Low LDL Cholesterol (Hypobetalipoproteinemia)

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Practice Essentials

Abetalipoproteinemia (ABL) and familial hypobetalipoproteinemia (FHBL) are relatively uncommon inherited disorders of lipoprotein metabolism that cause low cholesterol levels.[1]  Although persons whose low-density lipoprotein (LDL) cholesterol levels are moderately low (ie, individuals with FHBL) exhibit an enhanced tendency to develop fatty liver disease (FLD),[2]  persons with a profound reduction of LDL cholesterol may have a decreased risk for heart disease.

ABL is a rare disease associated with a unique plasma lipoprotein profile in which LDL and very low-density lipoprotein (VLDL) are essentially absent. The disorder is characterized by fat malabsorption, spinocerebellar degeneration, acanthocytic red blood cells, and pigmented retinopathy. It is caused by a homozygous autosomal recessive mutation in the gene for microsomal triglyceride transfer protein (MTP). MTP mediates intracellular lipid transport in the intestine and liver and thus ensures the normal function of chylomicrons (CMs) in enterocytes and of VLDL in hepatocytes.[3]

Affected infants may appear normal at birth, but by the first month of life, they develop steatorrhea, abdominal distention, and growth failure. Children develop retinitis pigmentosa and progressive ataxia, with death usually occurring by the third decade. Early diagnosis, high-dose vitamin E (tocopherol) therapy, and medium-chain fatty acid dietary supplementation may slow the progression of the neurologic abnormalities. Obligate heterozygotes (ie, parents of patients with ABL) have no symptoms and no evidence of reduced plasma lipid levels.

FHBL is also a rare disorder of apolipoprotein B (apoB) metabolism characterized by levels of plasma cholesterol and LDL cholesterol that are less than one-half normal in heterozygotes and are very low (< 50 mg/dL) in homozygotes. FHBL is caused by an autosomal, codominant mutation in the gene for apoB (APOB), which is carried on chromosome 2. This mutation results in a truncated form of apoB.[4, 5]  Homozygotes present with fat malabsorption and low plasma cholesterol levels at a young age. They develop progressive neurologic degenerative disease, retinitis pigmentosa, and acanthocytosis, similar to patients with ABL. Although heterozygotes are usually asymptomatic, they exhibit decreased LDL cholesterol and apoB levels and possibly have a decreased risk of atherosclerosis.[6, 7, 8]

The nonfamilial forms of hypobetalipoproteinemia are secondary to a number of clinical states, such as occult malignancy, malnutrition, and chronic liver disease.

Pathophysiology

Cholesterol and triglycerides are transported from sites of synthesis to sites of utilization in the form of lipoproteins. These particles consist of a core of cholesterol esters and triglycerides surrounded by a monolayer of free cholesterol, phospholipids, and proteins (apolipoproteins). The 4 major lipoproteins are very low-density lipoprotein (VLDL), low-density lipoprotein (LDL), high-density lipoprotein (HDL), and chylomicrons (CMs). VLDL and CMs are assembled within the lumen of the endoplasmic reticulum of hepatocytes and enterocytes, respectively, transported to the Golgi complex, and then secreted into the circulation.

Each lipoprotein is characterized by its lipid composition and by the type and number of apolipoproteins it possesses. CMs, VLDL, and LDL carry apolipoproteins on their surface; these apolipoproteins have lipid-soluble segments, the beta apolipoproteins, which remain part of the lipoprotein throughout its metabolism. Other apolipoproteins (A, C, D, E, and their subtypes) are soluble and are exchanged between lipoproteins during metabolism.

Beta apolipoproteins

Beta apolipoproteins are the largest of the apolipoproteins. They are critically important for the formation and secretion of CMs and VLDL; abnormalities that impede this process result in abetalipoproteinemia (ABL) and hypobetalipoproteinemia.

The 2 beta apolipoproteins are B-100 and B-48. ApoB-100 is carried on VLDL and the lipoproteins derived from its metabolism, including VLDL remnants or intermediate-density lipoprotein and LDL. ApoB-100, which is synthesized by the liver, is larger than apoB-48, being made up of 4536 amino acids. Unlike apoB-48, apoB-100 contains the binding site essential for LDL uptake by hepatocyte LDL receptors.[9] ApoB-48 is carried on CMs, is derived from the same gene as apoB-100, and is approximately half its size, consisting of 2152 amino acids.

Abetalipoproteinemia

MTP gene mutation

Formation and exocytosis of CMs at the basolateral membrane of intestinal epithelial cells is necessary for the delivery of lipids to the systemic circulation. One of the proteins required for the assembly and secretion of CMs is MTP. The gene for this protein (MTP) is mutated in patients with ABL.[10, 11]

Several mutations in the MTP gene have been described. In most patients with ABL, the mutation involves a gene encoding the 97-kd subunit of MTP. Consequently, children with ABL develop fat malabsorption and, in particular, suffer the results of vitamin E deficiency (ie, retinopathy, spinocerebellar degeneration).[12] Biochemical test results show low plasma levels of apoB, triglycerides, and cholesterol. Membrane lipid abnormalities also affect the erythrocytes, causing acanthocytosis (burr cells). Long-chain fatty acids are very poorly absorbed, and the intestinal epithelial cells become engorged with lipid droplets. Such children respond to a low-fat diet rich in medium-chain fatty acids, as well as to supplementation with high-dose, fat-soluble vitamins, especially vitamin E.[13]

Role of vitamin E

Most of the clinical symptoms of ABL are the result of defects in the absorption and transport of vitamin E. Normally, vitamin E is transported from the intestine to the liver, where it is repackaged and incorporated into the assembling VLDL particle by the tocopherol-binding protein. In the circulation, VLDL is converted to LDL, and vitamin E is transported by LDL to peripheral tissues and delivered to cells via the LDL receptor. Patients with ABL are markedly deficient in vitamin E because of the deficient plasma transport of vitamin E, which requires hepatic secretion of apoB-containing lipoproteins. Most of the major clinical symptoms, especially those of the nervous system and retina, are primarily due to vitamin E deficiency. This hypothesis is supported by the fact that other disorders involving vitamin E deficiency are characterized by similar symptoms and pathologic changes.[11]

Familial hypobetalipoproteinemia

APOB gene mutation

FHBL is a rare autosomal dominant disorder of apoB metabolism. Most cases of known origin result from mutations in the APOB gene, involving 1 or both alleles. More than 30 mutations have been described. Most often, a mutation involving a 4–base–pair deletion in the APOB gene prevents translation of a full-length apoB-100 molecule, leading to the formation of truncated apoB molecules (apoB-37, with 1728 amino acids; apoB-46, with 2057 amino acids; or apoB-31, with 1425 amino acids).[4, 5, 14, 15, 16]

Metabolic turnover studies indicate that in some persons, these APOB gene mutations result in impaired synthesis of apoB-containing lipoproteins, and that in other patients, they cause increased catabolism of these proteins. Overall, beta-lipoprotein levels remain low.

Heterozygotes may have LDL cholesterol levels less than or equal to 50 mg/dL, but they often remain asymptomatic and have normal life spans. In the homozygous state, the absence of apoB leads to significant impairment of intestinal CM formation, which in turn leads to impaired absorption of fats and fat-soluble vitamins. Cholesterol absorption may also be impaired. Subsequent vitamin E malabsorption results in low tissue stores of vitamin E and leads to the development of degenerative neurologic disease.[5]

Secondary causes

The secondary causes of hypobetalipoproteinemia include occult malignancy, as well as conditions such as malnutrition, liver disease, and chronic alcoholism. These conditions must be excluded before the diagnosis of FHBL can be made.

Epidemiology

Frequency

United States

Abetalipoproteinemia (ABL) and familial hypobetalipoproteinemia (FHBL) are rare inborn errors of lipoprotein metabolism. ABL occurs in less than 1 in 1 million persons. FHBL occurs in approximately 1 in 500 heterozygotes and in about 1 in 1 million homozygotes. Approximately one third of ABL and FHBL cases result from consanguineous marriages.

International

Frequency is similar to that reported in the United States.

Mortality/Morbidity

Abetalipoproteinemia (ABL)

Infants exhibit failure to thrive, with fat malabsorption and abdominal distention occurring during the first month of life. Spinocerebellar degeneration and pigmented retinopathy develop during childhood. Death usually occurs by the third decade. Obligate heterozygotes are asymptomatic and have normal plasma lipid levels; their risk of developing cardiovascular disease is probably lower than average.

The most prominent and debilitating clinical manifestations of ABL in adults are neurologic in nature and usually manifest for the first time in the second decade of life. Severe ataxia and spasticity develop by the third or fourth decade. Progressive central nervous system involvement is the eventual cause of death in most patients and often occurs by the fifth decade. Moreover, ophthalmic symptoms begin with decreased night and color vision, with progression to virtual blindness by the fourth decade.

Familial hypobetalipoproteinemia (FHBL)

Homozygotes are identified at a young age because of fat malabsorption and through the detection of decreased plasma cholesterol levels. A deficiency of fat-soluble vitamins may lead to retinitis pigmentosa, acanthocytosis (or burr cells due to altered red blood cell membrane lipids), and progressive, degenerative neurologic disease. Heterozygotes are asymptomatic and are often diagnosed when routine lipid screening discloses abnormally low plasma cholesterol levels. Fat malabsorption is rarely noted. Neurologic examination may reveal diminished or absent deep tendon reflexes and, less frequently, deficits in proprioception and ataxia. The syndrome is associated with normal longevity. Compound heterozygotes (ie, patients with mutations of the APOB gene at 2 different sites) have a clinical presentation similar to that of homozygotes.

Race

No race predilection for abetalipoproteinemia or familial hypobetalipoproteinemia has been described. Cases have been reported from every continent.

Sex

No sex predilection for abetalipoproteinemia or familial hypobetalipoproteinemia has been noted. Both disorders are caused by a mutation on an autosomal chromosome.

Age

The homozygous disorders are identified during infancy or childhood.

History

The phenotypic expression of homozygous abetalipoproteinemia (ABL) is essentially the same as that for homozygous familial hypobetalipoproteinemia (FHBL). Chylomicrons (CMs), very low-density lipoprotein (VLDL), and low-density lipoprotein (LDL) are essentially absent. Severe fat malabsorption and all of the sequelae of that condition are present during infancy and beyond. ABL, if left untreated, can result in early mortality.

A study by Sankatsing and colleagues of patients with FHBL evaluated (a) the arterial wall stiffness and carotid intima-media thickness (IMT), measured by B-mode ultrasonography, as noninvasive, surrogate markers for cardiovascular disease (CVD), and (b) the presence and severity of hepatic steatosis, as assessed by abdominal ultrasonography.[17] The hepatic transaminase levels were found to be only modestly elevated, although the prevalence (54% vs 29%; P = 0.01) and severity of steatosis were significantly higher in individuals with FHBL than they were in controls. Furthermore, despite similar IMT measurements, arterial stiffness was significantly lower in patients with FHBL (P = 0.04) than it was in controls, suggesting cardiovascular protection.

Heterozygotes with the mutation that leads to either ABL or FHBL are generally asymptomatic. However, because FHBL is a codominant condition (unlike ABL, which is a recessive disorder), carriers have half the normal levels of beta lipoproteins. Cholesterol levels range from 40-180 mg/dL. Some carriers may present with signs and symptoms of neurologic involvement.

Physical

The physical examination usually reveals fat malabsorption stigmata, spinocerebellar tract involvement, and ocular involvement. Some of the signs encountered due to fat malabsorption may include the following:

Causes

Abetalipoproteinemia (ABL) and familial hypobetalipoproteinemia (FHBL) are caused by genetic defects that encode for MTP or apoB molecules, respectively.

Laboratory Studies

Laboratory studies include the following:

Imaging Studies

Imaging studies include the following:

Other Tests

The molecular diagnosis of familial hypobetalipoproteinemia can be performed only in specialized laboratories; it is accomplished through the examination of the plasma apoB, using gel electrophoresis or deoxyribonucleic acid (DNA) analysis to identify specific mutations.

The demonstration of the molecular defect in persons with abetalipoproteinemia requires a specialized laboratory for the detection of low or absent MTP in intestinal biopsy specimens or DNA analysis to identify specific mutations.

Procedures

Intestinal biopsy may be needed, along with electron microscopy.

Liver biopsy is rarely needed but may become necessary to assess for fatty liver, chronic liver disease, or cirrhosis and to rule out other causes of hepatomegaly, fatty liver, and transaminase elevation.

Histologic Findings

Intestinal biopsy reveals the gross appearance of white mucosa, usually limited to the villi. Histologically, the villi are normal but are lined with fat-containing enterocytes (engorged with triglycerides). In specialized cases, light and transmission electron microscopy may show fat-loaded enterocytes.

Medical Care

Abetalipoproteinemia (ABL) and familial hypobetalipoproteinemia (FHBL) are rare genetic disorders. Infants and children who present with homozygous FHBL or ABL require early treatment with very high doses of vitamin E. Management in adults includes treatment of the complications of the disorders.

To prevent the neurologic manifestations that occasionally occur with FHBL, heterozygous patients receive modest supplementation with vitamin E.

Consultations

Patients who present with advanced complications of abetalipoproteinemia (ABL) or familial hypobetalipoproteinemia (FHBL), as well as the patients' first-degree relatives, require a comprehensive evaluation for the diagnosis and management of these conditions and for genetic counseling.[5] Expertise from the following consultants may be needed:

Diet

A low-fat diet, especially a reduction in the intake of long-chain fatty acids to less than 15 g per day, may alleviate intestinal symptoms.[21]

Oral supplementation of fat-soluble vitamins (ie, A, D, E, K) is needed.

Supplementation of vitamin E (alpha tocopherol in high doses) may prevent progression and may even reverse some of the stigmata of spinocerebellar degeneration. However, no randomized or case-controlled study is available to support this intervention.

Activity

No particular restriction in activity is recommended. Patients should be as active as dictated by their general health.

In patients with spinocerebellar degeneration or ataxia, only well-tolerated and supervised activity should be advised. Such patients may benefit from orthotic devices.

Medication Summary

Abetalipoproteinemia (ABL) and familial hypobetalipoproteinemia (FHBL) have no specific medical therapy other than vitamin supplementation, particularly high doses of vitamin E. Symptomatic medications for diarrhea and treatment of the cause of malabsorption may be needed. Dietary treatment related to ABL and FHBL is quite rigorous.

Vitamin E (Vita-Plus E Softgels, Vitec, Aquasol E)

Clinical Context:  Vitamin E protects polyunsaturated fatty acids in membranes from attack by free radicals and protects red blood cells against hemolysis.

Vitamin A (Del-Vi-A, Palmitate-A 5000)

Clinical Context:  Cofactor in many biochemical processes.

Class Summary

High-dose vitamin E therapy is used to raise tissue levels of tocopherol and to prevent the development of neurologic sequelae.

Further Outpatient Care

The following are elements of outpatient care:

Further Inpatient Care

Infants who present with failure to thrive may require additional monitoring and therapy in the hospital. This therapy may include the following:

Patients with spinocerebellar degeneration and severe gait disturbances may need supportive measures. They may also need orthotic appliances.

Visual assessment and therapy are indicated for retinal degeneration.

Inpatient & Outpatient Medications

See the medications below:

Transfer

Transfer is rarely required for patients who are finally identified as having abetalipoproteinemia (ABL) or familial hypobetalipoproteinemia (FHBL).

Patients with advanced spinocerebellar degeneration who are unable to walk may occasionally require transfer to a tertiary care facility. Any safe method of transfer is adequate for these patients.

Deterrence/Prevention

Abetalipoproteinemia (ABL) and familial hypobetalipoproteinemia (FHBL) are inherited disorders caused by genetic mutations.

Obligate heterozygotes (ie, parents or offspring of homozygote patients) and possible heterozygotes (ie, siblings) should be informed that if their spouse has a very low plasma cholesterol level, the possibility exists that their children could have homozygous or compound heterozygous hypobetalipoproteinemia. Such persons should be referred to a genetic counselor at a lipid clinic.[5]

Complications

Complications include the following:

Prognosis

Prognosis is reasonably good for most patients who are diagnosed early.

Patient Education

Educating patients about the implications of their disease is of paramount importance. They should be counseled about the possible long-term complications, including blindness and gait disturbances. The need for periodic monitoring should be emphasized.

Which syndromes cause low LDL cholesterol?What is abetalipoproteinemia (ABL)?What is familial hypobetalipoproteinemia (FHBL)?What is the pathophysiology of low LDL cholesterol syndromes?What is the role of beta apolipoproteins in the pathophysiology of abetalipoproteinemia (ABL)?What is the role of genetics in the pathophysiology of abetalipoproteinemia (ABL)?What is the role of vitamin E in the pathophysiology of abetalipoproteinemia (ABL)?What is the role of genetics in the pathophysiology of familial hypobetalipoproteinemia (FHBL)?What causes secondary hypobetalipoproteinemia?What is the prevalence of low LDL cholesterol syndromes in the US?What is the global prevalence of low LDL cholesterol syndromes?What is the morbidity and mortality associated with abetalipoproteinemia (ABL)?What is the morbidity and mortality associated with familial hypobetalipoproteinemia (FHBL)?What are the racial predilections of low LDL cholesterol syndromes?What are the sexual predilections of low LDL cholesterol syndromes?At what age are low LDL cholesterol syndromes typically diagnosed?What are the signs and symptoms of low LDL cholesterol syndromes?Which physical findings are characteristic of low LDL cholesterol syndromes?What causes low LDL cholesterol syndromes?What are the differential diagnoses for Low LDL Cholesterol (Hypobetalipoproteinemia)?What is the role of lab testing in the workup of low LDL cholesterol syndromes?What is the role of imaging studies in the workup of low LDL cholesterol syndromes?How is a diagnosis of a low LDL cholesterol syndrome confirmed?What is the role of biopsy in the workup of low LDL cholesterol syndromes?Which histologic findings are characteristic of low LDL cholesterol syndromes?How are low LDL cholesterol syndromes treated?Which specialist consultations are beneficial to patients with low LDL cholesterol syndromes?Which dietary modifications are used in the treatment of low LDL cholesterol syndromes?Which activity modifications are used in the treatment of low LDL cholesterol syndromes?What is the role of medications in the treatment of low LDL cholesterol syndromes?Which medications in the drug class Vitamins are used in the treatment of Low LDL Cholesterol (Hypobetalipoproteinemia)?What is included in the long-term outpatient care of low LDL cholesterol syndromes?When is inpatient care indicated for the treatment of low LDL cholesterol syndromes?Which medications are used in the treatment of low LDL cholesterol syndromes?When is patient transfer indicated for the treatment of low LDL cholesterol syndromes?How low LDL cholesterol syndromes prevented?What are the possible complications of low LDL cholesterol syndromes?What is the prognosis of low LDL cholesterol syndromes?What is included in patient education about low LDL cholesterol syndromes?

Author

Vibhuti N Singh, MD, MPH, FACC, FSCAI, Clinical Assistant Professor, Division of Cardiology, University of South Florida College of Medicine; Director, Cardiology Division and Cardiac Catheterization Lab, Chair, Department of Medicine, Bayfront Medical Center, Bayfront Cardiovascular Associates; President, Suncoast Cardiovascular Research

Disclosure: Nothing to disclose.

Specialty Editors

Francisco Talavera, PharmD, PhD, Adjunct Assistant Professor, University of Nebraska Medical Center College of Pharmacy; Editor-in-Chief, Medscape Drug Reference

Disclosure: Received salary from Medscape for employment. for: Medscape.

Yoram Shenker, MD, Chief of Endocrinology Section, Veterans Affairs Medical Center of Madison; Interim Chief, Associate Professor, Department of Internal Medicine, Section of Endocrinology, Diabetes and Metabolism, University of Wisconsin at Madison

Disclosure: Nothing to disclose.

Chief Editor

George T Griffing, MD, Professor Emeritus of Medicine, St Louis University School of Medicine

Disclosure: Nothing to disclose.

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