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CommentaryGenes, Geography and Geometry

The “Critical Mass” in Hypertrophic Cardiomyopathy

Nina Kaludercic,* Carlo Reggiani,†

and Nazareno Paolocci,*‡

From the Division of Cardiology,* Department of Medicine, The

Johns Hopkins Medical Institutions, Baltimore, Maryland; the

Department of Human Anatomy and Physiology,† University of

Padova, Padova, Italy; and the Department of Clinical Medicine,‡

Section of General Pathology, University of Perugia, Perugia, Italy

As recently stated by Elliott and Spirito, “the prevention ofpremature death from ventricular tachyarrhythmia, heart fail-ure and stroke remains a major aim of clinical managementin what is now called hypertrophic cardiomyopathy.”1 Hy-pertrophic cardiomyopathy (HCM) is an autosomal dom-inant disease, the first myocardial affliction for which agenetic basis was identified and, in essence, a disease ofthe contractile sarcomeric proteins. The stigmata of thedisease are myocardial hypertrophy, often asymmetricand with any possible diffuse or segmental pattern of leftventricle (LV) thickening, and impaired LV contractile anddiastolic function. HCM can be classified as obstructiveor non obstructive. In the 25% of HCM patients with LVoutflow tract obstruction, there is the presence of a dy-namic ventricular gradient. A pronounced LV outflowtract obstruction directly correlates with the severity of theclinical manifestations while adversely impacting theprognosis.2

HCM results from excessive cardiac growth, increasednumber of fibroblasts with secretion of collagen (fibrosis),and disruption of the characteristic cell-to-cell alignmentof the sarcomere and myocyte. Electrical instability (ie,atrial and ventricular arrhythmias) is another prominentfeature of HCM and a leading cause of death, particularlyin young athletes.3 Although HCM is a relatively benignsyndrome in adult populations, with an annual frequencyof sudden cardiac death (SCD) of 0.5% to 1%, HCM stillremains the most common cause of death among chil-dren and adolescents (1% to 2%).4 Early age of onset,family history of SCD, malignant arrhythmias and exer-cise-induced hypotension, in addition to specific geneticmutations, all contribute to a higher risk for SCD.5,6 As amatter of fact, “the major clinical challenge is the identifica-

tion of the small number of individuals who are prone toserious complications and rapid disease progression.”2

Typical clinical manifestations of HCM include chestpain, exertion-related dyspnea (exercise intolerance),palpitations and, less frequently, syncope. However,HCM is clinically variable, with some patients remainingasymptomatic throughout their lifetime, while others ex-perience the most serious complications. This heteroge-neity is likely one of the major reasons for the complexityof HCM management. Unfortunately, SCD is quite oftenthe presenting manifestation in young individuals.3

Current treatments focus on relieving the symptoms ofHCM, treating arrhythmias (ie, atrial fibrillation and non-sustained ventricular tachycardia) that occur commonlyin HCM, and, foremost, preventing SCD. These interven-tions include changes in lifestyle (ie, diet and exercise)and pharmacological tools such as �-blockers, Ca2�

channel blockers and diuretics. The implantation of car-diac defibrillators is also a valid option, particularly forthose patients that are at the highest risk for suddendeath.

HCM is caused by mutations in one of a number ofgenes. Approximately 450 different mutations have beendiscovered in genes for functional/structural proteins inthe sarcomere (13 related genes) and myofilaments.2

Most of the alterations are missense, with a single aminoacid residue substituted for another. The majority of HCMmolecular defects lie in genes encoding functional andregulatory sarcomeric proteins such as �-myosin heavychain (�-MHC), actin, cardiac troponin T and I, and tro-pomyosin, as well as structural proteins, ie, myosin bind-ing protein C (MYBPC) and titin.2

Identifying the specific gene mutation underlying thedisease in individuals has more than an etiological rele-vance, as specific gene mutations may contribute to thedifferent phenotypic and functional outcomes in patients

Supported by a Post-Doctoral Fellowship Grant from the American HeartAssociation (N.K.) and NIH (HL075265 to N.P.).

Accepted for publication October 24, 2008.

Address reprint requests to Nazareno Paolocci, M.D., Ph.D., 858 Ross,Division of Cardiology, Johns Hopkins Medical Institutions, 720 RutlandAvenue, Baltimore, MD, 21205. E-mail npaoloc1@jhmi.edu.

See related article on page 35Journal of Molecular Diagnostics, Vol. 11, No. 1, January 2009

Copyright © American Society for Investigative Pathology

and the Association for Molecular Pathology

DOI: 10.2353/jmoldx.2009.080138

12

suffering from HCM. For example, disease due to muta-tions in �-MHC appears to manifest at a younger age andis associated with more pronounced hypertrophy and ahigher risk of SCD when compared to HCM caused bymutations in MYBPC or �-tropomyosin genes.7 Heartsfrom subjects harboring a TnT mutation, however, exhibita mild hypertrophic phenotype, despite the associationwith high incidence of SCD.8 While HCM is a monogenicdisorder in nature, the ultimate phenotype is likely theresult of the complex blend of the primary causal muta-tion with alterations in other genes and the surroundingenvironment.6 To predict the severity of HCM outcome,possibly prevent SCD, and describe “genotype-pheno-type” connections, other “modifying factors” that mayinfluence HCM phenotypic expression should be consid-ered and studied as well.

In addition to the genetic defects affecting functionaland structural sarcomeric proteins, polymorphisms in therenin–angiotensin–aldosterone system are consideredpotential disease modifiers in HCM. The renin–angio-tensin–aldosterone system plays a pivotal role in cardio-vascular physiology and pathology. Gene polymor-phisms of the angiotensin-converting enzyme (ACE) andangiotensin II type 1 receptor (ATR-1) are associated withthe severity of hypertrophy9 and the prognosis of HCMpatients.6,10 ACE is a zinc metallopeptidase, convertingangiotensin I into the vasoactive and aldosterone-stimu-lating peptide angiotensin II (a potent pro-hypertrophicagent), and inactivating bradykinin (a weapon againsthypertrophy). ACE activity is highly variable among indi-viduals, mainly due to the presence of an insertion/dele-tion polymorphism in intron 16, consisting of a 287-bp Alurepeat sequence, with three genotypes: insertion, inser-tion/deletion, and deletion (DD). Patients with the DDgenotype have increased tissue and plasma levels ofACE and may therefore have increased levels of angio-tensin II, fueling hypertrophy and fibrosis. DD-ACE isconsidered a ‘pro-LVH’ modifier in HCM.

However, assessing the modulatory role of angiotensinII in the setting of HCM is more complex than a simple“gene specificity.” One study of families with mutations inMYBPC and �-MHC by Tesson and colleagues hasshown that a significant correlation between the DD-ACEgenotype and hypertrophy was observed in the presenceof Arg403 codon mutations of �-MHC, but not for othermutations of either MYBPC or �-MHC.11 Conversely, Per-kins and colleagues observed DD-ACE only in patientsharboring MYBPC.12 This inconsistency highlights an-other potential problem for the modulatory role of angio-tensin II in the HCM setting—the “codon-specificity”(within the same gene) for this angiotensin II effect. Im-portantly, the frequency of the ACE insertion/deletionpolymorphism shows wide diversity among geographicalareas and populations, thus making comparison of stud-ies performed in different countries more difficult.13

Inhibiting ACE or blocking ATR-1 might reverse hyper-trophy and fibrosis in experimental and human cardiacafflictions such as hypertension, congestive heart failureand metabolic syndrome. In these conditions, ACE inhib-itors are known to improve LV diastolic function andcoronary blood flow. Several studies have assessed the

potential benefits of ATR-1 antagonism in human HCM. Inone study, Araujo and colleagues have shown that 6months’ treatment with losartan (100 mg/day) improveddiastolic function and decreased LV filling pressure in 20patients with nonobstructive HCM, although no changeswere observed in the thickness of LV wall.14 More re-cently, the administration of losartan (50 mg/day) for a1-year period was able to reduce LV mass to some extent(6.4%).15 Kawano et al demonstrated that 12 months’treatment with valsartan (100 mg/day) reduced collagensynthesis in HCM patients, with no favorable effects onLV diastolic function, filling pressures, or degree of hy-pertrophy.16 In each of the studies mentioned above, theprimary HCM-causing genetic mutation was never re-ported. The different genetic backgrounds or ethnicity ofthe HCM patients enrolled in these studies may havecontributed to the divergent impact of this class of com-pounds on LV hypertrophy and could prove important inunderstanding different therapeutic outcomes with thesame drug regimen.

The study conducted by Penicka and colleagues17 inthe current issue of the Journal of Molecular Diagnosticsaddresses these issues. These authors first hypothesizedthat a long-term (12 months) administration of the angio-tensin II type I receptor antagonist candesartan in pa-tients with HCM is able to reduce LV hypertrophy as wellas improve LV function and exercise tolerance. Theyaimed to demonstrate that the magnitude of the impact ofthe AT1-blocker on patients harboring HCM is a functionof the specific sarcomeric mutation that is present. To testthis hypothesis, they performed a double-blind, placebo-controlled, randomized and multicenter study, enrolling24 genetically independent, adult patients with nonob-structive HCM, normal ejection fraction (�60%), and si-nus rhythm. HCM was diagnosed by echocardiographyshowing a non-dilated, hypertrophied LV (wall thickness�15 mm), and absence of LV hypertrophy caused byhypertension and valvular diseases. Treatment with ACEinhibitors or AT1-R antagonists at any time in the past wasone of the major exclusion criteria.

Investigating “genotype-phenotype” correlations inHCM has been a topic of intense study. These authorslook at the condition from a different angle, askingwhether “the efficacy of AT-1 blockade on LV mass andfunction in HCM subjects can be predicted on the basis ofthe intrinsic gene mutation harbored?” The investigatorsfirst performed baseline molecular genetic testing, re-vealing that the majority of the patients enrolled in thestudy were affected by mutations in �-MHC, MYBPC, andcardiac troponin T and I genes. Then, they assessedtolerance to exercise by bicycle ergometry, the presenceof malignant arrhythmias by Holter monitoring, the extentof LV hypertrophy by 2D echocardiography and LV out-flow tract pressure gradient by Doppler echocardiogra-phy. The patient cohort was subsequently divided in twogroups (n � 12 each), randomly assigned to receive theAT1-R blocker candesartan (32 mg/day) or matchingplacebo for 12 months. The authors performed a titrationof the candesartan dose to be used, with 67% of thepatients reaching the target dose of 32 mg daily. Thesame structural and functional end-points were re-evalu-

Genes, Geography, and Geometry 13JMD January 2009, Vol. 11, No. 1

ated after the drug (or placebo) treatment. Despite similarbaseline symptoms between the groups, including exer-cise tolerance, systo-diastolic LV function, and hypertro-phy magnitude, patients under candesartan treatmentshowed a significant reduction in mean LV thickness andmass when compared to those receiving placebo. Thesemorphological changes were concomitant with a betterfunctional outcome both in terms of systolic and diastolicfunction and with reduced LV filling pressures. This ben-eficial impact was absent in patients receiving placebo.Despite no change in LV ejection fraction was reportedbetween the groups, six patients receiving candesartanshowed a 1-point decrease in NYHA class compared toonly one patient receiving placebo. The reduction of LVmass (�15.5%) and improvement in LV systolic and di-astolic function in the candesartan group were also as-sociated with an increase in total exercise time.

Besides showing that long-term treatment with AT1-Rblockers are safe, in face of their vasodilative action, themost salient aspect of the present study is that the het-erogeneous response in terms of LV reduction after can-desartan treatment is in part dependent on the specificsarcomeric protein gene mutation. All patients displayingmutations in �-MHC showed the most marked decreasein LV mass, while carriers of the MYBPC genotypeshowed moderate responses. Conversely, no regressionof hypertrophy was observed in patients harboring thecardiac troponin I gene mutation. This pilot study is thefirst attempt to associate the effects of AT1-R blockadewith the significant mutation-specific regression ofhypertrophy.

Other studies have been published using different an-giotensin II receptor blockers from the same family, butthe present approach uses a new combination of tools,ie, cardiac functional assessment and mutation analysis.Mutational analysis by Penicka et al shows that the ben-eficial effects of the AT1-R blocker candesartan might bemutation specific, with better hypertrophy regression inpatients with mutations in �-MHC and MYBPC. This mustbe considered a pilot study, and increasing the popula-tion size is necessary to draw confident conclusions re-garding the correlation of AT1-R blockade and geneticmutations involved in HCM. The hypothesis of a “geneticbasis” as the explanatory factor for the conflicting resultsin AT1-R blockade on HCM progression still remainsspeculative, particularly because the causative geneticmutations identified here were not examined in the pre-vious studies,14,16,15 making the comparison betweenstudies rather problematic.

The concept that a heterogeneous genetic back-ground in HCM patients enrolled in long-term studies withAT1-R blockers is responsible for the different responsein terms of LV hypertrophy magnitude must be validated.Ideally, a comparison should be made studying differentpatient cohorts from the same geographical area. Theauthors noted that HCM-causing mutations may also de-pend on the genetic (and so “geographical”) backgroundof the cohort of individuals. Another major limitation is thatneither ACE nor AT1-R polymorphisms were assessed. Intheory, carriers of the DD-ACE or AT1-R C (increasedangiotensin II effect) should get the most beneficial ef-

fects after candesartan (or similar compounds). Finally,the specific molecular mechanisms linking beneficial ef-fects of candesartan to specific sarcomere protein genemutations should be explored.

Setting aside these intrinsic limitations, the work byPenicka and co-workers17 introduces a number of newintriguing questions and starting points for future morein-depth investigations. Half of the patients with unex-plained LV hypertrophy do not have a sarcomere orsarcomere-related gene mutation.2 For instance, recentstudies of mouse models of mutations in the �2 subunit ofAMP-dependent protein kinase and in the lysosomal-associated membrane protein 2 have been shown tocause unexplained LV hypertrophy.18 The �2 subunit ofAMP-dependent protein kinase mutations lead to markedaccumulation of glycogen within myocytes,19 whereas,lysosomal-associated membrane protein 2 mutationscause accumulation of authophagic vacuoles that con-tain undegraded cellular products.20 The rate of progres-sion from hypertrophy to dilation and overt heart failure ishigher in storage cardiomyopathies than in HCM,2 andinterstitial fibrosis is a major component in most of HCMcardiac phenotypes. Would AT1-R antagonists still beable to provide beneficial effects in patients affected byHCM based on non-sarcomeric mutation? Would colla-gen neo-synthesis and fibrosis still be a crucial factor? Isthe role of oxidative stress, particularly when the heartrapidly progresses from LV wall thickening to overt dila-tion, relevant?

Certainly, the balance between oxidants and antioxi-dants is important for maintenance of normal collagenhomeostasis. Thiol-reducing agents such as N-acetylcys-teine are effective in reducing myocardial oxidativestress, stress-responsive signaling kinases, and fibrosisin a mouse model of HCM.21 Based on existing literaturewith other ATR-1 blockers,22 one could anticipate thatpatients receiving candesartan may have better pre-served cardiac tissue redox conditions. However, there isno direct link between reactive oxygen species, changesin LV morphology, and gene mutations, at either thesarcomeric or the non-sarcomeric level. Other drugscould therefore be effective in reducing LV mass andimproving function in HCM patients on a “specific geneticmutation” basis. These issues highlight the need to un-derstand the exact mechanism by which gene mutationsare translated into clinical HCM phenotype.

A positive impact of candesartan on exercise tolerancehas been demonstrated in patients with mild diastolicdysfunction at rest and a hypertensive response to exer-cise.23 In the present study Doppler-echocardiographymeasurements were not performed immediately after ex-ercise, but only at rest. The data are consistent withprevious observations, which showed improved exercisetolerance correlated with an improvement in myocardialsysto-diastolic function.24 Again, decrease in LV massseems to be a major determinant for this change. Thismight be related to improved myocardial contraction andrelaxation in presence of decreased LV filling pressures.Moreover, the systolic blood pressure at peak exercisetended to be lower in the candesartan versus placebogroup. Ultimately, this systemic effect may have ac-

14 Kaludercic et alJMD January 2009, Vol. 11, No. 1

counted for some of the observed positive impact ofcandesartan on exercise tolerance. In addition, it is likelythat reduced collagen synthesis (deposition) associatedwith AT1-R antagonism could have contributed to in-creased myocardial function. These questions still remainunanswered, although the present study provides an ex-cellent and intriguing basis for a better understanding ofthe pathophysiological features of HCM and for improve-ments in its clinical management.

“Critical mass” is a concept used in different disci-plines, including sociodynamics and physics. It usuallydesignates the existence of a sufficient momentum or aminimum amount of given material, respectively, such thatthe momentum itself (or the material) becomes self-sus-taining and fuels further growth. In nuclear physics reach-ing this point starts a fission chain reaction under statedconditions. In both cases, this initial “critical” nucleus (orquantum) is, at the same time, igniting a reaction andsubjected to external factors that may influence the fur-ther development (orientation) of this “tipping point.”Adopting, by analogy, this conceptual frame in the HCMsetting is indeed very tempting. However, what if “massbecomes critical?” After all, the present study reiteratesone central mainstay of the pathophysiology, clinical out-come, and possible therapeutic success in HCM pa-tients: the magnitude of LV hypertrophy is one key deter-minant of symptoms and prognosis.

Young patients with “critical,” extreme hypertrophy(even with few or no symptoms at all) have substantialrisk for SCD.25 The present study suggests that �-MHC(and MYBPC to a lesser extent) may be more “critical”than other proteins, as all patients displaying a cande-sartan-induced marked decrease in LV mass (�100g)were positive for a mutation in �-MHC. Previous studieshave demonstrated that the magnitude of LV hypertrophyis not only a strong and independent predictor of prog-nosis, but in young patients the LV mass becomes really“critical” when, in absence of other generally acceptedrisk factors, the maximal wall thickness is �30 mm.25

Thus, we may have the threshold for its “criticality,” andpossibly a structural/molecular diagnostic marker for pre-dicting HCM outcome and the efficacy of a givenintervention.

We are left with several unresolved questions. Weknow that genetic mutations are major determinants ofHCM, but the molecular links between the causal sarco-meric mutations and the phenotype of HCM remain to befully understood. Putative mutations and the prevalenceof modifying factors, such as ACE and ATR-1 polymor-phisms, may differ between ethnic groups. However, inaddition to genetic and geographical influences, geom-etry should be also considered. More specifically, howwould the conceptual framework of predicting the effi-cacy of a given drug based on the specific gene mutationapply and work for HCM with LV outflow tract obstruc-tion? This dynamic condition exhibits poorer overall sur-vival, complex LV kinetics and requires a completelydifferent palette of drugs, avoiding agents endowed withsome vasodilative properties such as ACE inhibitors,ATR-1 blockers, and certain Ca2� channel blockers.26

Since �-adrenergic antagonists are the front-line therapy

in obstructive HCM, perhaps a polymorphism in �-adren-ergic receptor genes would apply as a modifying factor inpatients suffering from obstructive HCM. Whether andhow the possible cross talk between specific gene mu-tations, modifying factors and spatial distribution of theLV mass may influence HCM phenotype and the efficacyof a given intervention remains to be fully resolved.

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