apolipoprotein ( apo ) a - v - is an enigmatic modulator of plasma triacylglycerol ( tg ) homeostasis .
apoa - v is expressed solely in the liver and , following secretion , circulates at extremely low concentrations ( ~150400 ng / ml plasma ) .
this value is approximately 10,000 fold lower than that of apoa - i , the major apo of high density lipoprotein ( hdl ) . given the paucity of circulating apoa - v , it is logical to consider that it does not function as a typical member of the apolipoprotein class . despite this supposition ,
apoa - v possesses sequence homology with other apo s , is recovered in association with plasma lipoproteins and displays high lipid - surface seeking activity . using recombinant protein , it has been demonstrated that apoa - v binds heparan sulfate proteoglycans ( hspg ) , members of the low - density lipoprotein receptor ( ldlr ) family and the endothelial cell surface protein , glycosylphosphatidylinositol-anchored - high - density lipoprotein binding protein 1 ( gpih - bp1 ) .
this latter function has been invoked to explain the ability of apoa - v to enhance lipoprotein lipase ( lpl ) mediated hydrolysis of lipoprotein associated tg .
what is not explained , however , is how this can be achieved at such low circulating apoa - v concentrations .
for example , it has been estimated that , in the postprandial state , there is only enough apoa - v present to associate with ~1 in 24 apob containing lipoproteins .
thus , it would appear that apoa - v is either an exceptionally potent apolipoprotein or it possesses additional functionality that has yet to be revealed .  
genome wide association studies ( gwas ) have identified apoa5 as a determinant of plasma tg concentrations .
there is also strong evidence that abnormally high concentrations of tg are associated with atherosclerosis . given this connection ,
it is important to consider whether apoa5 may be a risk factor for disease processes that develop from chronic elevation of plasma tg .
the fact that ~  40 million adults in the united states have high tg ( 200 mg / dl ) and ~4 million of these have hypertriglyceridemia ( htg ; 500 mg / dl ) indicates the scale of this health problem . in the prospective cardiovascular munster study , a six - fold increase in coronary heart disease ( chd ) risk was measured in subjects with tg values > 200 mg / dl .
likewise , in the scandinavian simvastatin survival study , the authors reported increased risk of coronary events as tg levels increased above 220 mg / dl . in this study the 5- year event rate was significantly increased in untreated patients with mixed dyslipidemia ( including those with htg ) compared to those with elevated ldl - cholesterol alone . finally , the bezafibrate infarction prevention study found that , in men and women with established chd , elevated plasma tg increased the risk of cardiovascular mortality , stroke and transient ischemic attacks .
it is widely recognized that numerous genes play a role in determining plasma tg levels .
in addition to apoa5 , well - studied modulators of plasma tg homeostasis include lpl , apoc2 , apoc3 , gpihbp1 and others . according to prevailing models , plasma apoa - v functions to facilitate lipoprotein binding to gpihbp1 . as a component of tg - rich lipoproteins ,
apoa - v binding to gpihbp1 coordinates lpl and apoc - ii interactions in a manner that promotes efficient tg hydrolysis . indeed , when any one of these proteins is missing or defective , htg ensues .
further complexity is introduced when the effect of apoa - v on plasma tg is examined in different physiological settings . in mice , for example , an inverse correlation between apoa - v and tg is well documented .
these authors showed that gene disruption of apoa5 leads to a marked increase in plasma tg while transgenic overexpression of apoa5 induces a significant decline .
by contrast , in human populations with htg , plasma apoa - v and tg are oftentimes positively correlated !
thus , it is likely that , depending on the genetic background / physiological conditions that lead to htg , apoa - v may actually accumulate along with tg .
in a recent study , do et al . investigated the correlation between rare apoa5 variants and early onset myocardial infarction ( mi ) . in this tour de force study , the authors conducted exome sequencing on over 9,700 human subjects with premature mi ( 50 years of age in males and 60 years in females ) along with mi - free controls .
they sought to identify genes in which rare mutations contribute to the risk of early-onset mi in the population .
two genes ( ldlr and apoa5 ) were identified in which rare coding mutations were more frequent in mi cases compared to controls .
likewise , in apoa5 , those subjects with rare non - synonymous mutations were at 2.2-fold increased risk for mi .
when compared to non - carriers , ldlr mutation carriers had higher plasma ldl cholesterol while apoa5 mutation carriers had higher plasma tg . it may be anticipated that subjects identified to be at risk for premature mi as a result of harboring these rare apoa5 mutations could improve or normalize their plasma tg concentration , and reduce the risk of mi , by augmentation with wild type ( wt ) apoa - v .
a basic question emerging from the above investigation relates to the underlying mechanism whereby elevated plasma tg increases the risk of heart disease .
certainly the correlation between chronic elevated plasma tg and atherosclerosis is less clear than that for cholesterol .
while the tg content of plaque is far less prominent , evidence suggests that , in cells located in and around plaque deposits , tg - rich lipoproteins induce inflammation and related atherogenic processes .
elevated tg also correlates with the formation of small dense ldl , lipoprotein particles that are positively associated with chd risk and inflammation .
given the very low concentration of apoa - v in plasma under normal circumstances , any decrease will likely impact lipoprotein - associated tg hydrolysis by lpl , thereby interfering with tg - rich lipoprotein clearance .
the resulting increase in circulating tg - rich lipoproteins will promote inflammatory cytokine release , contributing to endothelial injury .
studies in mouse models of dyslipidemia have provided evidence that apoa - v is athero - protective .
crossed apoa5 transgenic mice with apoe2 knock in ( ki ) mice ( deficient in apoe and transgenic for apoe2 ) .
compared to control apoe2 ki mice , plasma tg levels were lower and atherosclerotic lesion size was reduced when apoa - v levels were increased .
subsequently , grosskopf et al . crossed apoa5 transgenic mice with apoe ( -/- ) mice . in this
study a significant decrease in vldl and remnant lipoproteins , together with a 70% reduction in aortic lesion area , was noted . in both of these studies ,
the apoa - v concentration in plasma was increased by expression of the transgene . of interest , however ,
a dysfunctional apoe protein is present while in apoe ( -/- ) mice no apoe is present .
it is conceivable that , since apoa - v and apoe share the ability to bind hspgs and members of the ldl receptor family , augmenting apoa - v levels in the absence of functional apoe compensates for the missing or defective apoe .
these mice manifest delayed catabolism of vldl and chylomicrons owing to an abundance of apoc - iii bound to the surface of these particles . to study the effect of apoa - v augmentation on apoc - iii overexpression - dependent htg , qu et al .
used adenovirus mediated gene transfer to increase apoa - v production in apoc3 transgenic mice .
apoa - v gene transfer into apoc3 transgenic mice caused a reduction in apoc - iii content on vldl that , in turn , led to an increase in lpl - mediated tg hydrolysis .
as discussed above , and previously by sharma et al . , relatively common apoa5 snps are associated with increased plasma tg .
for example , individuals homozygous for the c.553 g > t apoa5 snp ( rs2075291 ) have extremely elevated plasma tg levels .
recent studies have shown that htg in these subjects is due , at least in part , to production of a dysfunctional apoa - v protein .
it may be anticipated that an increase in circulating levels of wt apoa - v in homozygous carriers of this snp will induce tg lowering , thereby reducing the risk of related disease processes .
another coding snp strongly associated with elevated plasma tg is c.56c > g apoa5 ( rs3135506 ) .
this snp , which introduces an amino acid substitution ( ser19trp ) in the signal sequence of apoa - v , is thought to impede processing / secretion of the mature protein .
along the same lines , a rare homozygous apoa5 deletion mutation has been identified in the signal sequence ( c.16_39del ; p.ala6_ala13del ) , generating a variant apoa - v protein that is not secreted .
the missing amino acids are required for translocation of nascent apoa - v to the endoplasmic reticulum and , as a result , this protein accumulates in the cytoplasm in association with lipid droplets . in both c.56c 
>  g and c.16_39del , the mature protein is predicted to be identical to wt apoa - v .
mutation- and snp - induced effects on signal sequence function or cleavage decrease secretion efficiency resulting in diminished , or complete lack of , circulating apoa - v .
however , despite the fact that individuals harboring the c.56c  >  g snp may secrete less apoa - v , if this polymorphism is not the sole or primary underlying cause of htg , then apoa - v levels will likely accumulate in plasma along with tg and it is doubtful augmentation with apoa - v will induce tg lowering .
non - coding snps located within the apoa5 gene locus are also associated with elevated plasma tg , most likely due to effects on apoa5 gene expression .
for example , the -1131t > c snp ( rs662799 ) , located upstream of the transcription start site , has been proposed to reduce transcription efficiency .
likewise , an ivs3 + 3 g > c mutation causes a frameshift in the donor splice site of intron 3 creating a premature stop codon that results in a nonsense protein .
other rare mutations in apoa5 result in severe truncation of apoa - v and abolish function altogether .   given this , it may be anticipated that individuals with htg caused by deleterious apoa5 snps or mutations would benefit from increased circulating levels of wt apoa - v ( table1 ) . likewise ,
as seen above , if htg is caused by enhanced apoc - iii production , it may be anticipated that increased circulating levels of apoa - v would be beneficial .
on the other hand , if htg is associated with a diagnosis of metabolic syndrome , for example , simply adding to the pool of apoa - v is not expected to improve the tg profile owing to the multifactorial causation of this disorder . from a practical standpoint ,
if htg is observed , then the apoa5 gene should be examined for the presence of deleterious snps or loss of function mutations . if present , then wt apoa - v augmentation may induce tg lowering . on the other hand , subjects with htg unrelated to apoa5 or from a combination of apoa5 variation and other genes ,
this predicted differential response provides a plausible explanation for the conundrum emerging from analysis of apoa - v genetically engineered mouse models ( wherein a clear inverse correlation between apoa - v and plasma tg exists ) and human population studies that reveal a positive correlation between apoa - v and tg .
the importance of discriminating between predicted responders and non- responders is illustrated below . for subjects harboring apoa5 mutations ,
specific criteria must be met prior to consideration for supplementation with wt apoa - v .
these include persistent elevated plasma tg , continuing low plasma apoa - v protein levels and a lack of polymorphisms / mutations in other known tg modulating genes .
based on the above discussion , it is conceivable that deleterious apoa5 snps or mutations ( e.g. interfere with apoa - v secretion efficiency or produce an apoa - v protein with compromised function ) may increase the risk of atherosclerosis due to chronically elevated plasma tg levels . to investigate the potential benefit of direct addition of apoa - v , shu et al .
administration of apoa - v  containing reconstituted hdl induced a 60% decline in plasma tg after 4 h that was attributed to enhanced catabolism and clearance of vldl . despite the reduction in plasma tg observed in this experiment , the effect was short - lived , suggesting that this approach may not represent a feasible therapeutic strategy . in an effort to increase the duration of tg - lowering induced by apoa - v ,
aav2/8-mediated gene transfer of wt apoa - v was performed in apoa5 ( -/- ) mice . in this study ,
apoa - v expression lasted at least 8 weeks and induced a 50% decline in plasma tg levels , suggesting that gene therapy may be beneficial in some instances .
it is noteworthy that recent advances suggest gene transfer technology may constitute a feasible therapeutic option for a subset of individuals with chronic htg . for this purpose ,
vectors most commonly used for treatment of metabolic disorders are adeno - associated virus ( aav ) and lenti - virus . among these
furthermore , compared to other viral vectors , aav induces a minimal host immune response .
in fact , aav vectors have been used to deliver genes to over 500 study subjects by various routes of administration for potential treatment of genetic disorders including cystic fibrosis , hemophilia and canavan , batten , parkinson 's and alzheimer 's diseases , without significant safety concerns . among approved aav - based therapies , glybera ( uniqure )
long- term gene expression ( > 1.5 years ) has been demonstrated after aav transduction in animal models including canine , murine and hamster .
moreover , it has been demonstrated that aav can successfully be transduced into a variety of cell and tissue types , including brain , liver and muscle .
an advantage of the aav serotype 2/8 is that it efficiently targets liver . given that apoa - v is expressed solely in liver , aav2/8 is a strong candidate vector for studies involving apoa - v gene transfer .  
given the role of apoa5 in modulation of plasma tg documented in studies of genetically engineered mice , gwas , human population loss of function studies and large - scale exome sequencing , therapies designed to promote its athero - protective effects are very attractive . whereas it may be possible to develop a small molecule therapeutic capable of inducing endogenous apoa - v expression , individuals harboring common snps or rare mutations in apoa5 may not benefit from this approach . on the other hand ,
gene therapy represents a safe , robust and efficient means to achieve sustained expression of wt apoa - v . by controlling plasma tg homeostasis in this manner
 abstractapolipoprotein ( apo ) a - v is a novel member of the class of exchangeable apo 's involved in triacylglycerol ( tg ) homeostasis . 
 whereas a portion of hepatic - derived apoa - v is secreted into plasma and functions to facilitate lipoprotein lipase - mediated tg hydrolysis , another portion is recovered intracellularly , in association with cytosolic lipid droplets . 
 loss of apoa - v function is positively correlated with elevated plasma tg and increased risk of cardiovascular disease . 
 single nucleotide polymorphisms ( snp ) in the apoa5 locus can affect transcription efficiency or introduce deleterious amino acid substitutions . likewise , rare mutations in apoa5 that compromise functionality are associated with increased plasma tg and premature myocardial infarction . 
 genetically engineered mouse models and human population studies suggest that , in certain instances , supplementation with wild type ( wt ) apoa - v may have therapeutic benefit . 
 it is hypothesized that individuals that manifest elevated plasma tg owing to deleterious apoa5 snps or rare mutations would respond to wt apoa - v supplementation with improved plasma tg clearance . on the other hand , subjects with hypertriglyceridemia of independent origin ( unrelated to apoa - v function ) 
 may not respond to apoa - v augmentation in this manner . 
 improvement in the ability to identify individuals predicted to benefit , advances in gene transfer technology and the strong connection between htg and heart disease , point to apoa - v supplementation as a viable disease prevention / therapeutic strategy . 
 candidates would include individuals that manifest chronic tg elevation , have low plasma apoa - v due to an apoa5 mutation / polymorphism and not have deleterious mutations / polymorphisms in other genes known to influence plasma tg levels . 