Familial hypercholesterolemia in Sweden : genetic and clinical studies

Abstract

Prevention of premature disease and death from cardiovascular complications of atherosclerosis is an important goal for public health, and the early identification of individuals with increased risk is an important goal of modern medicine. Familial hypercholesterolemia (FH) is one of the most common monogenic diseases (1/250 to 1/300) where strong evidence of positive health effects of intervention has been established. In afflicted families, 50% of children or siblings of a carrier are expected to inherit the disease. FH is generally the consequence of reduced function of the LDL receptor pathway, either due to mutations in the LDLR gene itself or to mutations in the APOB or PCSK9 genes. Screening in a family where the molecular defect is known is relatively simple, but the large number of different FH-causing mutations makes genetic analysis complex. Consequently, most patients with FH are currently not diagnosed, and thus excluded from life-saving therapy.

Aims of this thesis: (a) Investigate if genotyping could be a simpler and less expensive alternative to sequencing when searching for the molecular defect in Swedish patients with suspected FH, and to describe the mutation pattern in this population including the frequency of mutation-negative patients with this phenotype. (b) Study the prevalence of FH in hypercholesterolemic patients with acute coronary syndrome. (c) Evaluate the possible role of polygenic inheritance in patients with the FH phenotype. (d) Use deep exome sequencing to explain a unique aggregation of homozygous FH in a family.

Results: (a) We constructed a genotyping panel by selecting 113 FH-causing mutations and investigated 300 unrelated Swedish patients with suspected FH. Complementary sequencing was performed in those not diagnosed by genotyping. Altogether 81% of those with mutations were identified using the platform; and ten mutations accounted for more than 50% of those patients. (b) In 116 patients with acute coronary syndrome and cholesterol ≥ 7 mmol/l, 8 (6.9%) had a mutation in the LDLR gene causing FH. This was more frequent at higher LDL levels. (c) We evaluated the influence of a polygenic 12-SNP score for LDL cholesterol in 88 mutation-negative patients and 57 mutation-positive patients with probable FH. Although the negative cohort had a slightly higher score, there was no correlation between LDL cholesterol and SNP score in either cohort. (d) With the use of haplotype construction from the heterozygous mother and her four homozygous children we made the postmortem diagnosis of homozygous FH in the father, explaining the extreme aggregation of this rare condition. Measurements of plasma markers of cholesterol and bile acid production showed that they are both normal in homozygote FH.

Conclusions: Genotyping can be used as a cost-effective first step in the molecular diagnosis of patients with suspicion of FH. The prevalence of FH is high among hypercholesterolemic patients with acute coronary syndrome. Polygenic mechanisms of inheritance are not relevant in explaining the FH phenotype where no mutation in the LDLR, APOB or PCSK9 genes can be found. Deep exome sequencing is useful to characterize genotype/phenotype patterns in rare forms of dyslipidemia and should now be applied in the search for novel mechanisms behind the “mutation-negative” FH phenotype in the Swedish population.