Unusual xanthomas in a young patient with heterozygous familial hypercholesterolemia and type III hyperlipoproteinemia

Searching for new genes associated with the familial hypercholesterolemia phenotype using whole-genome sequencing and machine learning

One of the most common congenital metabolic disorders is familial hypercholesterolemia. Familial hypercholesterolemia is a condition caused by a type of genetic defect leading to a decreased rate of removal of low-density lipoproteins from the bloodstream and a pronounced increase in the blood level of total cholesterol. This disease leads to the early development of cardiovascular diseases of atherosclerotic etiology. Familial hypercholesterolemia is a monogenic disease that is predominantly autosomal dominant. Rare pathogenic variants in the LDLR gene are present in 75-85 % of cases with an identified molecular genetic cause of the disease, and variants in other genes (APOB, PCSK9, LDLRAP1, ABCG5, ABCG8, and others) occur at a frequency of < 5 % in this group of patients. A negative result of genetic screening for pathogenic variants in genes of the low-density lipoprotein receptor and its ligands does not rule out a diagnosis of familial hypercholesterolemia. In 20-40 % of cases, molecular genetic testing fails to detect changes in the above genes. The aim of this work was to search for new genes associated with the familial hypercholesterolemia phenotype by modern high-tech methods of sequencing and machine learning. On the basis of a group of patients with familial hypercholesterolemia (enrolled according to the Dutch Lipid Clinic Network Criteria and including cases confirmed by molecular genetic analysis), decision trees were constructed, which made it possible to identify cases in the study population that require additional molecular genetic analysis. Five probands were identified as having the severest familial hypercholesterolemia without pathogenic variants in the studied genes and were analyzed by whole-genome sequencing on the HiSeq 1500 platform (Illumina). The whole-genome sequencing revealed rare variants in three out of five analyzed patients: a heterozygous variant (rs760657350) located in a splicing acceptor site in the PLD1 gene (c.2430-1G>A), a previously undescribed single-nucleotide deletion in the SIDT1 gene [c.2426del (p.Leu809CysfsTer2)], new missense variant c.10313C>G (p.Pro3438Arg) in the LRP1B gene, and single-nucleotide deletion variant rs753876598 [c.165del (p.Ser56AlafsTer11)] in the CETP gene. All these variants were found for the first time in patients with a clinical diagnosis of familial hypercholesterolemia. Variants were identified that may influence the formation of the familial hypercholesterolemia phenotype.

Xanthomas Can Be Misdiagnosed and Mistreated in Homozygous Familial Hypercholesterolemia Patients: A Call for Increased Awareness Among Dermatologists and Health Care Practitioners

Background

Familial hypercholesterolemia (FH) is an autosomal dominant inherited genetic disorder and results in the development of coronary artery disease (CAD). Clinical diagnosis of homozygous HH patients is usually straightforward because persistent hypercholesterolemia can produce xanthoma and corneal arcus. However, xanthoma may also be misdiagnosed as skin lesions and could therefore be mistreated. The aim of this case study report is to highlight the plight of patients with FH as means of raising awareness of the condition among dermatologists and health care practitioners, also to determine the genotype-phenotype correlation in severely affected homozygous FH proband patients.

Methods

Genetic screening of FH associated genes was performed by Ion Torrent next-generation sequencing and cascade screening by capillary sequencing.

Results

We present two clinical cases with prominent skin lesions seen in a dermatology clinic that were referred to plastic surgery for excision. Genetic testing was performed later, and confirmed common single nucleotide deletion variant (c.2027delG) in the LDLR alleles consequent to a frameshift mutation p.(G676Afs*33). In addition to the LDLR variant, two possibly damaging APOB variants p.(L3313I) and p.(L1212M) and three damaging variants p.(R19*), p.(G83Q) and p.(S474*) in APOC3, PON2 and LPL genes respectively were identified. The PON2 gene variant p.(G83Q) was found to be novel, while others have been previously reported. Both patients were refractory to pharmacological therapies and are currently on lipoprotein apheresis (LA).

Conclusions

The present report indicates the need for increased awareness of FH, among the public and healthcare practitioners and supports the need for diagnostic screening and cascade genetic testing of this high-risk condition, which could ultimately lead to better prevention of CHD in this lethal condition.

Case series of type III hyperlipoproteinemia in children

Type III hyperlipoproteinemia (type III HLP) rarely manifests in childhood. Long-term follow-up (37 years) of the first patient revealed hypothyroidism at diagnosis requiring thyroxine replacement, palmar xanthomas requiring surgical removal, splenomegaly requiring splenectomy, 18 episodes of pancreatitis and premature coronary artery disease. Investigation revealed an apolipoprotein E phenotype of E2/E2 and partial lipoprotein lipase deficiency. Investigation of the second patient revealed a combination of apoE2/E2 phenotype and heterozygous familial hypercholesterolaemia. The third patient had a complete deficiency of lipoprotein lipase activity, an abnormal thyroid stimulating hormone on diagnosis (with subsequent normalisation without treatment), and apoE2/E2 phenotype. Type III HLP is a serious disorder with lifelong consequences of premature vascular disease and recurrent pancreatitis. Early presentation of disease in our patients was associated with additional precipitating factors. Drug treatment of paediatric type III HLP is indicated if dietary modifications alone are insufficient in managing the dyslipidaemia.

Unmasking of type III hyperlipoproteinemia by hypothyroidism: a dramatic illustration of altered lipoprotein metabolism in a postpartum woman

OBJECTIVE

To illustrate the potential abnormalities in lipoprotein metabolism associated with type III hyper-lipoproteinemia and the modulation of their clinical expression by thyroid hormone and estrogenic status.

METHODS

An illustrative case, with associated clinical and laboratory data, is presented, and relevant clinical and pathophysiologic studies from the literature are reviewed.

RESULTS

A 35-year-old woman, at 7 months after delivery of her first child, presented to her family physician with a complaint of painful eruptions on the palms of her hands. On evaluation, she was found to have new hypothyroidism and severe hypertriglyceridemia (>1,569 mg/dL). Thyroxine replacement was initiated, and she was referred to the lipid clinic. When seen in the lipid clinic shortly thereafter, her triglyceride level had normalized, but her low-density lipoprotein (LDL) fraction was strikingly elevated (representing a combination of elevated intermediate-density lipoprotein and LDL cholesterol). On physical examination, palmar xanthomas were noted, suggestive of type III hyperlipoproteinemia. This diagnosis was further supported by homozygosity at the apolipoprotein E (apo E) gene locus for the apo E2 allele implicated in this condition. Ultimately, with attainment of euthyroidism in the subsequent weeks, the lipid profile normalized, with the LDL cholesterol concentration particularly reduced at 55 mg/dL.

CONCLUSION

Clinical expression of type III hyperlipoproteinemia necessitates interaction between an underlying genetic defect of lipoprotein metabolism (apo E2 homozygosity) and a secondary metabolic insult such as, in the current case, hypothyroidism and possibly breast-feeding-mediated hypoestrogenemia. As such, in patients with type III hyperlipoproteinemia, it is essential to search for exacerbating factors, particularly because the amelioration of such factors may rectify the effects of the underlying dyslipidemia.

Apolipoprotein E2/E2 genotype in combination with mutations in the LDL receptor gene causes type III hyperlipoproteinemia

The primary genetic cause of type III hyperlipoproteinemia is the homozygous presence of the apolipoprotein E2 allele. However, only approximately 1% of subjects with the apolipoprotein E2/E2 genotype develop type III hyperlipoproteinemia. Other factors are therefore necessary to express type III hyperlipoproteinemia. Two individuals were identified as having type III hyperlipoproteinemia (triglyceride to very low-density lipoprotein (VLDL) cholesterol ratio >0.3). However, in contrast to unchanged or slightly decreased low-density lipoprotein (LDL)-cholesterol levels typically observed in type III patients, elevated LDL-cholesterol levels were observed. The expected apolipoprotein E2/E2 isoform was confirmed by genetic analysis. To explain the elevated LDL-cholesterol level, single strand conformation polymorphism analysis was performed to screen for mutations in the LDL receptor gene. In both individuals, mutations causing an impaired LDL receptor function (2 bp insertion in exon 3 and Glu119 --> Gly mutation in exon 4) were identified. In six more unrelated individuals, these mutations combined with the common apolipoprotein E3/E3 genotype, resulted in an isolated, severe LDL-cholesterol elevation. Our results indicate that the level of LDL receptors plays an important role in remnant clearance, and that the combination of the binding-defective apolipoprotein E2 with a defective LDL receptor precipitate type III hyperlipoproteinemia.

Genetics of type III hyperlipoproteinemia

One hundred forty-seven relatives of 43 patients with "classical" type III hyperlipoproteinemia (HLP) having the apolipoprotein (apo) E2/2 phenotype were studied to determine the occurrence of hyperlipidemia and the presence of further possible genes for lipoprotein disorders in these families. In 12 pedigrees primary dyslipidemia was prevalent among patients and respective blood-relatives. In these kindreds the coexistent presence of genes for familial combined hyperlipidemia (n = 6), familial hypertriglyceridemia (n = 5), and familial hypercholesterolemia (n = 1), respectively, was supposed. Our results, therefore, confirm and extend previous data on the multifactorial genesis of the diseases. Besides homozygosity for a receptor binding-defective isoform of apo E (apo E2), additional genes for familial lipoprotein disorders might operate in the pathogenesis of type III HLP. This is the largest family study performed so far in this primary lipoprotein disorder.

Coexisting dysbetalipoproteinemia and familial hypercholesterolemia. Clinical and laboratory observations

Type III dysbetalipoproteinemia and familial hypercholesterolemia (FH) are two metabolic disorders giving rise to severe disturbances of lipid homeostasis and premature atherosclerosis. Both metabolic abnormalities have a genetic basis and co-occurrence in the same patient has seldom been described. Because of the unique structure of the French Canadian population, there was an opportunity to observe patients with both dysbetalipoproteinemia (E2/2 homozygotes) and FH (N=14) and to compare their clinical data with that of patients with type III (N=75), patients with FH (N0.7 and the presence of beta-VLDL on electrophoresis. Presence of a low density lipoprotein receptor, LDL-R, mutation should be suspected in a type III patient with a LDL-C level above 3.0 mmol/l and a family history of premature CAD. In the group of patients studied, the coexistence of dysbetalipoproteinemia and heterozygous FH does not appear to increase the prevalence of cardiovascular complications above that observed among control type III or control E3/3-FH patients. Thus, the presence of two epsilon2 alleles in these patients affects the expression of the abnormal LDL-R allele and the resulting phenotype substantiates the non additive effects of alleles at these two loci (epistasis).

Update on low density lipoprotein receptor mutations

Recent research has focused on the rapid detection of new LDL receptor gene variants and large scale screening for known mutations. Whether the nature of the mutation in the LDL receptor gene in familial hypercholesterolaemia determines clinical variability has been examined, as well as the potential value of detecting mutation carriers for clinical practice. There is also evidence that some patients with clinical familial hypercholesterolaemia do not have detectable defects in the LDL receptor or apolipoprotein B.