25th June 2023, Dr Chee L Khoo
A few weeks ago we explored familial hypercholesterolaemia (FH) primarily in adults. The issues we touched on were how not so rare FH really is and further, how aggressive we should target the LDL-Cholesterol (LDL-C). As we know, commencing lipid lowering agents is only the first step. The next step is to agree on what the LDL-C target should be in this patient based on the cardiovascular risk of the patient. We also touched on some targets for children and when we should start screening and start treating children. The recommendations are based primarily on the European Society of Cardiology and European Atherosclerosis Society guidelines. How did these guidelines evolve?
Worldwide, one baby with heterozygous FH (heFH) is born every minute, and one baby with homozygous FH (hoFH) every day [2], but fortunately, FH is well detectable and very easily treatable. An elevated LDL-C at a young age may make the diagnosis of FH suspicious but it is not a very reliable tool. Genetic screening is not always accessible nor affordable.
The Netherlands is one of the few countries that has been very active in FH screening. With the assistance of funding of the Dutch government in 1994, pro-active home visits by specialised nurses for genetic family screening started from the age of six years on. The program has evolved from a regional pilot research project funded by the Dutch Ministry of Health to full nationwide population screening, based on promising results in detecting and treating FH in adults.
Once a family member is identified with FH, other family members were offered screening. This cascade screening program was approved by the National Ethics Committee. Insurance companies promised not to charge penalties for having a mutation. On the contrary, they encouraged early detection and prevention as they actually, reduce insurance payouts in the longer term.
The estimated prevalence of FH in The Netherlands in 1994 was 1:500. With a population of 15 million, they expected a total number of 30,000 FH patients in The Netherlands. It was estimated that most of them would be traced in a time frame of twenty years [3]. They aimed to establish the feasibility of an active family screening supported by genetic studies and to assess whether the active identification of cases with FH would lead to improvements in preventive care. A further aim was to assess the specificity and sensitivity of cholesterol measurement by comparing molecular diagnosis with cholesterol measurements in families in which a pathogenic variant in the LDL receptor (LDLR) gene had been detected.
More than 1000 different pathogenic LDLR variants were identified and accounted for about 80% of patients of Dutch origin. By the beginning of 2014, when the program stopped, 64,171 subjects had undergone genetic testing for FH. Of these, 26,232 (40.8%) were heFH mutation carriers and 37,939 (59.2%) were unaffected relatives [3]. Together with the 4000 index cases, the program has led to the identification of over 30,000 FH cases.
Active cascade screening found at least 1500 FH cases each year, in 2015, only 360 new FH family members were identified, and numbers steadily rose towards 758 and 639 in 2019 and 2020, respectively. By 2014, with discovery of more cases, the estimated prevalence of FH was revised to 1:244.
Despite the successful program, the funding by the government was discontinued. a non-profit organisation, LEEFH foundation was launched to coordinate family screening for FH as well as in stimulating individual genetic screening for those at risk. The foundation played a facilitating role in a growing voluntary FH network of 29 Dutch hospitals that have a crucial role in the regional communication and cooperation with general practitioners in stimulating family screening.
Molecular biologists believed that detection in childhood instead of adulthood increases the chance of discovering pathogenic variants, because at a young age there are few secondary causes that interfere with a clear cut diagnosis. Each new case can be followed up with child–parent testing or reverse cascade screening. This turned out to be effective because, for every child with a molecular proven FH variant, one of both parents is affected as well.
Although the genetic screening for dyslipidaemia was restricted to LDLR promoter and coding region and exon 26 and 29 of APOB, later on, CNV analysis using MLPA (multiplex ligation-dependent probe amplification) became available and was applied to every individual that had significantly elevated LDL-C levels but lacked pathogenic variants. The panel was expanded in 2003, when PCSK9 gain-of-function (GOF) variants were discovered as a third causal mechanism for FH [4].
Whereas many laboratories worldwide still screen for a limited number of genes or variants only, next generation screening (NGS) was introduced in the Netherlands in 2016 as new diagnostic tool for dyslipidaemia. With this approach, genes beyond LDLR, APOB, and PCSK9 were included in the screening. As such, FH was not the only focus anymore, and patients with other heritable lipid disorders could also be identified and receive adequate treatment. At the moment, genetic testing for dyslipidemia in the Netherlands includes causality genes for familial dysbetalipoproteinaemia, hypertriglyceridemia, hyperalphalipoproteinemia, hypolipoproteinemia, and rare diseases sitosterolemia, recessive hypercholesterolemia, chylomicron retention disease, cerebrotendineous xanthomatosis, and cholesteryl ester storage disease (or Wolman disease in specific cases). The Dutch NGS panel currently includes 27 genes.
Although the introduction of NGS has led to great advances,
many dyslipidaemic cases are still left undiagnosed because pathogenic variants
are lacking [5]. In severe clinical FH cases, including Dutch patients with a
modified definite Dutch Lipid Clinic Network criteria score of definite FH, a
pathogenic FH variant was absent in 50–60% of cases [5,6]. We have a lot of
work to do yet.
The discovery of new molecular mechanisms and genetic variants that cause FH has also led to the development of effective therapeutic targets. One of the most well-known targets is PCSK9. This led to the discovery of PCSK9 inhibitors. A similar development recently took place for angiopoietin-like 3 (ANGPTL3). The ANGPTL3 protein inhibits the enzymes lipoprotein lipase (LPL) and endothelial lipase. LOF mutations in this gene associate with lower levels of triglycerides and LDL-c and fewer cardiovascular events [7]. This inspired researchers to develop pharmacological inhibitors of ANGPTL3.
Successful genetic screening for FH and other dyslipidaemias identifies many individuals each year who are at risk of cardiovascular disease. By identifying the underlying genetic cause, these individuals can receive the best treatment and cascade screening can identify additional family members in whom events can be prevented. Besides identifying these individuals, knowing the architecture of disease pathology provides opportunities for effective targeted drug development and individualised therapy and teaches us about new mechanisms for disease pathology.
References:
- Zuurbier, L.C.; Defesche, J.C.; Wiegman, A. Successful Genetic Screening and Creating Awareness of Familial Hypercholesterolemia and Other Heritable Dyslipidemias in the Netherlands. Genes 2021, 12, 1168. https://doi.org/10.3390/genes12081168
- Wiegman, A.; Gidding, S.S.; Watts, G.; Chapman, M.J.; Ginsberg, H.N.; Cuchel, M.; Ose, L.; Averna, M.; Boileau, C.; Borén, J.; et al. Familial hypercholesterolaemia in children and adolescents: Gaining decades of life by optimizing detection and treatment. Eur. Heart J. 2015, 36, 2425–2437
- Besseling, J.; Sjouke, B.; Kastelein, J.J. Screening and treatment of familial hypercholesterolemia—Lessons from the past and opportunities for the future (based on the Anitschkow Lecture 2014). Atherosclerosis 2015, 241, 597–606
- Abifadel, M.; Varret, M.; Rabès, J.-P.; Allard, D.; Ouguerram, K.; Devillers, M.; Cruaud, C.; Benjannet, S.; Wickham, L.; Erlich, D.; et al. Mutations in PCSK9 cause autosomal dominant hypercholesterolemia. Nat. Genet. 2003, 34, 154–156
- Reeskamp, L.; Tromp, T.R.; Defesche, J.C.; Grefhorst, A.; Stroes, E.S.; Hovingh, G.K.; Zuurbier, L. Next-generation sequencing to confirm clinical familial hypercholesterolemia. Eur. J. Prev. Cardiol. 2020, 2047487320942996
- Dron, J.S.; Wang, J.; McIntyre, A.D.; Iacocca, M.A.; Robinson, J.F.; Ban, M.R.; Cao, H.; Hegele, R.A. Six years’ experience with LipidSeq: Clinical and research learnings from a hybrid, targeted sequencing panel for dyslipidemias. BMC Med. Genom. 2020, 13, 23
- Dewey, F.E.; Gusarova, V.; Dunbar, R.; O’Dushlaine, C.; Schurmann, C.; Gottesman, O.; McCarthy, S.; Van Hout, C.V.; Bruse, S.; Dansky, H.M.; et al. Genetic and Pharmacologic Inactivation of ANGPTL3 and Cardiovascular Disease. N. Engl. J. Med. 2017, 377, 211–221