Low cholesterol states: clinical implications and management

ABSTRACT

Introduction

Hypocholesterolemia results from genetic – both monogenic and polygenic – and non-genetic causes and can sometimes be a source of clinical concern. We review etiologies and sequelae of hypocholesterolemia and therapeutics inspired from genetic hypocholesterolemia.

Areas covered

Monogenic hypocholesterolemia disorders caused by the complete absence of apolipoprotein (apo) B-containing lipoproteins (abetalipoproteinemia and homozygous hypobetalipoproteinemia) or an isolated absence of apo B-48 lipoproteinemia (chylomicron retention disease) lead to clinical sequelae. These include gastrointestinal disturbances and severe vitamin deficiencies that affect multiple body systems, i.e. neurological, musculoskeletal, ophthalmological, and hematological. Monogenic hypocholesterolemia disorders with reduced but not absent levels of apo B lipoproteins have a milder clinical presentation and patients are protected against atherosclerotic cardiovascular disease. Patients with heterozygous hypobetalipoproteinemia have somewhat increased risk of hepatic disease, while patients with PCSK9 deficiency, ANGPTL3 deficiency, and polygenic hypocholesterolemia typically have anunremarkable clinical presentation.

Expert opinion

In patients with severe monogenic hypocholesterolemia, early initiation of high-dose vitamin therapy and a low-fat diet are essential for optimal prognosis. The molecular basis of monogenic hypocholesterolemia has inspired novel therapeutics to help patients with the opposite phenotype – i.e. elevated apo B-containing lipoproteins. In particular, inhibitors of PCSK9 and ANGPTL3 show important clinical impact.

KEYWORDS:

• Abetalipoproteinemia
• ANGPTL3 deficiency
• hypobetalipoproteinemia
• LDL cholesterol
• monogenic hypocholesterolemia
• PCSK9 deficiency
• polygenic hypocholesterolemia

1. Introduction

Hypocholesterolemia, primarily due to depressed low-density lipoprotein (LDL) cholesterol levels, is often considered to be a benign condition that is protective against atherosclerotic cardiovascular disease (ASCVD). However, very low cholesterol states may sometimes be cause for alarm, especially when associated with underlying severe illness or when predisposing to certain debilitating neurological or other multisystemic conditions. Diagnosing the cause of hypocholesterolemia is important in order to commence appropriate treatment and monitoring [1]. Here, we review the non-genetic and genetic causes of low LDL cholesterol states, the various underlying genetic and biochemical etiologies, and associations of the genetic disorders with ASCVD, diabetes, fatty liver disease and multisystemic disorders and vitamin deficiencies. We suggest an approach for treatment of patients at risk for multisystemic complications and outline the current state of therapeutics targeted to treat high LDL cholesterol that were inspired from understanding the pathways underlying the monogenic hypocholesterolemia disorders.

2. Non-genetic causes of hypocholesterolemia

The majority of cases of hypocholesterolemia are caused by secondary non-genetic factors (Table 1) [22]. Lifestyle factors, such as a vegan, vegetarian [3] or low-fat diet, alcoholism [2] and malnutrition, can contribute toward low cholesterol. Hypocholesterolemia may be a sign of an infectious disease [15–17], underlying inflammatory state [12], or severe illness [4,7–9,23,11,13,14,18,19,24–26]. In some illnesses, the hypocholesterolemia may result from malabsorption [19,25]. In many critically ill patients, hypocholesterolemia is associated with a worse prognosis [5] and is an independent predictor of mortality [6,10,12]. In addition to lipid lowering therapies, other medications, such as interleukin-2 used for treating refractory metastatic cancer [20,27], and alternative therapies [19] may contribute toward hypocholesterolemia.

Table 1. Non-genetic secondary causes of low LDL cholesterol.

Secondary factor	Cause	Reference
Lifestyle	Alcohol intake Low-fat diet Malnutrition Vegan or vegetarian diet	[2] [3]
Non-communicable diseases	Acute renal failure Anemia Beta-thalassemia intermedia Cancer cachexia Cystic fibrosis Chronic pancreatitis Gaucher disease Hyperthyroidism Inflammatory response syndrome Malabsorption states [4,5] Sickle cell disease Smith-Lemli-Opitz syndrome	[6] [7] [4] [8] [23] [5] [9] [10] [11] [12] [5] [13,14]
Infectious diseases	Amebic liver abscess Hepatitis C Human immunodeficiency virus Visceral leishmaniasis	[15] [16] [17] [18]
Medications	Interleukin-2 Lipid-lowering medications (i.e. statins, PCSK9 inhibitors)	[19] [20,21]

3. Genetic causes of hypocholesterolemia

Numerous genetic etiologies can result in low LDL cholesterol (Table 2), including rare disorders causing a complete absence of apolipoprotein (apo) B-containing lipoproteins, an isolated absence of apo B-48 lipoproteins, and lastly disorders of reduced but not absent levels of apo B-containing lipoproteins. Figure 1 highlights the metabolic pathways of the main proteins involved in the monogenic hypocholesterolemia disorders.

Figure 1. Simplified lipid pathway demonstrating the role of the key proteins from each monogenic hypocholesterolemia disorder. The proteins involved in apo B lipoprotein production (apoB48, apoB100, MTP), secretion (SAR1B) and catabolism (LDLR) are shown in green. The proteins involved in modulating apoB lipoproteins (PCSK9 and ANGPTL3) are shown in red. Lipoproteins are labeled in blue, with smaller, denser lipoproteins corresponding with darker shades of blue. Abbreviations: ANGPTL3, angiopoietin-like 3; APOB, apolipoprotein B; HL, hepatic lipase; IDL, intermediate-density lipoprotein; LDL, low-density lipoprotein; LPL, lipoprotein lipase; LDLR, LDL receptor; LRP, low density lipoprotein receptor-related protein 1; MTP, microsomal triglyceride transfer protein; PCSK9, proprotein convertase subtilisin/kexin type 9; SAR1B, secretion associated Ras related GTPase 1B; TG, triglyceride; VLDL, very-low density lipoprotein.

Table 2. Summary of the epidemiology, genetic, and clinical features of the primary causes of low LDL cholesterol.

	ABL	Homozygous FHBL	CRD	Heterozygous FHBL	Polygenic hypocholesterolemia	PCSK9 deficiency	ANGPTL3 deficiency	LDLR gain of function
Epidemiology	<1 in 10^6	<1 in 10^6	<1 in 10^6	1 in 1000–3000	10% of the population	Het: 1–3 in 100 Homo: <1 in 10^5	Het: 1 in 300 Homo: <1 in 10^6	7 members of 1 Icelandic family
Genetic Basis	Autosomal recessive; biallelic variants in MTTP	Autosomal co-dominant; biallelic variants in APOB	Autosomal recessive; biallelic variants in SAR1B	Autosomal co-dominant; heterozygous variants in APOB	Polygenic; common variants associated with modulating LDL cholesterol levels	Autosomal semi-dominant; heterozygous or biallelic variants in PCSK9	Autosomal semi-dominant; heterozygous or biallelic variants in ANGPTL3	Autosomal dominant; GOF 2.5kb deletion spanning the LDLR 3’ UTR
Pathology	Impaired packaging and secretion of apoB lipoproteins		Impaired secretion of apo B lipoproteins	Reduced secretion of apo B lipoproteins	Dependent on carrier status of SNPs measured.	Reduced uptake and degradation of LDLR	Reduced lipoprotein and endothelial lipase inhibition	Enhanced lipolysis of apoB100 lipoproteins
Clinical Features	− Acanthocytosis		− Absent acanthocytosis	− Absent acanthocytosis − Increased risk of hepatic disease	None
	− Gastrointestinal signs − Hepatic disease (i.e. hepatic steatosis, fibrosis, cirrhosis, elevated serum transaminases) − Fat soluble vitamin deficiency causing multisystemic sequelae: - Ophthalmic (i.e. retinitis pigmentosa)^† - Musculoskeletal (i.e. myopathy, osteopenia) - Neurological (i.e. neuropathy)^† - Hematological (i.e. coagulopathy, bruising)
Biochemical Features	↓↓ LDL cholesterol ↓↓ total cholesterol ↓↓ apo B ↓↓ TG		↓↓LDL cholesterol ↓↓HDL cholesterol ↓↓total cholesterol ↓↓ apo B Normal TG	↓LDL cholesterol ↓total cholesterol ↓ apo B ↓ TG	↓ LDL cholesterol ↓ apo B	↓LDL cholesterol ↓total cholesterol ↓ apo B ↓ TG	↓LDL cholesterol ↓HDL cholesterol ↓total cholesterol ↓ apo B ↓ TG	↓LDL cholesterol ↓total cholesterol ↓ apo B Normal TG
Treatment	High dose oral vitamin therapy, dietary recommendations, and monitoring (Refer to Table 3)			Monitor for hepatic disease	No specific treatment

Symbols: ↓↓ very low; ↓ low; † milder ophthalmic and neurological complications for CRD patients; Abbreviations: ABL, abetalipoproteinemia; apo B, apolipoprotein B; CRD, chylomicron retention disease; FHBL, familial hypobetalipoproteinemia; GOF, gain-of-function; HDL, high-density lipoprotein; Het, heterozygous; Homo, homozygous; LDL, low-density lipoprotein; LDLR, LDL receptor; RBC, red blood cell; SNP, single nucleotide polymorphism; TG, triglyceride.

Table 3. Treatment and clinical follow-up of patients with monogenic hypocholesterolemia: abetalipoproteinemia, homozygous familial hypobetalipoproteinemia, and chylomicron retention disease.

	Evaluation				Frequency
Growth	Height and weight on track with growth curve for age				6–12 months
Diet	− Low fat diet (<10–30% of total caloric intake) − Ensure total caloric intake is adequate − Reduced long-chain fatty acid intake − Supplementation of essential fatty acids (i.e. 1–2 teaspoons or 5–10 ml of polyunsaturated fatty acid rich oils) − Supplementation of medium chain TG not required				Daily
Vitamin therapy	Vitamin A 100–400 IU/kg/day (15 000 IU/day for CRD)	Vitamin D 800–1200 IU/day	Vitamin E 100–300 IU/kg/day (50 IU/kg/d for CRD)	Vitamin K 5–35 mg/week	Daily – weekly
Laboratory investigation	Lipid levels − total cholesterol − TG − LDL cholesterol − HDL cholesterol − apo B − apo A1	Hepatic function − AST − ALT − GGT − total and direct bilirubin − ALP − Albumin	Vitamin levels − Beta-carotene − 25-OH vitamin D − Plasma or RBC vitamin E − INR	Other − CBC − Vitamin B12 − Folate − Calcium − Phosphate − Uric acid − TSH	Annual (Exception: lipid levels will not usually change on an annual basis)
Clinical features for monitoring (lifelong)	Gastrointestinal − Abdominal distention − Appetite − Diarrhea − Esophagitis − Jaundice − Nausea/Vomiting	Neurological − Monitor development and function − Ataxia − Dysarthria − Hyporeflexia − Muscle pain or weakness − Proprioception loss − Reduced or lost deep tendon reflex	Hepatic − Hepatomegaly Ophthalmological − Nyctalopia (night blindness) − Retinopathy − Retinitis pigmentosa − Xerophthalmia		6–12 months
Commence after age 10	Musculoskeletal − DXA for bone mineral density − Echocardiography to monitor heart activity and muscle	Neuromuscular − Electromyography to monitor skeletal muscle and peripheral nervous system activity	Hepatic− Ultrasonography or FibroScan) to screen for hepatic steatosis, fibrosis & cirrhosis− Fibrosis-4 (FIB 4) index to estimate fibrosis		3 years

Abbreviations: ALP, alkaline phosphatase; ALT, alanine aminotransferase; apo, apolipoprotein; AST, aspartate aminotransferase; CBC, complete blood count; DXA, dual-energy x-ray absorptiometry; GGT, gamma glutamyl transferase; FIB-4, Fibrosis-4; HDL, high-density lipoprotein; INR, international normalized ratio; LDL, low-density lipoprotein; RBC, red blood cells; TG; triglyceride; TSH, thyroid stimulating hormone. Adapted from Lee & Hegele, 2014 (35**) & Bredefeld et al. 2020 (42).

3.1. Complete absence of apo B-containing lipoproteins

3.1.1. Abetalipoproteinemia

Abetalipoproteinemia (ABL), or Bassen-Kornzweig syndrome [21], is a rare autosomal recessive disorder reported in over 50 families [28] with consanguinity in up to 75% of families [29–34]. ABL is caused by extremely rare biallelic (i.e. homozygous or compound heterozygous) variants within the MTTP gene located on chromosome 4q23 [35]. The gene codes for the 97 kDa microsomal triglyceride transfer protein (MTP) that forms a heterodimer with the 55 kDa protein disulfide isomerase (PDI) [35,36]. This heterodimer loads triglycerides (TG) onto apo B within enterocytes and hepatocytes [35,37,38]. More than 75 variants [29] within MTTP are causative for ABL [39] that impair the formation and secretion of apo B-containing lipoproteins, specifically chylomicrons and very-low density lipoproteins (VLDL) within enterocytes and hepatocytes, respectively [35,37,40]. The result is near absent or absent levels of chylomicrons, VLDL, LDL cholesterol (<0.1 mmol/L), apo B (<0.1 g/L), and TG (<0.2 mmol/L) in the blood [35,38]. Plasma cholesterol transport in ABL patients is largely carried out by high-density lipoprotein (HDL) particles [26]. Impaired chylomicron production and secretion, which is the primary vehicle of absorption and transportation of fat-soluble vitamins, eventually results in severe deficiencies of fat-soluble vitamins.

The clinical presentation of ABL is heterogeneous [41]. From infancy and early childhood, patients can exhibit diarrhea, oral fat intolerance, steatorrhea, and failure to thrive [33,37,42,43]. Nearly all patients exhibit fat malabsorption, acanthocytosis, and eventually severe deficiency of fat-soluble vitamins (A, D, E, and K) [26,38] that can cause multisystemic pathologies described in greater detail below. Hepatic sequelae are relatively common, including elevated serum transaminases, hepatomegaly, hepatic steatosis in about 15–30% of patients, as well as fibrosis and/or cirrhosis in <5% of patients [28,34,43,44]. There are infrequent reports of ileal adenocarcinoma and spinal cord glioblastoma in patients with ABL, although there is insufficient evidence for a clear convincing link of a higher cancer risk with ABL [45,46].

3.1.2. Homozygous Familial Hypobetalipoproteinemia

Familial hypobetalipoproteinemia (FHBL) is an autosomal co-dominant disorder caused by rare variants within APOB on chromosome 2p24.1, consisting largely of nonsense and frameshift variants, that result in early truncations [47]. Individuals carrying a single variant within APOB have heterozygous familial hypobetalipoproteinemia (HetFHBL), while carriers of biallelic variants have homozygous familial hypobetalipoproteinemia (HoFHBL). Loss of function variants within APOB impair the formation and secretion of apo B lipoproteins, specifically VLDL and chylomicrons from hepatocytes and enterocytes, respectively [37], resulting in absent or near absent levels of apo B-containing lipoproteins, in addition to very depressed levels of cholesterol and TG. Notably, truncated proteins larger than apo B-48 do not impede chylomicron secretion [48]. The clinical presentation and sequelae in HoFHBL are essentially identical to ABL [26]. A distinguishing feature is moderate hypocholesterolemia in parents of HoFHBL patients compared to no biochemical abnormalities in patients of ABL patients [35].

3.2. Isolated absence of apo B-48 containing lipoproteins

3.2.1. Chylomicron retention disease

Chylomicron retention disease (CRD), or Anderson’s Disease [49], is a very rare (approximately 55 cases described to date) [50] autosomal recessive disorder caused by biallelic variants within SAR1B on chromosome 5p31.1, which encodes the secretion associated Ras related GTPase 1B (SAR1B) protein [26]. SAR1B manages the formation of COPII vesicles required for proper trafficking of pre-chylomicron cargo within enterocytes from the endoplasmic reticulum to the Golgi; molecular defects within SAR1B impair chylomicron assembly and secretion, retaining pre-chylomicron transport vesicles in the cytoplasm [50] and resulting in the absence of apo B-48 lipoproteins in circulation [51]. Mutated SAR1B also impairs enterocyte production and secretion of HDL and apo A1 [51,52]. SAR1B has several functions, including roles in protection against inflammation, lipid homeostasis [53], and neuronal development [54], which may also affect the clinical presentation [50]. Patients with CRD have 50% reduced levels of total, LDL, and HDL cholesterol, while TG levels are normal [50]. Postprandial elevations in TG are blunted [50,51].

Patients with CRD can have similar but generally less severe clinical presentations to ABL and HoFHBL patients, including failure to thrive, malabsorption, diarrhea, steatorrhea, abdominal distention in infancy and early childhood and eventually fat-soluble vitamin deficiency that can lead to severe multi-systemic pathologies [26,51]. Hepatic steatosis and hepatomegaly [51] may stem from reduced SAR1B-dependent VLDL secretion [55]. Distinguishing features of CRD patients from ABL and HoFHBL patients include absence of acanthocytosis, milder neurological and ophthalmological complications [50,51], normal TG levels, low HDL cholesterol levels [50], and more frequent elevations in creatine kinase [50,55].

3.3. Low but not absent apo B-containing lipoproteins

3.3.1. Heterozygous familial hypobetalipoproteinemia

Heterozygous familial HBL (HetFHBL) occurs in patients carrying one copy of a deleterious variant within APOB [47]. Patients have depressed apo B (<30% of normal) and LDL cholesterol (<30% of normal) compared with noncarriers [56], with LDL cholesterol levels as low as 8 mg/dL (0.2 mmol/L) [57]. Unlike HoFHBL patients, severe malabsorption symptoms and flagrant vitamin deficiencies are largely absent in HetFHBL patients. However, neurological symptoms were described in an elderly 70-year-old HetFHBL patient, which may have resulted from the cumulative effects of life-long relatively low vitamin A and E levels [58].

The main clinical concern in patients with HetFHBL is predisposition to hepatic disease, which fortunately affects only a minority of patients. Impaired VLDL secretion, stemming from reduced apo B production and secretion, translates to reduced hepatic TG transport and fat retention [37]. HetFHBL patients have up to 2–3-fold higher hepatic fat content and approximately 2-fold [59,60] higher prevalence of hepatic steatosis than the general population [58,61–64]. Liver transaminases are frequently increased by 50% or more [37,60,62]. There have been a few reported cases with more severe liver disease, including fibrosis [58], cirrhosis [65], and hepatocellular carcinoma [37,66]. Pelusi et al. has also demonstrated a link between nonalcoholic fatty liver disease and carrier status of APOB pathogenic variants [67]. Despite the increased prevalence of hepatic steatosis, HetFHBL patients do not have a higher risk of insulin resistance, with a similar prevalence of diabetes as the general population [48,68].

As a result of the life-long reductions in apo B-containing lipoproteins, HetFHBL are protected from atherosclerosis, with an estimated 72% lower risk of coronary heart disease [69]. HetFHBL patients exhibit lower arterial stiffness, a marker of vascular function associated with vessel wall protection [60].

3.3.2. PCSK9 deficiency

PCSK9 deficiency is an autosomal semi-dominant disorder that results from loss-of-function variants within PCKS9 on chromosome 1p32.3. Loss of PCSK9 activity, which normally functions to direct the LDL receptor toward lysosomal degradation [26], results in increased LDL uptake. Carriers of PCSK9 loss-of-function variants have low levels of total and LDL cholesterol and TG, with the extent of the lipid reduction dependent on zygosity, although the lipid reductions are less severe than in patients with ABL, HoFHBL, or CRD [26]. The principle that PSCK9 loss-of-function variants offset genetically high levels of LDL cholesterol was observed in a familial hypercholesterolemia patient who had both a pathogenic LDLR variant and a PCSK9 variant, who had a near normal lipid profile and no complications [70].

Patients with PCSK9 deficiency are generally healthy and have a reduced risk of ASCVD [71], dementia [72], fatty liver, and hepatic disease [26,56,73,74]. The PCSK9 p.Q152H variant protects against liver injury caused by endoplasmic reticulum storage disease [75], and carriers of the p.R46L variant have a significantly lower risk of nonalcoholic fatty liver disease (OR = 0.42; 95% 0.22–0.81; p = 0.01), nonalcoholic steatohepatitis (OR = 0.48; 95% CI 0.26–0.87; p = 0.01), and fibrosis (OR = 0.55; 95% CI 0.32–0.94; p = 0.03) [76]. However, despite the hepatoprotective effects apparent with PCSK9 deficiency, it did not appear to compensate for genetic susceptibility to nonalcoholic fatty liver disease in a HetFHBL patient [77].

Both homozygotes and heterozygotes with PCSK9 deficiency are protected from ASCVD as a result of the lifelong reductions in LDL cholesterol levels, confirmed by Mendelian randomization and observational studies [78–80]. The estimated degree of risk reduction for coronary heart disease, ranging from 17% to 88% across numerous studies, is proportional to the extent of reduction in LDL cholesterol levels [71,78,79,81]. Carriers have a significantly lower mean intima-medium thickness, indicating reduced atherosclerosis risk [78]. Carriers of the PCSK9 p.R46L variant have a significantly lower age- and sex-adjusted risk for aortic valve stenosis (OR = 0.77; 95% CI 0.65–0.92) and myocardial infarction (OR = 0.76; 95% CI 0.64–0.89) [82]. PCSK9 deficiency mediated reductions in LDL cholesterol by 10 mg/dL (0.26 mmol/L) significantly reduce risk of abdominal aortic aneurysm (OR = 0.76; 95% CI 0.68–0.86) and ischemic stroke (OR = 0.93; 95% CI 0.88–0.99), each controlled for sex and age [71].

3.3.3. ANGPTL3 deficiency (familial combined hypolipidemia)

ANGPTL3 deficiency, also known as familial combined hypolipidemia, is an autosomal semi-dominant disorder [83] resulting from rare loss of function variants in the ANGPTL3 gene located on chromosome 1q31.3 [26]. In addition to reduced VLDL production, deficiency of ANGPTL3, which normally inhibits lipoprotein lipase and endothelial lipase activity [84], results in enhanced hydrolysis of chylomicrons, VLDL and HDL particles [85]. Carriers of biallelic variants have a near complete absence of circulating ANGPTL3, along with significant depressions in levels of all plasma lipids (i.e. LDL cholesterol, total cholesterol, apo B, TG, HDL cholesterol, and apo A1) [83,86]. Carriers of heterozygous variants have ANGPTL3 levels 42–50% lower than controls and have milder, yet significant, pan-hypolipidemia [86,87].

Patients with ANGPTL3 deficiency generally do not present with any adverse clinical findings [26,83]. They do not experience vitamin deficiencies [26], are not at an elevated risk of hepatic steatosis, and appear to have normal serum transaminase levels [60,86]. Compared to heterozygous carriers and noncarriers, homozygotes demonstrate higher insulin sensitivity [88].

Mounting evidence from Mendelian randomization studies suggest patients with ANGPTL3 deficiency are protected from ASCVD. In separate meta-analyses, Stitziel et al. and Dewey et al. found a 34% (OR = 0.66; 95% CI 0.44–0.98; p = 0.04) [89] and 39% (OR = 0.61; 95% CI 0.45–0.81; p < 0.001) reduced risk of CAD in their respective cohorts of ANGPTL3 loss of function variant carriers [87]. Individuals within the lowest tertile of circulating ANGPTL3 levels had a significantly reduced risk of myocardial infarction compared with the highest tertile (OR = 0.65; 95% CI 0.55–0.77; p = 2.2 × 10⁻⁷), which remained significant after adjustment for plasma LDL cholesterol and TG (OR = 0.71; p = 0.0001) [89]. Carriers of ANGPTL3 loss-of-function variants also demonstrated a reduced risk of myocardial infarction that trended toward significance (OR = 0.66; 95% CI 0.39 to 1.06; p = 0.09) [87]. In a family study, two out of three first-degree relatives had evidence of coronary atherosclerotic plaque while the three individuals with ANGPTL3 deficiency had none, including a middle-age individual with multiple risk factors for ASCVD [89]. It is notable that patients with ANGPTL3 deficiency also have life-long depressions in HDL cholesterol, suggesting that protection from lower LDL cholesterol supersedes possible deleterious effects of concurrently low HDL cholesterol [86].

3.3.4. LDLR gain-of-function variant

A novel large-effect gain-of-function variant within the LDLR gene on chromosome 19p13.2 discovered in an Icelandic family has become the latest identified monogenic cause of low LDL cholesterol. The LDL receptor is responsible for removal of LDL cholesterol in the blood. While LDLR gain-of-function variants have been reported in the past [90–92], to date, only a single one, which is a 2.5 kb deletion spanning the 3’ UTR in 7 heterozygous members segregating in three generations of a single Icelandic family, has shown marked reductions in LDL cholesterol levels [93]. This de novo variant, which caused a 1.79-fold increase in surface expression of the LDL receptor was unique to this family and segregated in an autosomal dominant manner. Carriers also had reductions in PCSK9 levels, which may have further contributed toward the depressed lipid profile. Mean levels of LDL cholesterol were 0.87 mmol/L among six heterozygous family members, which was within the 1st percentile for each carrier’s age and sex, and remained depressed over decades. Total and non-HDL cholesterol and apo B levels were also significantly lower compared to controls, while levels of HDL cholesterol and TG were normal. Noncarrier family members had normal lipid profiles. There were no detrimental effects among the carriers.

3.3.5. Polygenic hypocholesterolemia

While many hypocholesterolemia cases have a monogenic origin (16–52%), a considerable proportion of such patients have polygenic hypocholesterolemia (34–50%) [94–96], which is the result of the cumulative effects of common variants or polymorphisms throughout the genome discovered from genome-wide association studies. Each common variant is individually associated with marginally depressed levels of LDL cholesterol, but cumulatively these small effects add up to produce a potentially large effect. Polygenic susceptibility has also been captured in some cases of monogenic hypocholesterolemia and may explain some of the heterogeneity among cases with a common monogenic basis [94].

Polygenic hypocholesterolemia patients have LDL cholesterol and apo B levels depressed below the 5th percentile, although lipid reductions are attenuated compared with monogenic hypocholesterolemia [95,97]. The prevalence of hepatic disease is the same or lower. In a cohort study by Rimbert et al. of 38 patients with polygenic hypocholesterolemia and 40 patients with monogenic hypocholesterolemia, likelihood of hepatic steatosis was 87% lower in those with polygenic hypocholesterolemia [95]. Non-alcoholic steatohepatitis, measured by elevations in ALT, was reported absent in the 34 patients with polygenic hypocholesterolemia evaluated while it was present in 47% (16/34) of patients with monogenic hypocholesterolemia [95]. In contrast, a recent cohort study by Cefalù et al. found no difference in the prevalence of hepatic steatosis among 63 patients with polygenic hypocholesterolemia compared with 60 patients with monogenic hypocholesterolemia or 47 patients with hypocholesterolemia of unknown genetic origin [97]. These discordant results cannot be explained by metabolic differences between the monogenic hypocholesterolemia cohorts in the two studies as they are both comprised mostly of carriers of loss-of-function APOB variants [95,97].

3.3.6. Congenital disorder of glycosylation (CDG)

Congenital disorders of glycosylation (CDG) are rare, autosomal recessive disorders causing defective glycosylation of proteins and lipids. CDG type I disorders are a newly recognized cause of hypocholesterolemia [98]. Two prevalent CDG type I disorders that result in impaired N-glycosylation activity are asparagine-linked glycosylation protein 6 (ALG6)-deficiency and phosphomannomutase 2 (PMM2)-deficiency, caused by biallelic loss-of-function variants in ALG6 and PMM2, respectively [99]. Patients present with levels of LDL cholesterol, total cholesterol and apo B below the 5th percentile. The proposed mechanism is increased levels of sterol regulatory element-binding protein 2 (SREBP2) elevates the expression and abundance of the LDL receptor, enhancing the uptake of LDL cholesterol. There is conflicting evidence that lower levels and impaired activity of PCSK9, from defective N-glycosylation, also contribute to the depressions in LDL cholesterol [99,100].

Clinical presentations vary among CDG type I patients, with a wide range of neurological, ophthalmological, gastrointestinal, musculoskeletal, and hepatic features. Neurological deficits present in nearly all patients, resulting in diagnosis commonly in infancy and early childhood [100,101]. These complications primarily result from impaired glycosylation rather than severe vitamin deficiencies. Heterozygous carriers have attenuated levels of LDL cholesterol and apo B without any other clinical abnormalities [99].

4. Association with clinical phenotype and disease

We describe the association of the genetic disorders of low LDL cholesterol with ASCVD, diabetes, fatty liver disease, and multisystemic vitamin deficiencies.

4.1. Protection against ASCVD

Depressed levels of LDL cholesterol and apo B appear to be protective against ASCVD, a relationship supported by numerous Mendelian randomization and observational studies. Systematic evaluation of risk has not been completed for patients with ABL, CRD, LDLR gain-of-function, or polygenic hypocholesterolemia; however, the plethora of studies exploring risk in patients with FHBL, ANGPTL3 deficiency, and PCSK9 deficiency provide convincing evidence that genetically driven low levels of LDL cholesterol protect against atherosclerosis. Risk of coronary heart disease is 72% lower in FHBL patients [69], 34–39% lower in patients with ANGPTL3 deficiency [87,89], and ranges from 17% to 88% lower in studies of patients with PCSK9 deficiency [71,78,79,81]. However, it is important to remember this genetically driven reduction in risk is not absolute as some patients with these variants develop ASCVD [68,69].

4.2. Fatty liver disease

Susceptibility to fatty liver disease in low LDL cholesterol states depends upon the molecular defect underlying the hypocholesterolemia. ABL and FHBL patients have defective VLDL secretion that leads to hepatic fat accumulation [60], meaning some patients are predisposed to hepatic steatosis that sometimes develops into nonalcoholic steatohepatitis, fibrosis, and cirrhosis [28]. Carriers of rare variants in APOB, resulting in ABL, are significantly more likely to develop liver disease and have higher aminotransferase levels [102]. Patients with polygenic hypocholesterolemia also appear to have a similar risk of hepatic steatosis as monogenic hypocholesterolemia [97]. There are also few reported cases of hepatic steatosis although not cirrhosis in patients with CRD [50]. Patients with ANGTPL3 deficiency [86], PCSK9 deficiency [73], or LDLR gain-of-function [93] do not appear to be at a higher risk for fatty liver; each of these patients have defects that increase catabolism of apo B lipoproteins rather than impaired VLDL secretion from the liver. In fact, individuals with PCSK9 deficiency may experience hepatoprotective effects that reduce risk of nonalcoholic fatty liver disease [75,76]. Of note, rare variants in other genes, such as CIDEB which encodes a structural protein on lipid droplets, also modulate the risk of fatty liver [103].

4.3. Diabetes

Diabetes has sometimes been described in some patients with monogenic hypocholesterolemia [104], prompting the question of whether these patients are genetically predisposed to diabetes. Despite the high prevalence of fatty liver disease in patients with FHBL, there is not any increased prevalence of insulin resistance [105,106] and diabetes appears to be absent [48]. The prevalence of diabetes in patients with ANGPTL3 deficiency was 4.9%, relatively lower than the reference population at 9.2%, which may result from improved insulin sensitivity [48]. Mendelian randomization studies suggest that patients with PCSK9 deficiency may be at a higher risk of developing type 2 diabetes [107,108]. However, associations of specific variants with diabetes are conflicted; Lotta et al. found that the loss-of-function p. R46L variant confers a 9% increased risk of type 2 diabetes (OR = 1.09; 95% CI 1.01–1.17; p = 0.03) [109], while Bonnefond et al. found no significant associations [110]. Overall, patients with FHBL or ANGPTL3 deficiency do not appear have a higher susceptibility to developing diabetes, while patients with PCSK9 deficiency may have a higher risk that needs to be further evaluated. There is no increased diabetes risk among patients who take PCSK9 inhibitor drugs. For other low cholesterol state phenotypes, i.e. ABL, CRD, polygenic hypocholesterolemia, and LDLR gain-of-function, a systematic evaluation of risk remains to be completed though diabetes does not appear to be a common feature among these patients.

4.4. Rare multisystem vitamin deficiencies

Rare multisystemic fat-soluble vitamin deficiencies affect patients with ABL, HoFHBL, and CRD, who each have impaired absorption and secretion of chylomicrons, the main carrier vehicle of fat-soluble vitamins (A, D, E, and K). These rare multisystemic vitamin deficiencies cause various neurological, ophthalmological, musculoskeletal, and coagulation defects. Deficiency of vitamin A underlies retinopathy and retinitis pigmentosa, which can result in loss of night and peripheral vision [111,112]. Complete blindness may eventually result if the severe vitamin deficiency is left untreated [38,42,43]. Neuropathy, spinocerebellar ataxia, reduced proprioception, deficient deep tendon reflex, and dysarthria stem from vitamin E deficiency [26,38,43,113,114]. Demyelination can affect neurons within both the central and peripheral nervous system [38]. Beginning in the first or second decade of life, these neurological sequalae can lead to disability without treatment. Myopathy, rickets, osteopenia, osteomalacia, and osteoporosis [26,38,43] arise largely from deficiency of vitamin D [114], though vitamin K deficiency may also contribute to bone deterioration [115,116] in addition to coagulopathy with prolonged prothrombin time [38], susceptibility to bruising, and hemorrhage [114].

5. Approach to diagnosis and treatment

For patients with ABL, HoFHBL, and CRD, early diagnosis and treatment are essential for optimal prognosis [112,117]. Diagnosis can be based upon the presenting clinical features, the lipid profile, and genetic testing with next-generation DNA sequencing, when available. Prompt commencement of oral high dose vitamin therapy, consisting of 100–400 IU/kg/day vitamin A, 800–1200 IU/kg/day vitamin D, 100–300 IU/kg/day vitamin E, and 5–35 mg/week vitamin K, is essential to attenuate, halt, and potentially reverse the various severe multisystemic complications [43]. Gastrointestinal manifestations, such as steatorrhea and malabsorption, experienced in early childhood or infancy can be resolved with a low-fat diet (<30% calories from fat) [35,68]. To promote normal growth, care should be taken to ensure that daily caloric intake is adequate and essential fatty acids may also be orally supplemented through 1–2 teaspoons (5–10 ml) of polyunsaturated fatty acid rich oil. Patients should be consistently followed-up on a 6–12-month basis for a complete evaluation. A detailed breakdown of the elements to be clinically evaluated and investigated in the laboratory are outlined in Table 3. Overall, a low-fat diet, high-dose vitamin therapy, essential fatty acid supplementation and consistent follow-up are key elements of the life-long treatment regimen of patients with ABL, HoFHBL, and CRD. Though other complications are largely absent, patients with HetFHBL should also be monitored for nonalcoholic fatty liver disease; diet and exercise may be recommended to improve liver transaminase levels [113].

6. Translation to treatments for high cholesterol

Many of the monogenic hypocholesterolemia disorders have inspired novel therapies to treat elevated cholesterol in order to reduce ASCVD risk [74,118]. For instance, the small molecule inhibitor lomitapide targets MTP, mimicking the genetic defect in patients with ABL, while the antisense oligonucleotide mipomersen targets the messenger RNA of apo B-100, mimicking the genetic defect in FHBL patients with APOB truncations larger than apo B-48. Both are effective at lowering LDL cholesterol and other atherogenic lipids to reduce risk of ASCVD [119,120] on top of existing lipid-lowering interventions in patients with homozygous familial hypercholesterolemia [121–124]. Gastrointestinal side-effects for lomitapide, injection site reactions and flu-like symptoms for mipomersen, and concerns of elevated transaminases, hepatic steatosis and nonalcoholic steatohepatitis for both treatments call for monitoring, potential dosage adjustments and if necessary, discontinuation of treatment [120,125]. While lomitapide is available in many places, marketing of mipomersen was suspended in 2017 due to side effects.

In contrast, for hypercholesterolemic patients with at least one functional LDLR allele [126], the PCSK9 inhibitors, alirocumab, evolocumab [127], and inclisiran [128,129], have proven to be transformational. They have a favorable safety profile with minimal injection-site reactions and are very efficacious at lowering LDL cholesterol. Alirocumab and evolocumab are monoclonal antibodies that target PCSK9 to achieve reductions in risk of cardiovascular events, such as myocardial infarction and stroke [130,131]. Inclisiran is a small interfering RNA that prevents translation of PCSK9 messenger RNA. Each PCSK9 inhibitor displays a similar LDL cholesterol-lowering effect [126], but inclisiran may be advantageous for compliance as injections are required semiannually compared with every 2–4 weeks for the monoclonal antibodies [128]. Finally, inspired by patients with ANGPTL3 deficiency, evinacumab is a monoclonal antibody that inhibits ANGPTL3 and lowers LDL cholesterol with remarkable efficacy in patients with severe and refractory familial hypercholesterolemia [132,133].

7. Conclusions

Hypocholesterolemia, i.e. low levels of LDL cholesterol, is a complex trait that can be caused by many different genetic causes, although most cases likely result from non-genetic lifestyle factors such as diet, underlying illness, or medications. Monogenic hypocholesterolemia varies in severity based upon the underlying biochemical and physiological defect. ABL and HoFHBL patients have the most severe phenotypes with a complete absence of apo B-containing lipoproteins, followed by CRD patients who have an absence of apo B-48 lipoproteins; patients with these disorders can have severe gastrointestinal and hepatic sequalae, in addition to multisystemic manifestations (i.e. neurological, musculoskeletal, hematological, and ophthalmological) caused by severe deficiencies of fat-soluble vitamins (i.e. A, D, E, and K). It is imperative to identify and begin treatment regimens for these patients early.

Patients with PCSK9 deficiency, ANGPTL3 deficiency, polygenic hypocholesterolemia, and LDLR gain-of-function variant appear to have no negative clinical manifestations. Depressed LDL cholesterol levels are predicted to protect against ASCVD, based on evidence from patients with FHBL, ANGPTL3 deficiency and PCSK9 deficiency. This has incentivized the development of various lipid-lowering therapies with these hypocholesterolemia disorders in mind intended for patients with high LDL cholesterol, with inhibitors of PCSK9 and ANGPTL3 showing the greatest potential for wide usage with a viable safety profile.

8. Expert opinion

With increased clinician experience, familiarity, and ongoing research involving patients with hypocholesterolemia, treatment guidelines are being updated with more effective regimens for optimal prognosis [134]. But there remains room for further progress in understanding and adequately addressing low LDL cholesterol in patients. Early definitive diagnosis of the severe monogenic hypocholesterolemia disorders, as next-generation sequencing becomes more accessible and widely used, can pave the way for improved prognosis with early initiation of high-dose vitamin therapy and a low-fat diet. Barriers to effective care may include inadequate access to regular care and treatments due to a lack of financial coverage, as well as lack of access to genetic testing to confirm specific diagnoses.

Development of a PRS that captures more variation in the genome [73] and ancestry-specific PRS can help us better understand and identify polygenic susceptibility to low LDL cholesterol in different populations. Testing and refinement of the component single nucleotide polymorphisms (SNPs) is equally as important since simply increasing the number of SNPs does not necessarily improve the ability to diagnose polygenic hypocholesterolemia [97]. Hypocholesterolemia is often underinvestigated as some clinicians perceive that low cholesterol states are not associated with other diseases and are rather protective. Understanding how genetic status, LDL cholesterol, and apo B levels associate with the etiology of other diseases, such as breast cancer [135] or PCSK9 and diabetes [107–110], where the relationship is not clear can elucidate potential risks or benefits of the monogenic hypocholesterolemia inspired lipid-lowering therapies.

Treatment of the mirror-image condition, namely hypercholesterolemia, might benefit from future insights gained from studying natural human genetic variants. For example, a MTP inhibitor with similar effects as the p.G865V variant in MTTP, which blocks TG transfer but retains phospholipid transfer activity of MTP, may reduce risk of hepatic steatosis and gastrointestinal side-effects; this would be a significant improvement over the existing MTP inhibitor lomitapide [136].

Also, new methods of delivery could help patients with hypercholesterolemia. For instance, development of novel gene editing technologies that target the regulators of synthesis, secretion, and metabolism of apo B lipoproteins could also pave the way for a more permanent solution in patients with elevated LDL cholesterol. In non-human primates, a CRISPR-Cas9 adenine base editor delivered to the liver precisely altered a single base pair to create a loss-of-function variant in PCSK9 resulted in sustained reductions of 90% and 60% over 8 months in levels of PCSK9 and LDL cholesterol, respectively [137]. Vaccines also hold promise for generating long-lasting endogenous antibodies against PCSK9 [138] or ANGPTL3 [139,140] to help achieve sustained reductions in lipid levels.

Finally, ongoing research might uncover potential new causal genes, for instance loci associated with reduced LDL cholesterol and also reduced hepatic fat content. Targeting such a putative gene product would circumvent some of the adverse liver effects seen with earlier generation treatments such as lomitapide and mipomersen. However, all findings of putative novel genes for low cholesterol [141] will require calm circumspection and will need to be replicated and validated before they can be considered on a par with proven known causal genes.

In the next five to ten years, therapeutics for atherosclerosis prevention based on the hypocholesterolemia disorders, namely PCSK9 deficiency and ANGPTL3 deficiency, will reach the forefront of lipid management due to their efficacy and favorable safety profile, just as statins are currently. In addition to the mAb that already exist, which are currently already on the market, it is likely that at least some of the advanced therapeutics that are more immediately feasible, such as vaccines against PCSK9, will be at advanced stages of development or in clinical trials, for the management of high levels of LDL cholesterol.

Article highlights

• Fat malabsorption in abetalipoproteinemia, hypobetalipoproteinemia, and chylomicron retention disease causes severe symptoms from infancy or childhood, such as diarrhea and failure to thrive, and eventually fat-soluble vitamin deficiency and hepatic disease

• Fat-soluble vitamin deficiency results in progressively worsening neurological, hematological, musculoskeletal, and ophthalmological sequalae over a patient’s lifetime

• Early initiation of a low-fat diet and high-dose fat-soluble vitamin therapy is essential in abetalipoproteinemia, hypobetalipoproteinemia, and chylomicron retention disease to preserve long-term neurological and ophthalmological function

• PCSK9 deficiency and ANGPTL3 deficiency are protective against atherosclerotic cardiovascular disease

• PCSK9 inhibitors (alirocumab, evolocumab and inclisiran) and ANGPTL3 inhibitors (evinacumab) are effective therapeutic agents to lower LDL cholesterol levels

Declaration of interest

RA Hegele is supported by the Jacob J. Wolfe Distinguished Medical Research Chair, the Edith Schulich Vinet Research Chair, and the Martha G. Blackburn Chair in Cardiovascular Research. He holds operating grants from the Canadian Institutes of Health Research (Foundation award), the Heart and Stroke Foundation of Ontario (G-21-0031455) and the Academic Medical Association of Southwestern Ontario (INN21–011). RA Hegele reports consulting fees from Acasti, Aegerion, Akcea/Ionis, Amgen, HLS Therapeutics, Novartis, Pfizer, Regeneron, Sanofi, and Ultragenyx.

The authors have no other relevant affiliations or financial involvement with any organization or entity with a financial interest in or financial conflict with the subject matter or materials discussed in the manuscript apart from those disclosed.

Reviewer disclosures

Peer reviewers on this manuscript have no relevant financial or other relationships to disclose.