Cardiovascular diseases (CVDs), mainly atherosclerotic diseases (ASCVDs), remain the leading cause of premature morbidity and mortality worldwide. In the Czech Republic, they account for 45% of total standardized mortality.1 Among other factors, such as smoking or obesity, different types of dyslipidemias (DLPs) play a pivotal role in ASCVD manifestation. The vast majority of DLPs have a significant genetic background, with many polymorphisms individually playing a relatively small but highly important role in concert.2 In addition to polygenic DLPs, there are monogenically inherited disorders of blood lipid metabolism, among which familial hypercholesterolemia (FH) is the most common.3

Familial dysbetalipoproteinemia (FD), previously known according to the Fredrickson et al.4 classification as hyperlipoproteinemia type III (HLP III), seems to be the second most common, and FD subjects possess a risk of ASCVD that is comparable to that in subjects with heterozygous FH.5

Phenotypically, FD manifests as a mixed DLP with the presence of cholesterol-rich VLDL particles, with a VLDL-cholesterol/triglycerides (TG) ratio > 0.3.5 In clinical practice, the first clue to think about possible FD should be when a patient presents with mixed DLP and has a total cholesterol (TC)/TG ratio ≤ 2, under conditions of TC > 5 mmol/l, TG > 3 mmol/l).5 When FD is suspected, overall accepted diagnostic criteria can be used to help select suitable candidates for further (especially genetic) testing. Widely used criteria include the apolipoprotein B (apoB)/TC ratio < 0.15 g/mmol (sensitivity 89%, specificity 97%), the so-called apoB algorithm, defining FD as apoB < 1.2 g/l, TG > 2.3 mmol/l, TG/apoB < 10 and TC/apoB > 6.2 (sensitivity 93%, specificity 99%) or6,7 the non-HDL-cholesterol (non-HDL-C)/apoB ratio with comparable sensitivity and specificity and the cut-off value 3.69 mmol/g.8

FD is characterized by the accumulation of triglyceride-rich apoB-containing particles (mostly remnant particles) and, according to some sources, is associated with an up to 10-fold increase in the risk of developing premature CV events.5,9 In the study by Paquette et al.10 it was demonstrated that the risks of ASCVD and peripheral vascular disease (PVD) in FD are more than 3-fold and 13-fold higher, respectively, than in normolipidemic controls. Furthermore, the risk of PVD is approximately 4-fold higher in FD than in FH. Another potential clinical impact of FD or concomitant hypertriglyceridemia (HTG) is the risk of developing acute pancreatitis, a potentially life-threatening condition.5,9

FD is primarily determined by a polymorphism (rs429358) within apolipoprotein E (APOE; OMIM acc. ID 617347) gene. The most frequent alleles of the APOE gene are E2, E3 and E4, with the E3 allele being the most common in the general population (77-82%, sometimes incorrectly11 referred to as wild-type), followed by the E4 allele (11-15%) and the E2 allele (7-8%).9,12,13 In the majority of cases, FD is associated with the homozygous APOE2/E2 genotype, which is why it was long thought to be only an autosomal recessively inherited disease (see Supplementary Table S1 for other rare variants). Less than one-fifth of patients with APOE2/E2 manifest FD in the context of other environmental, metabolic, or yet undescribed genetic factors (see Supplementary Table S2 for more details).5,14, 15, 16

Considering the literature reporting the prevalence of APOE2/E2 (up to 1% in Caucasians), up to 10,000 patients with FD can be expected in the Czech Republic only.13 However, their detection rate is dramatically lower.9

Genetic predispositions to FD behind the APOE genotype remain almost completely unknown. Only a few papers have focused on other genetic variants that could participate in FD development. Potentially, only APOA5 variability seems to play an important role.14, 15, 16, 17

Genetic risk scores (GRS) have recently often been mentioned as a powerful tool for discriminating between patients and controls, but this concept has not yet been applied to FD.

Generally, the accumulation of risk alleles of several to thousands common DNA variants (mostly SNPs), each of which has a relatively small effect, occurs in the background of almost all phenotypes/diseases. As number of SNPs involved in disease development could reach hundreds, creation of GRS could express more complex risk estimation (based on the sum of risks caused by single SNPs) and seems to be a promising tool to implement complex results from genetic screening into the clinical practice.18

To date, dozens of SNPs associated with increased plasma TG levels in the general population have been detected using different approaches (candidate gene studies, comparative sequencing, genome-wide association studies (GWAS)).19, 20, 21 Based on these results and several confirmatory studies, we selected a set of 18 common SNPs whose accumulation of their risk alleles may lead to the clinical manifestation of FD.22,23

The aim of our study was to analyze the potential effect of preselected SNPs on the development of FD with an effort to create a specific unweighted GRS as an additional genetic determinant of FD development in subjects with APOE2/E2 genotype.