Apolipoprotein A deficiency
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Synonyms and keywords: Familial hypoalphalipoproteinemia, FHA, familial HDL deficiency, FHD, high density lipoprotein deficiency, HDLD

# Overview
Apolipoprotien A1 deficiency is a rare monogenic metabolic disorder resulting in undetectable Apo A1 levels and HDL C less than 20mg/dl. APOA1 gene encodes for the Apo A1 protein which is the major component of HDL C. It is synthesized in the liver and released into the circulation as very small discoid pre beta HDL, which picks up free cholesterol from the cells and macrophages. Apo A1 also activates LCAT which esterifies free cholesterol on the surface of alpha 4 HDL resulting in the formation of cholesterol esters. These two initial steps in the reverse cholesterol are dependent on a functional Apo A1 which is defective in Apo A1 deficiency. Apo A1 synthesis is affected leading to very low HDL levels. Worldwide, 82 cases and a variety of mutations are reported. The biochemical phenotype is always a low Apo A1 and low HDL C. Clinical phenotype varies with each mutation and is inconsistent. Symptomatic patients usually present with corneal opacities, xanthelasma and premature heart disease. Cardiovascular risk assessment and optimizing risk factors has an important role in the management.

# Historical Perspective
- In 1981, Vergani and Bettale described a familial syndrome with hypoalphalipoproteinemia.
The proband and his relatives had low levels of HDL C, Apo A1 with normal lipase and LCAT activity.
They reported a high prevalence of premature cardiac events without the presence of other established coronary risk factors and a shortened life expectancy on longevity analysis.
Based on the biochemical data and the pedigree they have described to have an autosomal dominant inheritance.
- The proband and his relatives had low levels of HDL C, Apo A1 with normal lipase and LCAT activity.
- They reported a high prevalence of premature cardiac events without the presence of other established coronary risk factors and a shortened life expectancy on longevity analysis.
- Based on the biochemical data and the pedigree they have described to have an autosomal dominant inheritance.
- In 1982, Breslow identified the gene sequence of human Apo A1.
- In 1982, Karathanasis isolated and described the characteristics of the human Apo A1 gene.
- In 1982, Daniel described cerebrovascular abnormalities and clinical status of eight children with history of familial lipoprotein disorders and evidence of thromboembolic cerebrovascular disease. Six of the eight children had low levels of plasma HDL C.
They have speculated that the vascular events are due to lipoprotein mediated endothelial damage and thrombus formation.
- They have speculated that the vascular events are due to lipoprotein mediated endothelial damage and thrombus formation.
- In 1983, Brewer reported that Apo A1 mRNA codes for a precursor apolipoprotein-preproapoA1 by nucleic acid sequence analysis.
- In 1986, Borecki described the possibility of genetic heterogeneity and provided clear evidence of a major gene involved in hypolipoproteinemia after studying 64 individuals in 14 nuclear families.
- In 1986, Jose described a polymorphic site on the 3' end of the Apo A1 gene and reported that the patients with this finding had lower HDL C levels. They have also suggested the polymorphism as a useful marker for the risk of premature coronary artery disease and familial hypoalphalipoproteinemia.
- In 1988, LiWH speculated that the gene coding for Apo A1 is a member of apolipoprotien multigene superfamily, which include genes encoding for Apo A1, Apo-A II, Apo C and Apo E.
- In 1998, Gillotte described the mechanism apolipoprotein mediated cellular lipid efflux.
- In 2006, crystal structure of Apo A1 and the description of the electrostatic features of Apo A1 which are crucial in understanding the interactions of Apo A1 with ABCA1 and SR-B1 were described by Ajees.

# Classification
- Apolipoprotein A1 deficiency can be classified based on the type of mutation and the genes affected as follows:
Familial apolipoprotein A1/ Apo C III/Apo IV deficiency
ApoA1 and Apo C III deficiency
Isolated Apo A1 deficiency
Apo A1 variants
- Familial apolipoprotein A1/ Apo C III/Apo IV deficiency
- ApoA1 and Apo C III deficiency
- Isolated Apo A1 deficiency
- Apo A1 variants

## Familial apolipoprotein A-I/C-III/A-IV deficiency
- In 1982, Schaefer and colleagues reported a 45 year old female proband with HDL deficiency, undetectable plasma Apo A1, low triglyceride, normal LDL C, corneal arcus, planar xanthomas and premature CVD. The patient had severe CVD with no known CVD risk factors and died during coronary artery bypass surgery at age 43 years.
- Her plasma LCAT activity was normal.
- The defect was identified as a homozygous deletion of the entire APOA1 /C III/A IV gene complex.
- Heterozygotes in the kindred had 50% of normal plasma HDL C, Apo A1, Apo AIV, and Apo CIII levels.

## ApoA1/ApoC-III Deficiency
- In 1982, Norum and colleagues described two sisters with HDL deficiency, undetectable plasma Apo A1, Apo C III, planar xanthomas, and premature CVD requiring coronary bypass surgery at ages 29 and 30 years.
- These patients had low triglyceride, normal LDLC and enhanced clearance of VLDL associated Apo B.
- The defect was identified as a homozygous DNA re-arrangement affecting the ApoA1 and Apo CIII genes.

## Apo A1 Deficiency
- In 1991, Matsunaga and colleagues described a 56-year-old Japanese woman with premature CVD, planar xanthomas, normal triglyceride, LDL C, marked HDL C deficiency and undetectable plasma Apo A1 levels. The defect was identified as a homozygous Apo A1 codon 84 nonsense mutation, resulting in a lack of normal Apo A1 production.
- In 1994, Ng and colleagues reported a Canadian kindred with an isolated mutation in the apolipoprotein A1 gene. The proband was a 34-year presented with bilateral retinopathy, bilateral cataracts, spinocerebellar ataxia, and tendon xanthomas.
HDL C was very low and Apo A1 was undetectable. Genomic DNA sequencing of the proband's Apo A1 gene had a nonsense mutation at codon , which was designated as QX.
Genotyping of the kindred showed four homozygotes, four heterozygotes and two unaffected subjects.
Heterozygotes had 50% of normal HDL C and Apo A1.
- HDL C was very low and Apo A1 was undetectable. Genomic DNA sequencing of the proband's Apo A1 gene had a nonsense mutation at codon , which was designated as QX.
- Genotyping of the kindred showed four homozygotes, four heterozygotes and two unaffected subjects.
- Heterozygotes had 50% of normal HDL C and Apo A1.
- In 2008, Santos reported a kindred with the similar mutation that was identified in the Canadian kindred in two homozygous brothers presenting with tubo-eruptive, planar xanthomas, corneal arcus, mild corneal opacification, HDL C <5 mg/dL, normal LDLC and triglyceride levels. They had no detectable Apo A1 containing HDL. Multiple heterozygotes in this kindred had HDL C 50% of normal levels.
- In 2009, Wada and colleagues reported a Apo A1 mutation (ApoA-I Tomioka) in  a 64 year old with corneal opacities and prior history of myocardial infarction. He had marked plasma HDL C (4 mg/dl) and Apo A1 (5mg/dl) deficiency. Genomic sequencing revealed a homozygous deletion of successive adenine residues in codon 138 in Apo A1 gene, resulting in a frameshift mutation.
- In 2010, Al-Sarraf and colleagues reported an Iraqi kindred with two probands in 2010 with complete Apo A1 deficiency, marked HDL C deficiency, normal LDL C and triglyceride levels  caused by a homozygous nonsense mutation with a stop codon at Arg10. One proband was a 35 year old woman with xanthelasma and xanthomas with no CVD, while her 37 year old brother had planar xanthomas and sustained a myocardial infarction at age 35 years.

## Apo A1 Variants
- Apo A1 variants are heterozygous premature terminations, frameshift mutation or amino acid substitutions in the 243 amino acid Apo A1 sequence.
- These patients may have HDL C levels that are low or normal, plasma LCAT activity that is normal or reduced, may develop premature CVD or amyloidosis.
- Six heterozygous Apo A1 missense mutations with low HDL C and decreased LCAT activity are reported. They are not at increased risk of developing premature CVD.
- Few mutations resulting in low HDL C with normal LCAT function have an increased risk of coronary artery disease at a young age.
- Few mutations in Apo A1 are associated with familial visceral amyloidosis.
- Below is a list of few selected Apo A1 variants which support the inconsistency in the biochemical and clinical phenotype:
In 1980, Franceschini reported significant hypertriglyceridemia and marked decrease of HDL C (7-14 mg/dl) with no signs of coronary atherosclerosis in the father, son, and daughter of an Italian family. They had normal lipoprotein lipase, LCAT activity and a reduced Apo A1 on 2D gel electrophoresis. He suggested the finding was probably due to a change in the amino acid composition and it was designated as Apo A1 Milano.
In 1991, Funke and colleagues reported a 42-year-old German patient with corneal opacification, marked HDL deficiency, Apo A1]]deficiency, decreased plasma LCAT activity, increased non-HDL C and triglyceride, and lack of CVD. Sequencing of LCAT gene was normal, but the patient was found to be homozygous for an Apo A1 frameshift mutation resulting in a truncated 229 amino acid protein instead of full length Apo A1.
In 1995, Takata and colleagues reported a 39-year-old Japanese man with corneal opacification, HDLC of 6 mg/dL, Apo A1 level of <3.0 mg/dL, increased LDL C, with normal levels of plasma triglyceride, phospholipid, Apo B, Apo C III, and ApoE levels and no coronary artery lumen narrowing on angiography. LCAT activity was about 50% of normal. The patient was homozygous for a codon 8 nonsense mutation in exon 3 of the Apo A1 gene. Heterozygotes in the family had normal HDLC levels.
In 2013, reported a 61-year-old male with significant coronary heart disease from the age of 42, corneal arcus, combined hyperlipidemia, HDL C of 1 mg/dL, Apo A1 of 23 mg/dL, normal LCAT acticity  and only pre 1 and -2 HDL particles present on electrophoresis. He had a novel heterozygous inframe insertion mutation with a duplication of nucleotides called as Apo AI Nashua.
In 2014, Anthanont and colleagues reported a Apo A1 mutation in a  68-year-old male and two other family members with premature CVD, corneal arcus, HDL C 14 mg/dL, Apo A1 57 mg/dL, normal triglyceride, LDL C levels and lack of very large -1 HDL. Genotypic sequencing revealed a heterozygous nonsense mutation (Gln216termination) resulting in a truncated Apo A1 containing only 215 amino acids. This mutation is designated as Apo AI Mytilene. Kinetic studies showed proband Apo A1 production to be 40% of normal, cellular cholesterol efflux capacity 65% of normal, and normal LCAT activity.
In 2015, Anthanont and colleagues reported a mutation in a 68-year female and her two sons with severe HDL deficiency, mild hypertriglyceridemia, and detectable large alpha-1 and alpha-2 HDL particles on 2D gel electrophoresis. Genomic sequencing revealed a a heterozygous missense mutation of Apo A1, designated as Apo AI Boston. They had decreased LCAT function and cholesterol efflux.
- In 1980, Franceschini reported significant hypertriglyceridemia and marked decrease of HDL C (7-14 mg/dl) with no signs of coronary atherosclerosis in the father, son, and daughter of an Italian family. They had normal lipoprotein lipase, LCAT activity and a reduced Apo A1 on 2D gel electrophoresis. He suggested the finding was probably due to a change in the amino acid composition and it was designated as Apo A1 Milano.
- In 1991, Funke and colleagues reported a 42-year-old German patient with corneal opacification, marked HDL deficiency, Apo A1]]deficiency, decreased plasma LCAT activity, increased non-HDL C and triglyceride, and lack of CVD. Sequencing of LCAT gene was normal, but the patient was found to be homozygous for an Apo A1 frameshift mutation resulting in a truncated 229 amino acid protein instead of full length Apo A1.
- In 1995, Takata and colleagues reported a 39-year-old Japanese man with corneal opacification, HDLC of 6 mg/dL, Apo A1 level of <3.0 mg/dL, increased LDL C, with normal levels of plasma triglyceride, phospholipid, Apo B, Apo C III, and ApoE levels and no coronary artery lumen narrowing on angiography. LCAT activity was about 50% of normal. The patient was homozygous for a codon 8 nonsense mutation in exon 3 of the Apo A1 gene. Heterozygotes in the family had normal HDLC levels.
- In 2013, reported a 61-year-old male with significant coronary heart disease from the age of 42, corneal arcus, combined hyperlipidemia, HDL C of 1 mg/dL, Apo A1 of 23 mg/dL, normal LCAT acticity  and only pre 1 and -2 HDL particles present on electrophoresis. He had a novel heterozygous inframe insertion mutation with a duplication of nucleotides called as Apo AI Nashua.
- In 2014, Anthanont and colleagues reported a Apo A1 mutation in a  68-year-old male and two other family members with premature CVD, corneal arcus, HDL C 14 mg/dL, Apo A1 57 mg/dL, normal triglyceride, LDL C levels and lack of very large -1 HDL. Genotypic sequencing revealed a heterozygous nonsense mutation (Gln216termination) resulting in a truncated Apo A1 containing only 215 amino acids. This mutation is designated as Apo AI Mytilene. Kinetic studies showed proband Apo A1 production to be 40% of normal, cellular cholesterol efflux capacity 65% of normal, and normal LCAT activity.
- In 2015, Anthanont and colleagues reported a mutation in a 68-year female and her two sons with severe HDL deficiency, mild hypertriglyceridemia, and detectable large alpha-1 and alpha-2 HDL particles on 2D gel electrophoresis. Genomic sequencing revealed a a heterozygous missense mutation of Apo A1, designated as Apo AI Boston. They had decreased LCAT function and cholesterol efflux.

# Demographics, Epidemiology
- Worldwide, 82 Apo A1 mutations have been reported.
- The prevalence of Apo A1 deficiency is estimated to be less than 1/1,000,000 population.
- Apo A1 deficiency accounts for 6% of Japanese population with low HDL C.
- Genomic sequencing of Apo A1 gene in 10,330 population based participants in the Copenhagen City Heart study revealed:
In the study, only 0.27% of the individuals in the general population were heterozygous for non-synonymous variants which were associated with significant reductions in Apo A1 and HDL C.
In the study,  only 0.41% of the population was heterozygous for variants predisposing to amyloidosis.
- In the study, only 0.27% of the individuals in the general population were heterozygous for non-synonymous variants which were associated with significant reductions in Apo A1 and HDL C.
- In the study,  only 0.41% of the population was heterozygous for variants predisposing to amyloidosis.

# Pathogenesis
Apolipoprotein A1 deficiency is caused by mutation in the APOA1 gene encoding ApoA1 protein, a major transport protein of reverse cholesterol transport.

## Pathophysiology

## Reverse Cholesterol Transport
- HDL C is synthesized and secreted from the liver as nascent very small discoid pre--1 HDL, predominantly composed of apolipoprotein A1.
- Apo A1 is a predominant lipoprotein of HDL and plays an important role in maturation of HDL and reverse cholesterol transport by:
Apo A1 is important for mediating the efflux of cholesterol from peripheral tissues.
Apo A1 interacts with ABCA1 and accepts free cholesterol.
Apo A1 is a potent activator of LCAT, enzyme helpful in the formation of cholesterol esters.
Delivery of cholesterol esters to the liver is mediated by scavenger receptor class B type I (SR-B1).
- Apo A1 is important for mediating the efflux of cholesterol from peripheral tissues.
- Apo A1 interacts with ABCA1 and accepts free cholesterol.
- Apo A1 is a potent activator of LCAT, enzyme helpful in the formation of cholesterol esters.
- Delivery of cholesterol esters to the liver is mediated by scavenger receptor class B type I (SR-B1).
- Genetic factors regulate the circulating levels of HDL and its functionality, mutations in the Apo A1 gene affect the total plasma levels of Apo A1 leading to low undetectable levels of HDL C.
- Majority of clinical and epidemiological studies like the Framingham Heart Study, Emerging Risk Factor Collaboration, Munster Heart Study, INTERHEART Study have proved an inverse relationship between HDL C concentration and cardiovascular risk.
- The atheroprotective function of HDL C is determined by measuring the cholesterol efflux from the cells and its anti-oxidative ability.
- In Apo A1 deficiency there is complete absence of Apo A1 and HDL C in homozygotes and less than 50% normal in heterozygotes, this disrupts the reverse cholesterol transport by :
Change of chemical compositon in sub-populations of HDL C.
Decrease in the cholesterol efflux.
Failure of cholesterol ester formation as LCAT function is compromised.
- Change of chemical compositon in sub-populations of HDL C.
- Decrease in the cholesterol efflux.
- Failure of cholesterol ester formation as LCAT function is compromised.
- The changes in the reverse cholesterol transport predispose the patients to premature heart disease.

### Genetics
- Apolipoprotein A1 deficiency is caused by mutation in the Apo A1 gene (11q23-q24) which encodes for the apolipoprotein A1.
- Mutations in the gene result in decreased production, impaired function or increased Apo A1 catabolism.
- Clinical phenotype varies with individual mutation and the type.
- Frameshift mutations, nonsense mutations, genomic rearrangements, deletions are more commonly associated with premature CVD and undetectable Apo A1 levels.
- Patients with missense mutations usually have detectable plasma Apo A1, low HDL C and can present with cardiovascular symptoms, amyloidosis or are healthy patients with no signs of atherosclerosis.

# Natural History, Prognosis, Complications
- The age of symptom onset in patients with Apo A1 deficiency and the clinical presentation varies with different mutations.
- Few patients remain asymptomatic into adulthood and few individuals may present from adolescence with symptoms of blurred vision due to corneal opacities or cataract, tubero-eruptive, tendinous, palmar and/or planar xanthomas, xanthelasmas and premature CVD and carotid atherosclerosis.
- Individuals with certain mutations present with signs such as cerebellar ataxia, hearing loss, proliferative retinopathy or manifestations of secondary amyloidosis such as hepatomegaly, nephropathy and cardiomyopathy.
- If left untreated the major complication is development of premature CVD.
- Prognosis depends on occurrence of premature CVD and end-stage organ failure in individuals with amyloidosis.

# History and Symptoms
- Age of of symptom onset and age of clinical presentation varies as many patients can remain asymptomatic into adulthood. Majority of  patients are diagnosed for the first time with a cardiovascular event at a young age.
- Patients who are symptomatic usually present with:
Blurry vision due to corneal opacities
Yellowish orange lumps in the skin, palms and feet
Coronary heart disease - History of angina or MI when younger than 60 years, history of premature heart disease in siblings and first-degree relatives.
Congestive heart failure
Peripheral vascular disease - History of claudication
Cerebrovascular disease
History of stroke
History of transient ischemic attack
History of carotid endarterectomy
- Blurry vision due to corneal opacities
- Yellowish orange lumps in the skin, palms and feet
- Coronary heart disease - History of angina or MI when younger than 60 years, history of premature heart disease in siblings and first-degree relatives.
Congestive heart failure
Peripheral vascular disease - History of claudication
- Congestive heart failure
- Peripheral vascular disease - History of claudication
- Cerebrovascular disease
History of stroke
History of transient ischemic attack
History of carotid endarterectomy
- History of stroke
- History of transient ischemic attack
- History of carotid endarterectomy
- Less common symptoms in Apo A1 deficiency include:
Ataxia
Hearing loss
Manifestions of amyloidosis:
Nephropathy presents with hematuria, generalized body swelling, shortness of breath on exertion.
Cardiomyopathy can present with chest pain, shortness of breath on exertion, syncope, pedal edema.
- Ataxia
- Hearing loss
- Manifestions of amyloidosis:
Nephropathy presents with hematuria, generalized body swelling, shortness of breath on exertion.
Cardiomyopathy can present with chest pain, shortness of breath on exertion, syncope, pedal edema.
- Nephropathy presents with hematuria, generalized body swelling, shortness of breath on exertion.
- Cardiomyopathy can present with chest pain, shortness of breath on exertion, syncope, pedal edema.

# Physical Examination
Physical examination findings in Apo A1 deficiency include:
- Corneal opacities, corneal arcus
- Tubero-eruptive,  palmar or planar xanthomas
- Cerebellar ataxia
- Sensorineural hearing loss
- Hepatomegaly

# Diagnosis
- Apo A1 deficiency is diagnosed by combination undectectable Apo A1 and HDL C levels.

## Lipid Analysis
- Laboratory features consistent with the diagnosis of Apo A1 deficiency include:
Undetectable Apo A1
HDL C less than 10mg/dl
Normal or elevated triglyceride
Normal or elevated LDL C
- Undetectable Apo A1
- HDL C less than 10mg/dl
- Normal or elevated triglyceride
- Normal or elevated LDL C

## 2D Electrophoresis
- 2D gel electrophoresis with anti-apo A1 immunoblotting is very useful in differentiating the diseases with low HDL C. It is based on the distribution of Apo A1 in different sub-populations of HDL C.
- The normal values and distribution of Apo A1 in HDL C are as follows:
Normal plasma Apo A1 is 140mg/dl
10% is found in small discoidal pre beta HDL and alpha-1 HDL C.
90% is found in alpha-2 and alpha-3 HDL C.
- Normal plasma Apo A1 is 140mg/dl
- 10% is found in small discoidal pre beta HDL and alpha-1 HDL C.
- 90% is found in alpha-2 and alpha-3 HDL C.
- In Apo A1 deficiency, a total absence of Apo A1 containing HDL C is demonstrated on 2D electrophoresis.

## Molecular Gene Sequencing
- The gold standard for diagnosis of Apo A1 deficiency is molecular gene sequencing for identification of the mutation.

# Differential Diagnosis

# Approch to a patient with low HDL C

# Treatment

## Medical Therapy
The mainstay of therapy for Apo A1 deficiency includes:
- Patients with low HDL C and Apo A1 should be treated with statins for optimizing the level of LDL C.
- Patients with Apo A1 variants do not develop clinical sequelae generally to need specific treatment.
- Apo A1 infusion therapy is the future of treatment, which helps in  improving the cholesterol efflux and reduce the plaque burden in patients who undergo interventions for CAD.

## Surgical Therapy
- Patients presenting with myocardial infarction should undergo coronary bypass or PCI with stent.

# Primary Prevention
- Assessment of cardiovascular risk in patients diagnosed with Apo A1 deficiency and Apo A1 variants.
- The goal of LDL C should be targeted below 70mg/dl according to the ATP III guidelines with high intensity statin therapy.
- All the traditional risk factors of CVD should be identified and addressed.
- Sub-clinical atherosclerosis can be identified by imaging with coronary artery calcium or carotid media thickness assessment which helps in guiding the lipid lowering therapy and assess the cardiovascular risk.