The Potential Role of Universal Genetic Screening for Familial Defective Apolipoprotein B-100 in Southern Pennsylvania

Familial defective apolipoprotein B-100 (FDB) is an autosomal dominant disorder of lipid metabolism considered by international guidelines to be a genetically defined cause of familial hypercholesterolemia (FH).^1–3 FDB arises from mutations in apolipoprotein B (apoB) affecting the affinity of apoB-containing low-density lipoprotein particles for the low-density lipoprotein receptor (LDL-R) and, subsequently, their rate of endocytosis and catabolism in hepatocytes.⁴ FDB is associated with both hyperlipidemia and increased risk of atherosclerotic cardiovascular disease (ASCVD), although the hypercholesterolemia caused by FDB is often milder than that because of mutation in LDL-R, the most frequent cause of autosomal dominant FH, and is therefore frequently underdiagnosed by standard diagnostic criteria such as the Dutch Lipid Clinic Network and US MEDPED criteria.^5–9 Recent results from the Copenhagen General Population Study report that on average, individuals affected by FDB suffer myocardial infarction nine years earlier than non-carriers, and R3500Q carriers have been reported to display increased coronary artery calcification (CAC) even when compared with non-carriers presenting with equivalent levels of LDL-C.^9,10

Compared with LDL-R, relatively few pathogenic mutations have been identified in apoB, and, unlike LDL-R, most fall within a mutation “hotspot” affecting the conformation of the protein.^11,12 Interestingly, of mutations associated with FDB, the two most common, known as R3500Q and R3500W, affect the same site within apoB.^1,13 The R3500Q mutation is frequently reported in European cases of clinical heterozygous FH, and is thought to have arisen in a common central European ancestor due to the elevated prevalence reported in the region; its twin mutation, R3500W, was recently reported to comprise a plurality of mutations affecting subjects with clinical FH of Han Chinese background.^14–18 Furthermore, an association study of genetic variants affecting lipid levels in over 50,000 subjects reported R3500Q to be the single variant most strongly associated with elevated LDL-C in Americans of European background, leading to an average increase in LDL-C of 71 mg/dL.¹⁹

In 2010, the Amish Research Clinic in Lancaster, Pa., found that approximately 12 percent of members of the Old Order Amish, an Anabaptist group of Swiss- German origin, carried the R3500Q mutation, roughly 60 times the rate found in the general Caucasian population.¹⁰

The investigators who discovered the high prevalence of FDB among the Amish described the phenomenon as a “founder effect,” wherein a population bottleneck and subsequent endogamy over generations led to a dramatic increase in the prevalence of the R3500Q mutation. Founder effects for mutations causing FH have been reported in several other populations, such as French Canadians and Afrikaners (South African ethnic group descended from predominantly Dutch settlers); however, the reported population prevalence rates of FH mutations within these groups are significantly lower than that of FDB among the Amish, a rapidly growing demographic group.^20,21 Furthermore, approximately 10 to 20 percent of the Amish enter the general population each generation, leaving not only present members of the Amish community but also the many individuals of extended Amish background at risk.²²

To provide early preventive care and reduce lifelong disease burden in the public, the Pennsylvania Department of Health has undertaken since 1965 a newborn child genetic screening protocol. Last updated in 2014, the Newborn Child Testing Act, also known as P.L. 497, Number 251, established provision for “screening tests of newborn children” for several inherited diseases, including those more common among the Old Order Amish and other Anabaptist groups such as phenylketonuria (PKU) and glutaric acuduria (GA).²³ As a disorder of lipid metabolism conferring elevated lifelong risk for atherosclerosis and myocardial infarction, FH due to the R3500Q mutation poses a significant burden on the population health of special groups and the general population in Pennsylvania, however, screening and early treatment for FDB in this region presents a unique opportunity in lipid-focused preventive medicine. In addition to the benefits of lifelong treatment and management of the disorder, identification of newborns with FDB could serve as a starting point for “reverse” cascade screening of parents and other affected relatives.

As a single nucleotide polymorphism (SNP), diagnostic testing for the R3500Q mutation can be completed inexpensively and quickly en masse, and with currently available lipid-lowering treatment, the disease burden of FH due to mutations in apoB can be significantly reduced. As an internationally accepted, frequently reported cause of FH, FDB diagnosed as part of universal screening would also qualify those affected for specialty care, including therapy with monoclonal antibody inhibitors of proprotein convertase subtilisin/kexin type 9 (PCSK9).^24,25  Low rates of correct diagnosis according to standard clinical FH diagnostic criteria further underscore the importance of molecular diagnosis for FDB. The unique population of southern Pennsylvania thus presents an opportunity to implement individualized therapy based on mutational status on a population scale, a generational goal not only for lipid specialists, but for the entire medical field. We therefore call for the addition of the R3500Q mutation to universal genetic screening protocols in Pennsylvania and support for addressing FDB as an issue of public health.

Disclosure statement: Dr. Andersen has no disclosures to report. Lars Andersen has no disclosures to report.

References are listed on page 46 of the PDF.