing for genes associated with FH. The third group was a control cohort (n=1,732). The fourth group consisted of cascade-referred parents (n=220). We conducted whole exome sequencing in 424 children and adolescents who met the established criteria and in participants from the control cohort. In cascade-referred parents, we sequenced only target regions using Sanger sequencing.
Results: 91.1% of children had measured TC values during their routine checkup at the age of 5. We observed a weak positive correlation between their TC values and BMI, but extreme phenotypes were found in children with normal BMI. Through genetic testing in a cohort of children and adolescents with hypercholesterolemia, we identified 395 children with a pathogenic variant and 113 children with a variant of uncertain significance (VUS) in one of the genes associated with FH. 4 participants were homozygotes, 8 were compound or double heterozygotes, and the rest were heterozygotes. By expanding the panel to 15 additional genes, we found pathogenic variants and VUS in an additional 73 participants. 4 participants were homozygotes, 5 were compound heterozygotes, and the remaining 64 were heterozygotes. Causal variants were confirmed in 5 participants by expanding the panel to 283 genes. We found a statistically significant difference between the participants and the negative cohort in calculating the polygenic hypercholesterolemia burden. Through an exome-wide association study of rare variants, we identified 4 candidate genes that could be associated with higher cholesterol levels: TTC17, NRXN3, RAD51B, and PRKCD. 74.5% of the tested parents were positive for a familial variant.
Conclusions: The universal screening program in preschool children for hypercholesterolemia proved to be effective in detecting children and their parents with FH. At the same time, this approach identified patients with rare dyslipidemias, determined the burden of polygenic hypercholesterolemia in our population, and identified 4 new candidate genes that could be associated with hypercholesterolemia.
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