Original ArticlePhenotypic variability in 4 homozygous familial hypercholesterolemia siblings compound heterozygous for LDLR mutations
Introduction
Homozygous familial hypercholesterolemia (HoFH) is a rare clinical phenotype of severe low-density lipoprotein (LDL) hypercholesterolemia that is inherited in an autosomal dominant pattern with low-density lipoprotein receptor (LDLR) mutations. The high levels of low-density lipoprotein cholesterol (LDL-C) lead to cholesterol buildup in the skin and tendons right from early infancy. Cholesterol deposition in the intima of the arteries causes devastating atherosclerotic lesions and premature coronary heart disease (CHD). CHD and aortic stenosis (due to cholesterol deposition in aortic root and valve) may become detectable already during childhood. HoFH patients, if untreated, have a greatly reduced life expectancy, with a large proportion of patients suffering from fatal coronary insufficiency or myocardial infarction before the age of 20 years.1, 2, 3 More than 95% of patients with HoFH have mutations in both LDLR alleles.4, 5 Patients may carry 2 identical mutant alleles (true homozygotes) or 2 different mutant alleles (compound heterozygotes). The assay of LDLR activity in cultured skin fibroblasts is used to distinguish receptor-negative patients (<2% of residual LDLR activity) from receptor-defective patients (2%–30% LDLR activity). The residual LDLR activity is negatively correlated with the plasma level of LDL-C and the severity of clinical manifestations.4, 5, 6
Mutations in other genes, such as apolipoprotein B (APOB), proprotein convertase subtilisin/kexin type 9 (PCSK9) or, rarely, the LDLR adaptor protein 1 (LDLRAP1; autosomal recessive hypercholesterolemia) are also found in patients with a clinical phenotype consistent with HoFH.4, 7, 8 Patients may also be double heterozygotes when they carry mutations in LDLR in combination with mutations in either APOB or PCSK9 gene.
From the clinical standpoint, the criteria commonly adopted for the diagnosis of dominant HoFH are based on LDL-C level ≥12 mmol/L, presence of cutaneous or tendon xanthomas in infancy (often associated with aortic stenosis), and hypercholesterolemia in both parents.2, 3, 9 The prevalence of dominant HoFH had been estimated to be 1:1.000.000 and the prevalence of heterozygous carriers 1:500.1 Recently, population studies in Denmark indicated that the prevalence of clinically defined heterozygous familial hypercholesterolemia (FH; based on Dutch Lipid Clinic criteria) was ∼1: 200.10 An extensive molecular study conducted in the Netherlands indicated that the prevalence of heterozygous carries of FH candidate gene mutations is ∼1:244.5 In view of these findings, the estimated prevalence of dominant HoFH would be between 1:160.000 and 1:360.000. The Dutch study also revealed that patients with molecularly defined HoFH have a variable clinical phenotype ranging from mild and/or moderate to the extremely severe.5 This variability may depend on the candidate gene involved and the type of mutation (eg, LDLR-negative vs LDLR-defective), as well as the contribution of variants in modifying genes and possibly lifestyle and dietary habits. In this context, one should consider the presence of rare variants in APOB gene (resulting in truncated apoBs or in single amino acid substitutions) that are the cause of familial hypobetalipoproteinemia type 1 (FHBL1; OMIM # 1077309).11 FHBL1 is an autosomal dominant disorder characterized by plasma levels of total cholesterol (TC), LDL-C, and ApoB > the 5th percentile, with a heterozygous frequency of 1:1000. Mutations in APOB gene associated with FHBL1 might contribute to the phenotypic variability in FH by exerting a strong LDL-lowering effect.12 This study was prompted by the observation of identical twin sisters in whom the clinical diagnosis of definite HoFH was made in the second decade of life when they were found to be compound heterozygotes for 2 mutations in the LDLR gene. The phenotypic expression of the disease in these twin sisters was more severe than that observed in their 2 siblings who were compound heterozygotes for the LDLR mutations but were also heterozygous for a rare missense variant of apoB with a possible LDL-lowering effect.
Section snippets
Plasma lipids
Plasma levels of TC, high-density lipoprotein cholesterol (HDL-C), and triglycerides (TGs) were determined by standard enzymatic techniques (Roche Diagnostics GmbH, Mannheim, Germany). LDL-C was calculated by Friedewald's formula. Lp(a) was measured by rate nephelometry (BN ProSpec; Siemens Healthcare Diagnostics, Italy).
Gene resequencing
Genomic DNA was extracted from peripheral blood leukocytes,6 and exon by exon sequencing of LDL-related genes (LDLR, PCSK9, APOB, APOE, ANGPTL3, and APOC3) and HDL-related
Clinical features of the probands
The probands were identical twin sisters (subjects III.2 and III.3 in Fig. 1) referred to the lipid clinic at the age of 24 years for the presence of xanthomas in the Achilles tendons, which had appeared during adolescence but had been overlooked until the age of 19 years when their presence was found to be associated with elevated levels of plasma cholesterol (reported TC ∼13 mmol/L). However, at that time, only dietary and lifestyle changes had been suggested, and no decision had been taken
Discussion
The index cases of the present study were 24-year-old identical twin sisters presenting a phenotype that was somewhat milder than most HoFH but nevertheless more severe than average for HeFH. Although their father displayed typical LDL-C concentration of HeFH, the LDL-C level in their mother did not suggest heterozygous FH. Furthermore, the phenotype in the 2 twin sisters' siblings was compatible with that of heterozygous FH. On first examination, we assumed that the twin sisters had a severe
Conclusions
The FH family described in this study reveals that a severe phenotype as HoFH may be overlooked for many years despite the presence of early clues for its clinical identification. The study also emphasizes that: (1) HoFH patients sharing the same LDLR mutations may present with a variable phenotypic expression in terms of plasma LDL-C level and clinical manifestations; (2) patients may not be fully compliant with the suggested treatment for several reasons such as putative “drug intolerance,” a
Acknowledgments
C.R., F.B., and M.P. performed the analysis of candidate genes and mutant mRNA characterization. P.T. supervised the molecular investigations. T.S. and F.S. were in charge of the patients management and clinical data collection. S.C. and S.B. contributed to the design of the study and wrote the article. All the authors read and approved the final version of the article.
References (35)
- et al.
Homozygous familial hypercholesterolemia: current perspectives on diagnosis and treatment
Atherosclerosis
(2012) - et al.
Spectrum of mutations and phenotypic expression in patients with autosomal dominant hypercholesterolemia identified in Italy
Atherosclerosis
(2013) - et al.
Genotypic and phenotypic features in homozygous familial hypercholesterolemia caused by proprotein convertase subtilisin/kexin type 9 (PCSK9) gain-of-function mutation
Atherosclerosis
(2014) - et al.
Hypobetalipoproteinemia: genetics, biochemistry and clinical spectrum
Adv Clin Chem
(2011) - et al.
Additive effect of mutations in LDLR and PCSK9 genes on the phenotype of familial hypercholesterolemia
Atherosclerosis
(2006) - et al.
Flow-mediated dilation, carotid wall thickness and HDL function in subjects with hyperalphalipoproteinemia
Nutr Metab Cardiovasc Dis
(2014) - et al.
An apparent inconsistency in parent to offspring transmission of point mutations of LDLR gene in familial hypercholesterolemia
Clin Chim Acta
(2009) - et al.
Abnormal splicing of ABCA1 pre-mRNA in Tangier disease due to a IVS2 +5G>C mutation in ABCA1 gene
J Lipid Res
(2003) - et al.
Genetic screening of patients with familial hypercholesterolaemia (FH): a New Zealand perspective
Atheroscler Suppl
(2004) - et al.
Autosomal recessive hypercholesterolemia (ARH) and homozygous familial hypercholesterolemia (FH): a phenotypic comparison
Atherosclerosis
(2006)