Vol. 14, Spring 1997
Molecular Genetic Testing in Mainstream Medicine
Apo E Genotype Testing for Broad Beta Disease
(Type III Hyperlipoproteinemia)
- Introduction
Broad beta disease (type III hyperlipoproteinemia) is a familial dyslipidemia characterized by the combination of elevated serum cholesterol and triglycerides and the presence of the apolipoprotein E (apo E) genotype E2/E2. This lipid disorder is associated with high risk of coronary heart disease and peripheral vascular disease. The E2 variant of apolipoprotein E is defective in binding to receptors that normally clear harmful lipid particles called b-VLDL from the circulation. One percent of the general population has the E2/E2 genotype, and development of the frank lipid disorder occurs in 1-5% of these predisposed individuals, triggered by secondary genetic, hormonal or environmental factors.
Broad beta disease should be considered in all patients with elevated cholesterol and triglycerides, and demonstration of the apoE genotype E2/E2 is diagnostic of broad beta disease in individuals who have elevated cholesterol and triglycerides and accumulation of the abnormal lipoprotein, b-VLDL. Distinguishing this disorder from other causes of elevated cholesterol is important because effective treatment of broad beta disease to prevent atherosclerosis often requires a different approach than treatment of other dyslipidemias.
- Indications for apo E genotype testing
Patients may present with:
- No symptoms
- Peripheral vascular disease
- Coronary artery disease (coronary heart disease)
- Cutaneous xanthomas
- Diabetes mellitus, obesity or hypothyroidism (conditions which can trigger broad beta disease in individuals with the apo E genotype E2/E2)
- Relatives of individuals diagnosed with broad beta disease, especially siblings
- Dyslipidemia
Dyslipidemia, of which broad beta disease is one of numerous varieties, is an excessive accumulation of cholesterol or triglyceride in the blood, or a decrease in HDL. Dyslipidemia is a major cause of coronary atery disease (CAD) and other vascular disease involving atherosclerosis. Genetic disorders of lipid metabolism contribute to a significant proportion of premature CAD cases.
Dyslipidemia is extremely common in this country. Twenty percent of adults have high cholesterol (>240 mg/dl) and an additional 30% of adults have borderline high cholesterol (200- 239 mg/dl). Overall, about 37% of adults (~65 million) are candidates for dietary therapy or lipid-lowering drugs. The National Cholesterol Education Program has established guidelines for routine lipid screening, risk assessment and treatment of dyslipidemia.
- Broad beta disease
Elevated triglycerides are present in combination with elevated cholesterol in an estimated 10% of people with high or borderline high cholesterol. This combination is suggestive of broad beta disease, and because this dyslipidemia is easily diagnosed and effectively treatable, pursuit of a diagnosis is recommended.
Broad beta disease (also referred to as type III hyperlipoproteinemia, familial dysbetalipoproteinemia, or remnant removal disease) is characterized by abnormal build-up in the blood of remnant chylomicrons and VLDL particles containing cholesterol and triglycerides. Accumulation of these remnants (collectively called b-VLDL) can be seen as a "broad beta" band when lipoproteins are analyzed by electrophoresis. The excess b-VLDL is taken up in blood vessel walls and can lead to atherosclerosis.
Clinical aspects
Broad beta disease generally has an adult age of onset, varying from late teens to old age. In women, it is usually seen after menopause. Risk for cardiovascular disease is high. The main clinical manifestation, seen in a third of patients, is premature or accelerated atherosclerosis leading to coronary artery disease or peripheral vascular disease involving the lower extremities. Symptoms include angina, heart attack, claudication, and leg pain. Peripheral vascular disease is more common in this disorder than in other hypercholesterolemias.
Before vascular disease develops there are usually no symptoms, and most patients with broad beta disease are ascertained only from elevated cholesterol and triglycerides discovered during a routine screen. In untreated patients, xanthomas (fat deposits) are occassionally seen (flat ones in palmar creases or tuberous ones on joints).
Incidence
Broad beta disease has an incidence of 1/2,000 - 1/10,000 but the genetic predisposition (E2/E2 genotype) is present in 1% of the general population. Therefore only 1-5% of people with the genetic predisposition eventually develop the disorder. A number of secondary factors are known to provoke broad beta disease in people with the E2/E2 genotype. These include glucose intolerance, diabetes mellitus, hypothyroidism, obesity, low estrogen levels, and excessive intake of alcohol.
Diagnosis
Diagnosis of broad beta disease involves a series of laboratory tests culminating in the apo E genotype test (see below for details). Apo E is a component of several lipoproteins and plays an important role in lipid metabolism. There are three main apo E isoforms reflecting three alleles (E2, E3 and E4), of which E3 is the most common. The E2 variant has an Arg to Cys change at amino acid position158 which severely reduces binding to receptors that clear remnants from the circulation. Ninety- five percent of patients with broad beta disease are homozygous for E2. The remainder have rare mutations in one copy of the apo E gene some of which are not detectable by the apo E genotype test. Apo E phenotyping by isoelectric focusing is an alternative approach to the apo E genotype test, but is considered less accurate and has a longer turnaround time.
The apo E genotype E2/E2 (diagnosis of broad beta disease) is expected in:
- 5% of patients with total cholesterol >240 mg/dl and triglyceride level >150 mg/dl.
- 30% of patients with a VLDL cholesterol to triglyceride ratio between 0.3 and 0.4.
- 90% of patients with a VLDL cholesterol to triglyceride ratio above 0.4.
- 50% of patients with detectable b-VLDL.
- Recommended sequence of tests for workup of patients with elevated cholesterol and triglycerides
- Lipoprotein analysis (lipid panel) after a 12 hour fast. Determines levels of total cholesterol, triglycerides, HDL cholesterol and estimated LDL cholesterol.
- VLDL cholesterol quantitation is useful to calculate the VLDL cholesterol /TG ratio. A ratio of >0.3 is highly suggestive of broad beta disease. This test also includes: Lipoprotein electrophoresis to identify b-VLDL (broad beta band).
- Apo E genotype DNA test to identify the apo E2/E2 genotype. This genotype is diagnostic for broad beta disease when associated with combined elevation of cholesterol and TG. The test is typically performed by polymerase chain reaction followed by restriction enzyme cleavage and gel electrophoresis. Accuracy of E2/E2 detection is >99%.
- Why is identification of broad beta disease important?
a) Treatment: Early diagnosis of broad beta disease is important because certain therapeutic interventions tailored to this disorder have an excellent response and can prevent atherosclerosis. The most effective drugs for broad beta disease are fibrates or nicotinic acid which decrease production of VLDL and can lower both cholesterol and triglyceride levels substantially. Treatment of broad beta disease also includes elimination of secondary factors that exacerbate the hyperlipidemia.
b) Identification of relatives at risk: Family members of affected individuals should be tested for the Apo E2/E2 genotype and screened for hyperlipidemia. This will identify any members with broad beta disease who should be treated and any unaffected members with the genetic predisposition. Because inheritance of the predisposition is recessive, each sibling has at least a 25% chance of sharing the E2/E2 genotype. Family members with this genotype should be counseled to avoid secondary risk factors that can trigger broad beta disease.
A 56-year-old woman was referred to the Lipid Clinic for evaluation of persistent combined dyslipidemia. Cholesterol and triglyceride elevations each above 300 mg/dl had been discovered five years prior during a routine lipid screen. Since that time, a number of drug treatments were attempted (Probucol, Lovastatin, Simvastatin), some of which lowered her cholesterol and to a lesser extent her triglycerides. However, she suffered significant gastrointestinal distress from all of the drugs and was unable to maintain treatment for longer than a few months at a time.
At the time of referral, she had no chest pain or other symptoms suggestive of cardiac ischemia and no symptoms of peripheral vascular disease. Physical exam showed moderate obesity (weight 190 lbs). No xanthomas were present. Family history was negative for both coronary artery and peripheral vascular disease. Family history was significant for breast cancer in the patient's mother and maternal aunt.
Lipoprotein analysis utilizing ultracentrifugation was performed to further characterize her dyslipidemia. Results showed (in mg/dl) a total cholesterol of 415, triglycerides of 461; HDL cholesterol of 37; LDL cholesterol of 84, and VLDL cholesterol of 313. The VLDL cholesterol to TG ratio was 0.68. Lipoprotein electrophoresis detected a b-VLDL band. Both the VLDL cholesterol/TG ratio and detection of b-VLDL are highly suggestive of broad beta disease. Subsequent apo E genotype testing by DNA analysis revealed the E2/E2 genotype, confirming the diagnosis of broad beta disease. Tests to rule out secondary causes including hypothyroidism and glucose intolerance were normal.
Treatment with clofibrate was initiated and a low fat diet and exercise were advised. The drug was tolerated well and after six weeks both cholesterol and triglyceride levels had decreased to <250 mg/dl. Estrogen therapy was contraindicated due to family history of breast cancer.
This case illustrates:
a) Obesity associated with manifestation of broad beta disease,
b) Lack of family history of cardiovascular disease is not uncommon for cases of broad beta disease because the predisposition is recessive,
c) Diagnosis of broad beta disease can point to the most effective treatment.
Contributed by Annette K. Taylor, PhD (CO)
The Genetic Drift Newsletter is not copyrighted. Readers are free to duplicate all or parts of its contents. The Genetic Drift Newsletter is published semiannually by the Mountain States Genetics Network for associates & those interested in Human Genetics. In accordance with accepted publication standards, we request acknowledgement in print of any article reproduced in another publication. The views expressed in the newsletter do not necessarily reflect local, state, or federal policy. For additional information, contact Carol Clericuzio, M.D., Editor, Department of Pediatrics, The University of New Mexico, Albuquerque, NM, 87131
Table of Contents
Molecular Genetic Testing in Mainstream Medicine
Introduction
Venous Thrombosis and the Factor V (Leiden) Mutation
DNA Testing for Hereditary Hemochromatosis
APO E Genotype Testing for Broad Beta Disease (Type III Hyperlipoproteinemia)
Fetal Rh Testing for Maternal-Fetal Incompatibility
Type 1 (insulin-dependent) Diabetes Mellitus
Adult Onset Neurodegenerative Disorders: CAG Triplet Repeat Expansion Mutations
Genetic Testing for Prader-Willi and Angelman Syndromes
Clinical and Applied Molecular Genetics Laboratories - MoSt GeNe Region
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