Genetics of Familial Hypercholesterolemia: new diagnostic and research approaches

Introduction: Familial Hypercholesterolemia (FH, OMIM #143890) is an autosomal dominant disorder of lipoprotein metabolism characterized by high plasma levels of LDL cholesterol (LDL-C) with an estimated prevalence of 1 in 250 individuals in Western Europe. Lifelong exposure to elevated plasma levels of LDL-C leads to atherosclerosis at an early age, and as such FH patients are at a 2.2 to 25.8 fold higher risk for cardiovascular disease (CVD). FH is caused by genetic mutations that affect the structure and/or function of the LDL receptor (LDLR), a cell-surface receptor that removes LDL from the circulation. Variants in LDLR, APOB, and PCSK9 have been shown to result in clinical FH. To date, FH is an underdiagnosed and undertreated disease as exemplified by the fact that less than 1% of the worldwide familial hypercholesterolemia population is detected, diagnosed or treated according to current guidelines. Aims: The primary aim was to promote a combined clinical and genetic approach for the diagnosis of FH in order to achieve a definitive and early diagnosis in patients, allow a prompt treatment and, thus, prevent the occurrence of CVD. Since a mutation in one of the three FH main genes is found in only 60-80% of patients with a definitive clinical diagnosis, the second aim of this PhD was to exploit genetic data, derived from the use of new DNA high-throughput sequencing (HTS) technologies, to find new molecular pathways (major or modifier genes) causing the disease. In particular, the possible association of genetic variants in ABCG5/ABCG8 and STAP1 with FH-like phenotype was investigated. Methods and Results: Through a 55 genes targeted HTS, we set out a fast and cost-effective genetic screening for patients with a clinic suspect or a definite diagnosis of FH. In the first 2 years we analysed 32 patients and we were able to make a molecular definitive FH diagnosis in 14 of them. Our HTS design allowed also to obtain information about pharmacogenetics and genetic predisposition to other forms of dyslipidemia. The application of the 12 SNPs gene score in our FH cohort confirmed, as previously shown in literature, that the high LDL-C levels in FH patients could be due to an accumulation of several common genetic variants of small effect supporting the presence of the polygenic form of FH. Through a lipid profiles evaluation in a big cohort of clinical FH patients and a co-segregation analysis in 4 FH families, it was then demonstrated that heterozygous variants in ABCG5/ABCG8 genes do not cause monogenic FH, although it cannot be ruled out that they can have a role in the regulation of serum cholesterol levels. Lastly, the screening of plasma lipids and B cell profile of STAP1 variant carriers coupled with in vitro co-culture experiments allowed to demonstrate that variants in STAP1 are not associated with elevated LDL cholesterol in FH patients. Thus, in contrast to the previous literature findings, STAP1 cannot be considered the fourth FH gene. Conclusions: This PhD thesis provides a large view on familial hypercholesterolemia, a disease with an ever higher estimated worldwide prevalence, and remarks the importance and feasibility of an early diagnosis supported by a cost-effective genetics diagnosis approach to manage and prevent CVD. Indeed, genetics can definitely help to improve diagnosis efficiency and to quickly identify new patients within the families. Moreover, research of new genetic FH causes and the identification of novel molecular pathways causing the disease may be the groundwork to the development of novel and ever more safety and effective lipid lowering drugs.