Cardiovascular Journal of Africa: Vol 26 No 2 (March/April 2015)

CARDIOVASCULAR JOURNAL OF AFRICA • Volume 26, No 2, March/April 2015

AFRICA

ethnically matched control group (

100). Genotypes of

pedigree 1 for the p.Leu144His mutation were further confirmed

by bi-directional semi-automated DNA sequencing (Fig. 1).

Identification of previously documented

de novo

missense mutation

The mutation p.Arg170Gln (c.509G

A) has previously been

associated with HCM in children younger than 13 years.

Pst

restriction enzyme digestion confirmed that this mutation was

not present in the other available family members of pedigree

2, including the parents of individual 2.II.3. Haplotype analysis

of pedigree 2 derived from four microsatellite markers in the

TNNI3

-located chromosomal region shown in Fig. 3 indicated

that the parents (2.I.1 and 2.I.2) are definitely the biological

father and mother of all three of their offspring (2.II.1, 2.II.2 and

2.II.3). The p.Arg170Gln (c.509G

A) mutation is considered to

have occurred

de novo

in the affected patient (2.II.3), since this

mutation was absent in all other family members available for

testing, and no family members presented with symptoms.

Discussion

Inthisarticlewedescribethescreeningof apanelof HCM-affected

probands for

TNNI3

, a gene that has been associated not only

with HCM but also with dilated cardiomyopathy and RCM.

The screening was performed because two unrelated individuals,

referred with HCM, had features resembling that of RCM.

Of the sequence variants identified, four caused amino acid

changes, of which two were present in our probands with RCM;

one novel (p.Leu144His) and one a previously described

de novo

mutation (p.Arg170Gln). The p.Leu144His mutation was present

in the first actin-binding domain and overlapped with the ATPase

inhibitory region, and p.Arg170Gln was located in the second

actin-binding domain.

The other two, namely p.Arg162Gln and

p.Pro82Ser, were observed in two patients diagnosed with HCM.

One patient presented with symptoms of atrial fibrillation (male,

61 years) and the other was an unexplained death diagnosed

post mortem (male, 41 years). Both mutations have previously

been associated with HCM. Other family members were either

unaffected by HCM or not available for further testing.

RCM segregated in pedigree 1 for two generations, while in

pedigree 2 an isolated case presented with RCM with no evidence

of clustering within his immediate family. Two individuals

also fulfilled the diagnostic criteria for HCM by way of focal

‘hypertrophy’ for HCM. The p.Leu144His mutation was also

present in two brothers (1.III.1 and 1.III.2, Fig. 1) and most likely

in the father of the proband who died from the same disease. The

p.Arg170Gln mutation was identified in a single individual (2.II.3,

Fig. 3). We have subsequently shown that this mutation arose

novo

on the background of a haplotype inherited from his father.

In a previous study investigating the genetic aetiology of HCM in

pre-adolescent children between 1989 and 2007, the p.Arg170Gln

mutation was the only mutation identified in

TNNI3

The proband and her father in pedigree 1 were severely

affected but only became symptomatic in their mid-twenties,

and had had children before then. The other two siblings

of the proband voluntarily had no children. In most cases

of sarcomere-associated RCM, individuals with mutations in

TNNI3

in particular, present with likely

de novo

mutations,

as affected individuals tend to die young.

3,4

This pattern was

observed in 2.II.3 (Fig. 3), who died at age 16 years, and had a

novo

mutation on an allele inherited from his father.

The marked bi-atrial dilatation in the presence of normal-

to-small left and right ventricular dimensions and generally

preserved left ventricular systolic function (as measured by

left ventricular ejection fraction in three of four cases) argues

strongly for a restrictive process involving both ventricles in these

cases. Left ventricular diastolic function measurements were

universally abnormal, as expected, and this process associated

with high filling pressures most likely explains the bi-atrial

dilatation (inferior vena cava data for high right ventricular

filling pressures – see Table 2). These ‘restrictive hearts’ show

two additional morphological features of interest, namely focal

hypertrophy, which often does not reach the cut-off values for the

diagnosis of HCM by current criteria (15 mm), and prominent

involvement of the right ventricle in terms of hypertrophy

(three of four patients). The prominence of right ventricular

involvement is not only echocardiographic, but translates into

clinical involvement in terms of prominent right heart failure.

The index cases from both pedigrees demonstrated evidence

of advanced long-axis systolic impairment of both ventricles

and were worst affected from the viewpoint of symptomatic

congestive heart failure. The elder brother of the proband from

pedigree 1 (1.III.1, Fig. 1), with sparing of long-axis systolic

function of both ventricles, seemed least affected clinically and

in fact was relatively asymptomatic. The brother from pedigree 1

was intermediate, with relatively preserved left ventricular long-

axis function but significantly impaired right ventricular long-

axis function. The right ventricular long-axis function in all these

cases, as assessed by TAPSE, appeared to predict the presence

of right heart failure, or vice versa. Furthermore, interestingly,

both these features tracked the presence of right ventricular

hypertrophy in all three individuals exhibiting this.

Empirical data on outcomes related to right ventricular

structure is generally lacking because of the unique geometry of

the right ventricle.

TAPSE estimates longitudinal function, but

is not comprehensive.

However, a recent magnetic resonance

imaging-based study showed a higher risk in persons free of

cardiovascular disease at baseline, but with an increased right

ventricular mass.

The novel mutation p.Leu144His involves the same amino

acid residue as p.Leu144Gln, although it results in a different

amino acid change. The latter is one of several RCM-causing

mutations studied at laboratory level (p.Arg145Trp, p.Ala171Thr,

p.Lys178Glu and p.Arg192His). The amino acid involving these

mutations is located within the actin-binding domain (residue

130–148, 173–181) and the troponin C-binding domain (residue

113–164) of cardiac troponin I.

In principle, this mutation could affect the normal function

of cardiac troponin I since it is located in the sequence

(residues 137–148) required for inhibition of human cardiac

troponin I actomysin ATPase activity

and has been shown

to cause excessive inhibition in assays. However, all of the

mutations tested, including p.Ala171Thr, which is in the second

actin-binding domain and is next to the

de novo

mutation,

p.Arg170Gln, also have similar effects on basal actin-myosin–

ATPase activity. One suggestion is that the second binding

domain increases the cardiac troponin I concentration on actin.

All of these mutations exhibit an increased Ca

sensitivity