Familial hypercholesterolaemia with high triglycerides: A diagnostic challenge

Abstract

Combined or mixed hyperlipidaemia is characterised by hypercholesterolaemia together with high triglyceride concentrations. It is found in approximately 1 in 100 people in the United Kingdom. Most cases are secondary to an underlying condition such as the metabolic syndrome, diabetes mellitus (especially poorly controlled) or individuals with a high alcohol intake. Mixed hyperlipidaemia is also a feature of some primary hyperlipidaemia conditions such familial combined hyperlipidaemia (FCH) or type III hyperlipidaemia (dysbetalipoproteinaemia). One differential diagnosis for mixed hyperlipidaemia that can easily be overlooked is a patient with an underlying diagnosis of familial hypercholesterolaemia (FH) who also has a hypertriglyceridaemia due to any other cause. Those patients may have very high total and low-density lipoprotein cholesterol concentrations (LDL-C) with a moderately elevated triglyceride concentration. In this article, we report 4 cases of familial hypercholesterolaemia, confirmed by genetic testing, in patients initially presenting with hypertriglyceridaemia in addition to high total cholesterol and LDL-C. This article discusses the diagnostic challenges associated with this presentation and highlights the key role of directly measuring LDL-C to aid diagnosis in these specific situations.

Introduction

Familial hypercholesterolaemia (FH) is a group of inherited genetic defects that lead to the severe elevation of serum cholesterol concentrations. ¹ The current clinical practice ² to categorise the causes of hyperlipidaemia based on which component(s) of the lipid profile is raised has the potential to miss the diagnosis of FH in a patient when their lipid profile also shows a high triglyceride concentration (usually above 4.5 mmol/L) ² since FH tends to be excluded as a cause of hyperlipidaemia with such a finding. This potentially exposes the patient to risks of delayed diagnosis and failed treatment which, in turn, might lead to premature cardiovascular disease (CVD). An obvious family history of CVD in some of these patients may point towards an inherited hyperlipidaemia; however, in many cases, this is inconclusive, unavailable or may be attributed to the mixed lipid picture.

The finding of high LDL-C in these patients is therefore crucial to alert the clinician to the possibility of an FH diagnosis. However, the result for LDL-C is often unavailable in patients with hypertriglyceridaemia if the clinical laboratory uses the Friedewald ³ or the original Martin-Hopkins formula ⁴ to estimate the LDL-C concentration. This is because neither equation can reliably calculate LDL-C if the triglyceride concentration exceeds 4.5 mmol/L. By comparison, LDL-C calculated using the more recent Sampson-NIH equation ⁵ and the updated Martin-Hopkins ⁶ approach have both been found to be robust up to triglyceride concentrations of 9 mmol/L.

There is limited literature to highlight the presentation of FH in the presence of high triglycerides. ⁷ Here we report 4 cases presenting with hypertriglyceridaemia who were subsequently found to have a genetic diagnosis of FH. The cause of the high triglycerides was variable in each case, with some having no obvious underlying secondary cause. Clinical details for the cases are summarised in Table 1.

Table 1.

Summary of the 4 cases’ clinical findings.

Clinical data	Case 1	Case 2	Case 3	Case 4
Risk factors for raised triglycerides	Overweight, BMI 29 kg/m2. Poor diet	High BMI of 32.8 kg/m2	-	Poor diet. Alcohol intake: 18 units/week (above recommended limits⁸)
Family history	Father died of MI aged 46 years. Brother died of MI aged 43 years. A paternal uncle had an MI aged 45 years. Both paternal grandparents died aged 60 years	Grandfather and uncle: MI aged 50. Mother: high total cholesterol - around 12.0 mmol/L. Father: high triglycerides + MI at the age of 72 + triple bypass surgery	Father died aged 55 of query MI. Paternal uncle died suddenly aged 50	Brother’s and sister’s cholesterol about 11 and 9.0 mmol/L respectively. Mother: high cholesterol on a statin
Genetic test result	Heterozygous for a pathogenic LDLR variant c.682 G>T p. (Glu228Ter), autosomal dominant familial hypercholesterolaemia	Heterozygous for the LDLR c.1816 G>T p.(Ala606Ser) variant. Monoallelic pathogenic LDLR variants cause autosomal dominant familial hypercholesterolaemia	Heterozygous for the PCSK9 variant c.1486 C>T p.(Arg496Trp), autosomal dominant familial hypercholesterolaemia	Heterozygous for a pathogenic LDLR variant c.6delG p.(Trp4fs), autosomal dominant familial hypercholesterolaemia
Current treatment	Evolocumab in addition to ezetimibe and atorvastatin 80 mg	Pravastatin	Evolocumab	Evolocumab

Case 1

A 51-year-old male presented with raised total cholesterol and triglycerides (Table 2). His LDL-C could not be calculated using the Friedewald equation at the time because of the high triglyceride concentration. Highest and lowest triglyceride concentrations are shown in Table 3. He had a strong family history of early ischaemic heart disease (Table 1) with his father, one of his brothers and a paternal uncle all having myocardial infarctions (MIs) before the age of 50 years, being fatal events in the former two. His paternal grandparents both died at the age of 60 years but the causes are unknown. He had several relevant lifestyle factors for hyperlipidaemia including being borderline obese and having a relatively poor diet which included lots of red meat and chocolate. He also smoked for around 40 years; mostly 30 cigarettes a day. His alcohol intake was within recommended limits at about 8 units a week. ⁸ He was normotensive, non-diabetic and euthyroid. Despite his high triglyceride concentration, and the presence of secondary risk factors, the patient was diagnosed as possible FH and treated with a statin based on a literal interpretation of the Simon Broome criteria ⁹ (i.e. total cholesterol of >7.5 mmol/L and a family history of MI in a second-degree relative aged younger than 50 years or a first-degree relative aged younger than 60 years). FH was confirmed later with a genetic test, reported as heterozygous for a pathogenic LDLR variant c.682G>T p. (Glu228Ter), consistent with a diagnosis of autosomal dominant familial hypercholesterolaemia with cascade screening then being initiated. His apolipoprotein E (Apo E) genotype did not support a diagnosis of type III hyperlipidaemia.

Table 2.

Lipid profiles on presentation (results in mmol/L).

Lipid profile	Case 1	Case 2	Case 3	Case 4
Total cholesterol	11.7	8.0	7.4	12.1
LDL cholesterol	N/A	N/A	4.69 (Calculated LDL-C using Friedewald equation)	6.19 (direct LDL-C)
HDL cholesterol	0.8	1.1	1.11	-
Triglycerides	10.3	5.0	3.5	3.1

Table 3.

Highest and lowest triglyceride concentrations (in mmol/L).

	Case 1	Case 2	Case 3	Case 4
Highest triglycerides concentration in mmol/L	11.1	8.4	7.0	4.5
Lowest triglycerides concentration in mmol/L	3.0	3.7	1.4	2.2

Unfortunately, he had an MI whilst solely taking a statin, after which ezetimibe was added. His lipid profile did not improve significantly on this combination, so a PCSK9 inhibitor (evolocumab) was started in addition to his existing ezetimibe 10 mg and atorvastatin 80 mg daily. He managed to reduce his smoking to five cigarettes per day. His lipids then responded well as shown in Table 4.

Table 4.

Lipid profiles post treatment (results in mmol/L).

Lipid profile	Case 1	Case 2	Case 3	Case 4
Total cholesterol	4.2	6.8	2.7	5.7
LDL cholesterol (direct)	2.1	4	1.1	3.7
HDL cholesterol	1.1	1.2	1.2	1.4
Triglycerides	6.1	3.9	2.1	2.4

Case 2

A 60-year-old female was diagnosed with mixed hyperlipidaemia and fatty infiltration of the liver. Her lipid profile is shown in Table 2. Her highest and lowest triglyceride concentrations are shown in Table 3.

It proved impossible to obtain a calculated LDL-C concentration on this patient due to the high triglyceride concentration. Her Apo E genotype was not consistent with type III hyperlipidaemia.

Her family history was significant (Table 1) as her grandfather and uncle experienced MIs at the age of 50 years and her mother had been hypercholesterolaemic at around 12.0 mmol/L before treatment with a statin. Her father was also hypertriglyceridaemic and had an MI at the age of 72 years requiring triple coronary artery bypass surgery. She was initially diagnosed with familial combined hyperlipidaemia (FCH) which was reviewed later with the availability of genetic testing for FH. The result reported that this patient was heterozygous for the LDLR c.1816G>T p. (Ala606Ser) variant. Monoallelic pathogenic LDLR variants cause autosomal dominant familial hypercholesterolaemia. This variant is considered to potentially be a mild pathogenic variant or a susceptibility variant which in combination with other factors may lead to hypercholesterolaemia. Therefore, the result was considered to be consistent with a diagnosis of familial hypercholesterolaemia with variable phenotype and reduced penetrance.

She was obese despite her diet being almost vegetarian and including minimal alcohol. However, although active, this was limited as she had multiple sclerosis. She did not have diabetes and was euthyroid. Regarding lipid lowering treatment, she could not tolerate simvastatin or atorvastatin due to muscle aches and joint pains but was able to tolerate pravastatin. Her lipid profile showed some response as shown in Table 4.

Case 3

A 45-year-old female was first seen with a mixed hyperlipidaemia and lipid profile values as shown in Table 2. Her highest and lowest triglyceride concentrations are shown in Table 3. Her father had died at the age of 55 years from what was likely an MI, although no definite diagnosis was made. Her paternal uncle died suddenly around 50 years of age. She had five siblings who were all well. It was initially felt that her family history was inconclusive for FH and in this individual the LDL-C result was unavailable due to a hypertriglyceridaemia. She had a normal BMI and drank no alcohol. There was no obvious cause for her hypertriglyceridaemia. An Apo E genotype test was requested and the result did not support the diagnosis of type III hyperlipidaemia.

She was reviewed again and a decision to perform a genetic test for FH was taken because of her high LDL-C identified following the availability of directly measured LDL-C. Her genetic test for FH was reported as positive with a PCSK9 variant c.1486C>T p. (Arg496Trp).

She was unable to tolerate statins and was subsequently started on evolocumab, to which she responded well as shown in Table 4.

Case 4

A 43-year-old male presented with mixed hyperlipidaemia as shown in Table 2. His highest and lowest triglyceride concentrations are shown in Table 3. His brother and sister’s total cholesterols were markedly raised, being around 11 mmol/L and 9.0 mmol/L, respectively. His mother was also initially hypercholesterolaemic before taking a statin.

In view of the family history and high directly measured LDL-C, he was diagnosed with possible FH and a genetic test requested. The result showed that he was heterozygous for a pathogenic LDLR variant c.6delG p. (Trp4fs).

He admitted to having a poor diet and his average alcohol intake was 18 units a week. He was unable to tolerate statins and was eventually started on evolocumab after which the lipid profile improved as shown in Table 4.

Discussion

Given the prevalence of combined or mixed hyperlipidaemia, the presence of hypertriglyceridaemia in patients with FH may cause diagnostic difficulty as hypertriglyceridaemia is not an uncommon feature of secondary hyperlipidaemias. A review article by Cuchel et al. that discussed the diagnosis and management of FH stated that the condition is often underdiagnosed and undertreated, and that the presence of high triglycerides can complicate the diagnosis, as it may suggest other causes of hyperlipidaemia, such as the metabolic syndrome, diabetes or excess alcohol consumption. ⁷

Combined or mixed hyperlipidaemia is characterised by hypercholesterolaemia and high triglyceride concentrations and is commonly acquired secondary to conditions such as those already just mentioned. Primary causes of mixed hyperlipidaemia include familial combined hyperlipidaemia (FCH) and type III hyperlipidaemia. Whilst Apo E genotype can be used to exclude type III hyperlipidaemia, the underlying genetic basis of FCH is not known, with it most likely a polygenetic disorder. ¹⁰

FH, on the other hand, is a genetic disorder caused by mutations in the genes related to the low-density lipoprotein receptor (LDLR) pathway and is transmitted by autosomal dominant inheritance. Patients with FH have markedly raised LDL-C and usually triglycerides are within the normal range. In contrast, patients with a mixed hyperlipidaemia have an elevated triglyceride concentration ¹¹ which is generally due to an increase in triglyceride-rich very low-density lipoprotein (VLDL) particles and so the total cholesterol concentration is usually only moderately elevated. However, this case report identified four patients presenting with high total cholesterol and triglyceride concentrations found to have a genetic diagnosis of FH. Apo E genotype was performed to exclude type 3 hyperlipidaemia and found to be negative in the first three cases. In case number 4, hypertriglyceridaemia was attributed to a high alcohol intake. So whilst the likely cause for the hypertriglyceridaemia was identified in some of these cases, others had no obvious cause.

The strong family history of CVD and/or the availability of LDL concentration measurement at presentation have helped to point towards the diagnosis of FH earlier in 2 of the 4 cases meaning the diagnosis was not missed.

All these cases emphasise the importance of not attributing every mixed hyperlipidaemia simply to possible secondary causes and associated lifestyle factors. This is important as heterozygous FH is relatively common, with its prevalence estimated as being between 1 in 250 and 1 in 500 of the UK population. ¹¹ Despite these theoretical estimated prevalences, fewer than 1% are diagnosed in most countries. ¹¹

The identification of FH is crucial as this will define patients with high CVD risk who will need treatment for their hyperlipidaemia to reduce the risk of cardiac events. It will also potentially lead to tracing of possible family members also affected. Family history of CVD in some patients is so evident as to trigger applying the Simon Broome criteria and diagnosing FH; however, in many cases, family history is inconclusive or unavailable.

Decisions on how to manage the hyperlipidaemia can also be directly affected by LDL concentration. For example, in cases 2 and 3 neither of the patients would have been eligible for PCSK9 inhibitor treatment according to NICE guidelines if the diagnosis of FH had not been confirmed. ¹²

These findings also highlight the potential usefulness of direct LDL-C measurement. The initial result for calculated LDL-C was unavailable in three of these patients due to hypertriglyceridaemia. Some laboratories have now introduced a direct LDL-C assay to measure the actual concentration which enables detection of a possible FH diagnosis despite high triglycerides. However, in the United Kingdom, LDL-C is still predominantly estimated by calculation based on the measurement of total cholesterol, HDL cholesterol and triglycerides: in a recent UK NEQAS external quality assessment scheme report, only 61 out of the 524 laboratories participated in the LDL cholesterol scheme were obtained by a directly measured LDL-C assay, with the remainder being obtained using the calculated Friedewald method. ¹³ Since this equation is unreliable if the patient is non-fasting or has a triglyceride concentration beyond 4.5 mmol/L, direct LDL-C assays may prove useful in this situation in distinguishing increased LDL from VLDL cholesterol when calculated LDL-C is not valid. Whether more recent LDL-C formulae such as Sampson-NIH or updated Martin-Hopkins can serve a similar purpose requires further investigation.