(Circulation. 1999;99:3172-3180.)
© 1999 American Heart Association, Inc.
Basic Science Reports |
From the Department of Medicine and Epidemiology, School of Veterinary Medicine, University of California, Davis (M.D.K., J.A.K., P.D.P.); the Department of Clinical Sciences, College of Veterinary Medicine, The Ohio State University, Columbus (K.M.M.); The Animal Medical Center, Caspary Research Institute for Veterinary Research, New York, NY (S.K.L.); and Baylor College of Medicine, Houston, Tex (J.A.T.).
Correspondence to Dr Mark D. Kittleson, Department of Medicine and Epidemiology, School of Veterinary Medicine, University of California at Davis, Davis, CA 95616. E-mail mdkittleson{at}ucdavis.edu
| Abstract |
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Methods and ResultsA proband was identified, and related cats were bred to produce a colony. Affected and unaffected cats were bred to determine the mode of inheritance. Echocardiography was used to identify affected offspring and determine phenotypic expression. Echocardiograms were repeated serially to determine the natural history of the disease. Of 22 offspring from breeding affected to unaffected cats, 12 (55%) were affected. When affected cats were bred to affected cats, 4 (45%) of the 9 were affected, 2 (22%) unaffected, and 3 (33%) stillborn. Findings were consistent with an autosomal dominant mode of inheritance with 100% penetrance, with the stillborns representing lethal homozygotes that died in utero. Affected cats usually did not have phenotypic evidence of HCM before 6 months of age, developed HCM during adolescence, and developed severe HCM during young adulthood. Papillary muscle hypertrophy that produced midcavitary obstruction and systolic anterior motion of the mitral valve was the most consistent manifestation of HCM. Cats died suddenly (n=5) or of heart failure (n=3). Histopathology of the myocardium revealed myocardial fiber disarray, intramural coronary arteriosclerosis, and interstitial fibrosis.
ConclusionsHCM in this family of Maine coon cats closely resembles the human form of FHCM and should prove a valuable tool for studying the gross, cellular, and molecular pathophysiology of the disease.
Key Words: cardiomyopathy hypertrophy heart diseases genetics
| Introduction |
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40%
have histological evidence of myocardial fiber
disarray, and no heritable basis for the disease has been
described.4 5 This report describes a colony of Maine coon cats with a heritable form of HCM that mimics human FHCM. It is characterized by moderate to severe papillary muscle and left ventricular (LV) concentric hypertrophy, systolic anterior motion (SAM) of the mitral valve, and the typical histopathological features seen in human FHCM. The natural history of the disease mimics that seen in humans, and it is inherited as an autosomal dominant trait.
| Methods |
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Animal Procurement
Initially, a proband and several related cats with HCM were
identified. Echocardiograms were performed on 35 cats from the cattery
of origin. Seven were diagnosed with HCM (affected cats). One
unaffected male (the father of the proband) was bred to 3 affected
females (the proband's mother, the mother's sister, and a sister of
the proband). The breeding between this male and 2 of the affected
females had produced affected kittens previously. Eight kittens from
these breedings and 1 affected female (cat 1 in Figure 1
) were obtained to start a breeding
colony (Figure 1
).
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Determination of Inheritance Pattern
Heritable traits were determined through planned breedings
between affected and unaffected colony cats (group 1), affected and
affected cats (group 2), and unaffected and unaffected cats (group 3).
Affected cats were identified through careful and repeated
echocardiographic examinations, as described below.
Relationship data were collated and analyzed to determine the
mode of inheritance.
Determination of Phenotypic Expression
Standard feline echocardiographic studies were
performed with an Acuson 128XP/10 ultrasound machine and a 7-MHz
transducer. The cats were unsedated and restrained in right and left
lateral recumbency on a Plexiglas table. The transducer was introduced
from below through a medium-sized rectangular hole in the table to
maximize the echocardiographic window. Standard right
parasternal long-axis and short-axis views plus left apical and left
cranial views were examined.6 Papillary muscle and left
atrial sizes were judged subjectively by 1 investigator (M.D.K.) as
normal or mildly, moderately, or severely enlarged. Measurements of
diastolic LV wall thickness were made from the
2-dimensional image.
Cats were suspected of having HCM when moderate papillary muscle
hypertrophy or SAM was present without LV wall
hypertrophy (Figure 2
). Cats
were definitively diagnosed with HCM when severe papillary muscle
hypertrophy was present or a region of the LV wall or
the entire wall of the LV was
6 mm thick.4 The
disease was considered severe when papillary muscle
hypertrophy was severe or at least 1 LV wall was
7
mm thick.
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The presence or absence of SAM was recorded. Cats with SAM had a
grade II to IV/VI left apical systolic heart murmur that was
dynamic (loud when the cat was excited, softer when relaxed). SAM was
most reliably identified from a right parasternal long-axis view of the
LV, LV outflow tract (LVOT), and aorta; a cine loop; and the motion of
the anterior mitral valve leaflet with respect to the LVOT and
interventricular septum from the early part of systole
through end systole (Figure 3A
and 3B
).
On color flow Doppler echocardiography, 2
turbulent jets, 1 of dynamic subaortic stenosis (SAS) and the
other of mitral regurgitation, were seen originating
from the region where the anterior mitral valve leaflet encroached on
the LVOT (Figure 3C
). The peak velocity of the dynamic SAS jet
was obtained with continuous-wave Doppler
echocardiography and converted to a pressure
gradient by use of the modified Bernoulli equation (Figure 3D
).
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Natural History
Echocardiograms were first performed at 3 to 6 months of age.
The cats underwent ultrasonography every 4 to 8 months to identify cats
that developed HCM and to document the progression of the disease. The
ages at which HCM was diagnosed and at which moderate to severe HCM was
definitively diagnosed were recorded. Congestive heart failure was
diagnosed when radiographic evidence of pulmonary
edema or pleural effusion was present. Congestive heart failure was
treated until the cat became refractory to therapy. Sudden death was
defined as a witnessed event of sudden death in a cat or as finding a
cat with HCM dead but without pathological evidence of severe heart
failure or systemic disease. The age and mode of death were
recorded.
Pathological Examination
Hearts from cats that died were trimmed, weighed, and placed in
formalin. Maximal septal and LV free wall measurements were obtained.
Sections of the LV free wall and septum were cut in a plane
perpendicular to the long axis of the LV, and the microscopic
abnormalities were categorized and quantified as reported
previously.5 7
Statistical Analysis
The differences in occurrence and progression of disease and in
sex between groups 1 and 2 were analyzed by Student's unpaired
t tests. The null hypothesis was discarded at
P<0.05.
| Results |
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Unaffected x Affected Breedings (Group 1)
Seven litters and 28 kittens were the result of mating an affected
cat with an unaffected cat (Figure 1
). Twenty-two kittens
survived. Five kittens had omphaloceles and 1 had pectus excavatum. No
stillborn fetuses were present.
Twelve of the 22 surviving kittens developed HCM (7 males and 5
females). No evidence of HCM was present in any group 1 cat before
6 months of age. Papillary muscle enlargement, usually along with SAM
of the mitral valve, developed first, most commonly between 9 and 21
months of age (16±5 months; Figure 2
). One female did not have
any change until 38 months of age. The age at which LV wall thickening
or papillary muscle hypertrophy became moderate to severe
was between 13 and 48 months (24±13 months). The wall thickening was
often localized primarily to the papillary muscles and the
posterolateral free wall, between the papillary muscles (Figures 2D
, 4
, and 5
). The disease became severe earlier in
males (n=5; 19±6 months) than in females (n=5; 29±13 months).
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Five group 1 cats died of HCM: 2 of heart failure and 3 suddenly. Four of the 5 were males. The age at which heart failure was first noted was 33±10 months. The age of death (sudden or heart failure) was 32±12 months.
Affected x Affected Matings (Group 2)
Two litters containing 10 kittens or stillborn fetuses resulted
from breeding 2 affected cats. One litter had 3 kittens: 1 with pectus
excavatum, 1 stillborn fetus, and 1 male that developed HCM. The other
litter had 5 kittens that survived and 2 stillborn fetuses. One male
and 2 females developed HCM, and 2 females did not develop HCM (Figure 1
). Both males and 2 of the 4 females developed severe HCM in
group 2. Disease in this group appeared significantly earlier and
progressed more rapidly than in group 1. The
echocardiographic findings were suggestive of HCM when
cats were 4 to 6 months of age (5±1 months). The disease was severe by
7 to 12 months of age (9±2 months). All 4 affected cats developed
heart failure and were started on heart failure therapy between 18 and
24 months of age (20±3 months). The 2 males died suddenly at 18 and 25
months of age (Figure 6
). One female died
of heart failure at 25 months of age. The other female is alive (31
months old). Age of death was 23±4 months. The ages at which HCM was
diagnosed, at which HCM was severe, and at which heart failure was
noted were significantly less for group 2 than for group 1.
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Unaffected x Unaffected Matings (Group 3)
Breeding one unaffected to another unaffected cat produced 2
unaffected cats. Both survived and were 3 years old at the time the
study terminated.
Assessment of Inheritance Pattern
The following findings are compatible with an autosomal dominant
mode of inheritance with 100% penetrance. Whenever an affected cat was
bred to another cat, at least 1 affected cat was produced and
successive generations were affected (Figure 1
). Nine affected
cats were male and 7 female (no sex predilection). Three females and 4
males were the affected parent in group 1 (males and females were
equally likely to transmit the disease). Several instances of
male-to-male transmission occurred. Of the 22 kittens born from
affected (A/a) to unaffected (a/a) matings (group 1), 12 (55%)
developed HCM (A/a). Four of the 9 kittens (45%) produced from mating
affected (A/a) to affected (A/a) cats (group 2) developed HCM (A/a) and
2 (22%; a/a) did not. Three stillborn fetuses (33%) (assumed to be
lethal homozygotes: A/A) were produced from these breedings. Neither
group 3 cat developed HCM.
Phenotypic Expression
Papillary muscle hypertrophy was the predominant and
most consistent abnormality noted. It produced
midventricular obstruction in some cats (Figures 5B
and 7
). Separating the hypertrophied
papillary muscles from adjacent hypertrophied myocardium
was sometimes difficult, but often a clear separation could be
identified (Figure 4
). The hypertrophy, however, was
often not confined to the papillary muscles. For example,
hypertrophy was often noted at the basilar portion of the
interventricular septum, a region remote from any papillary
muscle influence.
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LV wall thickness achieved a maximum and then stabilized in all cats,
as it does in humans.8 Left atrial size was normal in cats
with mild to moderate hypertrophy and when severe
hypertrophy was first identified. Left atrial size then
increased in the cats with severe hypertrophy. The
exception to this was 1 cat who had 1 massively hypertrophied papillary
muscle, no wall hypertrophy, and no left atrial enlargement
for 4 years (Figure 5
).
SAM was identified in most cats (13 of 16 cats) from the time papillary
muscle enlargement was noted. In 2 cats, the SAM disappeared after LV
hypertrophy developed. The blood flow velocity across the
region of dynamic SAS was highly variable within the population,
and within each cat, it was dependent on the level of excitement. An
attempt was made to excite each cat to increase flow velocity at the
time of examination, but these maneuvers were variably successful. The
maximum pressure gradient ranged from 10 to 92 mm Hg (51±26
mm Hg). Cats with moderate to severe disease also had
end-systolic cavity obliteration in the region of the LV cavity
that contained the papillary muscles (Figure 4C
).
Pathological Examination
Of the 8 animals that died, 7 had severe HCM. The heart weight was
29 to 37 g (normal is <20 g) in these cats.9 10 11 The
remaining cat died suddenly before exhibiting
echocardiographic evidence of HCM (heart weight,
24 g). The ratio of heart weight to body weight was severely
increased in the cats with severe HCM (8.1 to 10.6 g/kg; normal, 3.0 to
5.0 g/kg) and was mildly increased (6.0 g/kg) for the cat that died
suddenly without severe HCM. Maximal ventricular free wall
thickness was significantly increased to 10.7±1.3 mm (normal,
5.0±0.2 mm) and interventricular septal thickness to
9.6±0.7 mm (normal, 5.0±0.3 mm).11 Because
cardiac muscle often undergoes postmortem contracture, the wall
thicknesses at postmortem examination were often greater than those
measured by echocardiography.
Hypertrophy consistently involved the papillary
muscles along with substantial portions of the LV walls. Moderate to
marked (>5% of the tissue section) myofiber disarray (predominantly
type I) of the interventricular septum and LV free wall,
moderate to severe intramural coronary
arteriosclerosis, and moderate to severe
interstitial fibrosis were present in all cats (Figure 8
).12
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| Discussion |
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HCM in these cats appears to be inherited as an autosomal dominant trait, as it is in human FCHM, with penetrance that increases to 100% in adulthood. Although penetrance may not be 100% in some human families with FHCM, often it is complete, as in our cats.17 18 19
HCM in our colony of Maine coon cats has most of the common
morphological characteristics present in human FHCM. It is
characterized by gross LV wall thickening, dynamic LVOT obstruction,
myocardial fiber disarray, small coronary artery disease, and
myocardial fibrosis. The one difference is that the LV wall thickening
is more commonly confined to or more severe in the papillary muscles
and LV free wall than in the interventricular septum.
Asymmetric hypertrophy of the free wall has also been
identified in a murine transgenic model of HCM in which the
-myosin
heavy chain gene (MHCG) contains a mutation commonly identified in
humans with HCM.20 Consequently, this finding may be
purely a species difference in the expression of the disease. However,
a rare form of HCM involving primarily the papillary muscles exists in
humans and is caused by mutations in either the essential or regulatory
myosin light chains.21 It is characterized by
midventricular obstruction due to the papillary muscle
hypertrophy.
The natural history of FHCM in Maine coon cats is similar to human
FHCM, in which the disease commonly becomes evident during childhood or
adolescence and progressively worsens during young
adulthood.22 In our colony, most cats first had definitive
evidence of HCM between 8 and 24 months of age. The disease usually
reached its nadir between 1.5 and 3 years of age. Cats generally become
mature adults between 12 and 18 months of age, so these periods
correspond to adolescence and young adulthood. In a few female cats,
however, the disease was not evident until
3 years of age. The
progressive nature of the disease in Maine coon cats is also similar to
that seen in transgenic mice with a
-MHCG mutation in which HCM is
not demonstrable at 5 weeks old but is by 15 weeks.20
Sudden cardiac death is another feature common to both humans and Maine coon cats with HCM.23 Five of our 16 affected cats died suddenly. One 16-month-old cat without severe hypertrophy but with myocardial fiber disarray died suddenly, as has been described in humans.24
FHCM is caused by sarcomeric protein gene mutations in humans.1 Because the clinical, pathological, and heritable characteristics of our feline model are similar to those of human FHCM, it is possible that a sarcomeric gene mutation causes FHCM in this model. It is this targeted group of genes that we are currently screening for mutations. If a mutation could be identified, the model could then be used to test diagnostic methods and therapeutic strategies, including means to prevent the progression and complications of myocardial hypertrophy and to examine gene therapy.
As in transgenic mice, HCM in our cats progressed to severe HCM and death more commonly in males than in females.20 In 1 study, male transgenic mice were affected at an earlier age than females. In addition, left atrial enlargement was present in 7 of 11 mutant males but only 2 of 12 female 15-week-old mice and in all 7 males but only 6 of 18 female 30-week-old mice. More male mice also had myocardial fiber disarray. It appears that male influences in some way modify the phenotypic expression of HCM. This could occur through modulation at the gene level or could be due to known sex differences in myocardial performance.25
In the pedigree analysis, the pattern of inheritance was
consistent with an autosomal dominant trait when the stillborn
fetuses were considered lethal homozygotes. Only a few examples of
human homozygous individuals who have lived exist in the
literature.26 27 In contrast, mice that are homozygous for
an
-MHCG mutation consistently die before 7 days
old.25 It is well recognized that mutations on the same
gene, such as the ß-MHCG, alter the severity and age of onset of the
disease and result in different prognoses.28 Because the
type of mutation in a heterozygote determines longevity, it is
reasonable to assume that the type of mutation in a homozygote could
also influence survivability. If a mutation of a sarcomeric protein is
involved in Maine coon cats with HCM, it clearly results in malignant
disease. Consequently, one would expect to see markedly reduced
survival in cats homozygous for the mutation.
The expressivity of the disease in our colony was influenced by the type of breeding. When affected cats were bred to affected cats, their offspring developed HCM at an earlier age, always developed severe disease, and developed severe disease at an earlier age. One explanation is that these individuals were the homozygotes instead of the stillborn fetuses. If this were true, all of the affected cats in group 2 were homozygotes. This is statistically improbable. Another possible explanation is that modifying factors were altered by breeding affected to affected cats. For example, the extent of hypertrophy in humans with HCM is influenced by the angiotensin I converting enzyme insertion/deletion (I/D) polymorphism.29 Individuals with the DD genotype have severe hypertrophy, whereas individuals with the ID or II genotype have mild or no hypertrophy. It is possible that in our cats, this or another polymorphism was encountered in these 2 breedings by chance or that a polymorphism or other modifying factors are coinherited with the mutation responsible for HCM in these cats.
Although the papillary muscles were most consistently and severely hypertrophied, myocardial fiber disarray was often also found in the LV free wall and interventricular septum. A previous study in humans with HCM has shown that myocardial fiber disarray is not confined to the severely thickened regions of the LV.30
We conclude that HCM in Maine coon cats may be an appropriate model of human disease. Access to this model could benefit investigators interested in examining the gross, cellular, and molecular pathophysiology of HCM. The ability to selectively breed affected to unaffected individuals could provide clues regarding the natural history of the disease in humans, especially during the emerging phase of the disease in adolescence, and might provide clues about why the disease is not expressed at birth. It could also allow investigators to study the offspring of matings of affected to affected individuals, something that occurs only rarely in humans. The model might also be useful to determine why the hypertrophy in HCM is so often asymmetrical instead of symmetrical, as it is in a pressure overload.
In conclusion, HCM in Maine coon cats mimics the hereditary aspects, phenotypic expression, natural history, and pathological characteristics of the human familial form of the disease. Consequently, it is likely that FHCM in Maine coon cats is caused by a mutation identified in humans with HCM. This animal model of FHCM may be a valuable tool for studying the gross, cellular, and molecular pathophysiology of the disease.
| Acknowledgments |
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| Footnotes |
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Received July 29, 1998; revision received March 17, 1999; accepted March 24, 1999.
| References |
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