(Circulation. 1995;91:2302-2305.)
© 1995 American Heart Association, Inc.
Articles |
-Tropomyosin That Causes Hypertrophic Cardiomyopathy
From the Cardiology Division, Brigham and Women's Hospital, and Harvard Medical School, Boston, Mass (H.W., R.A., C.E.S.); the Department of Cardiological Sciences, St George's Hospital Medical School, London, England (H.W.); the Institute of Biology and Genetics, University of Genoa (Italy) (D.A.C.); the Division of Cardiology, Ospedali Galliera, Genoa, Italy (P.S.); and Howard Hughes Medical Institute and Department of Genetics, Harvard Medical School, Boston, Mass (J.G.S.).
Correspondence to Hugh Watkins, Department of Genetics, Harvard Medical School, 200 Longwood Ave, Boston, MA 02115.
| Abstract |
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-tropomyosin have been described that segregate with hypertrophic
cardiomyopathy in single families. To confirm that these mutations are
the cause of the disease, we have investigated the origins of one of
these mutations, Asp175Asn, in a third and unrelated family.
Methods and Results The presence or absence of an
-tropomyosin mutation and the haplotypes of the flanking chromosomal
regions were determined for members of a family with hypertrophic
cardiomyopathy. Haplotypes were constructed by use of an intragenic
polymorphism and 10 flanking polymorphisms spanning a region of 35
centimorgans. The Asp175Asn missense mutation was present in the
proband and his two affected offspring but not in any of the proband's
three siblings. Although both parents were deceased, the haplotypes of
the four parental chromosomes could be reconstructed. One parental
chromosome was transmitted to two offspring: one bearing the Asp175Asn
mutation (the affected proband) and one clinically unaffected sibling
who lacked the
-tropomyosin mutation. Thus, the Asp175Asn mutation
must have arisen de novo.
Conclusions De novo mutations in the
-tropomyosin gene can
result in hypertrophic cardiomyopathy that may appear to be sporadic
but in subsequent generations gives rise to familial disease.
Individuals with sporadic hypertrophic cardiomyopathy should be advised
of the risk of transmission to offspring. In addition, these findings
provide the strongest genetic evidence that mutations in the
-tropomyosin gene are directly responsible for hypertrophic
cardiomyopathy.
Key Words: cardiomyopathy hypertrophy genes
| Introduction |
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-tropomyosin gene
mutations have been identified in families with hypertrophic
cardiomyopathy. Each of the missense mutations (Asp175Asn and
Glu180Gly) affects a conserved residue and has not been found in the
genomes of unaffected individuals.1 These data suggest
that
-tropomyosin gene mutations are a cause of familial
hypertrophic cardiomyopathy. However, some features of the mutations in
the
-tropomyosin gene contrast with those of mutations in the other
disease genes for hypertrophic cardiomyopathy. Unlike the ß-cardiac
myosin heavy chain2 and cardiac troponin T
genes,1 the
-tropomyosin gene is expressed
ubiquitously, yet the disease phenotype is limited to cardiac muscle.
In addition, screening of the
-tropomyosin gene sequences has
revealed only one further mutation in more than 120 unrelated
individuals with hypertrophic cardiomyopathy; an unrelated proband (DB)
was identified with the Asp175Asn mutation.3 Because of
the remote possibility that these
-tropomyosin mutations are only
linked polymorphisms, further evidence demonstrating a cause-effect
relation with hypertrophic cardiomyopathy would be desirable.
Spontaneously arising, or de novo, germ-line mutations in a
postulated disease gene are sufficiently rare that when they coincide
with the new development of an inherited trait in an individual, this
provides strong evidence that the mutation causes the
trait.4 5 We therefore have studied the proband with
the
Asp175Asn mutation, along r with all available family members (family
DB), to determine whether the
-tropomyosin gene mutation arose de
novo. Such analyses typically require the availability of both parents
of an individual with sporadic disease to demonstrate that the
chromosome on which the mutation arises was previously normal and to
exclude nonpaternity as an alternative explanation. Although the
proband's parents were not available for study, by constructing
chromosomal haplotypes in the extended pedigree, we are able to
demonstrate that the Asp175Asn mutation the proband carries did arise
de novo. Demonstration of a de novo occurrence in a family with
hypertrophic cardiomyopathy of a mutation previously seen to segregate
with disease provides the strongest evidence that the
-tropomyosin
gene is itself a disease gene for hypertrophic cardiomyopathy.
| Methods |
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Detection of the Asp175Asn Mutation
Exon 5 of the
-tropomyosin gene was amplified from genomic
DNA of each family member by polymerase chain reaction (PCR) as
described.1 The presence or absence of the G
A
transition that encodes the Asp175Asn mutation was determined in the
PCR-amplified DNA of each individual by cycle
sequencing.1
Analysis of Short Tandem Repeat Polymorphisms
Short tandem
repeat (STR) polymorphisms were typed to identify
each chromosome by definition of the alleles present at each locus
(ie, to define the haplotype of each chromosome). The previously
described intragenic STR polymorphism,
HTM
CA,1 was used, along with 10
flanking polymorphic markers. These polymorphisms were selected for
their high heterozygosity and appropriate map locations as indicated by
our analyses7 and published linkage maps of chromosome
15.8 9 10 11 Alleles were
typed by PCR amplification with end
labeling of the forward primer, followed by denaturing PAGE, as
described.1 All STR polymorphisms were run against
sequencing ladders to allow precise identification of individual
alleles by size.
Construction of Haplotypes
STR polymorphisms were typed at
increasing genetic distances
from the
-tropomyosin gene to delineate the extent of the
chromosomal region inherited without recombination in family DB. Once
the alleles carried by each family member at each polymorphism were
identified, the alleles specific to each parental chromosome were
deduced by inspection of inheritance patterns in nuclear families. The
order assumed for the STR markers was based on published map
data8 9 10 11 and the
recombinations observed in our own data
(Reference 6 and unpublished data). This order was (from centromeric to
telomeric) D15S118, THBS, CYP19, D15S126, D15S209, D15S98, D15S117,
HTM
CA, D15S159, D15S108, D15S125. STR loci within
two clusters (D15S98-D15S117 and HTM
CA-D15S159-D15S108)
were inseparable from each other, and the order used was assigned
arbitrarily. Because no recombination events were seen between any of
these STR loci in the four chromosomes inherited from the grandparents,
the actual order is not of consequence for these analyses.
Confirmation of Paternity
The possibility that individuals
with the same haplotype had
inherited these chromosomes from different fathers was assessed by
estimation of the probability of an identical match by chance. To
minimize the unknown effects of linkage disequilibrium, probabilities
were calculated only for loci separated by a minimum of 3 centimorgans
(cM). Allele frequencies were taken from published data.
| Results |
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Mutation Analysis
The aspartate-to-asparagine substitution in
exon 5 of the
-tropomyosin gene (Asp175Asn, numbered according to Reference 15),
which was previously identified in the proband, results from a
guanine-to-adenine transition at nucleotide residue 579 (G579A). Direct
sequencing of exon 5 of the
-tropomyosin gene was performed on
amplified DNA from all available first-degree relatives. The G579A
mutation was present in the proband and his two affected sons and
was absent in all other family members (Fig 2
). Thus,
the diagnoses of disease status based on genetic analyses were
concordant with the clinical studies. Of particular importance for this
study was the finding that individual III-4 (the proband's brother)
did not carry the Asp175Asn mutation.
|
Haplotype Analysis
Haplotype analysis was performed to define
the extent of the
chromosome 15 region surrounding the
-tropomyosin gene that was
shared by each family member. Analysis of the alleles present at
each STR locus in the nuclear family comprising the proband, his wife,
and his children (III-2, III-3, IV-1, IV-2, and IV-3) indicates that
the three affected individuals share the same haplotype:
2.9.2.2.2.1.7.2.1.3.5, designated haplotype A (Fig 1A
).
Analysis of the
alleles present in the proband's three siblings (III-4, III-5, and
III-7) indicates the other haplotypes (B, C, and D) that together
identify each of the four chromosomes inherited from the grandparents
(II-3 and II-4). No recombination events affect the inheritance of
these chromosomes within the region identified by the 11 STR loci
(approximately 35
cM,8 9 10 11 Fig
1B
). Only the configuration
of haplotypes in Fig 1
fitted the observed data; no other
permutation
was compatible with the inheritance patterns seen.
Haplotype A, which
identifies the chromosome carrying the Asp175Asn
mutation in the three genetically and clinically affected individuals,
was also inherited by individual III-4 (Fig 1A
). Individual
III-4 is an
unaffected male who does not have the Asp175Asn mutation. One possible
explanation for the differences at the Asp175 residue between
individuals III-2 and III-4 is that they have different fathers who by
chance have transmitted an identical haplotype. The probability that
different fathers might transmit identical alleles at each marker was
estimated from the published frequencies of the specific alleles.
Because particular combinations of alleles at closely linked loci might
be coinherited, we used data from polymorphisms spaced across the
region and excluded data from loci tightly linked to each other (Fig
1B
; see the "Methods" section). Based on the
frequencies of
alleles at eight loci spaced across 35 cM, the chance that individuals
III-2 and III-4 would inherit the identical haplotype if they had
different fathers is approximately 1 in 1 million. Further, 10 very
highly polymorphic STR markers from other chromosomes were typed and
did not reveal evidence of nonpaternity (data not shown). We conclude
that nonpaternity does not explain the discordant finding of a shared
haplotype (A) in two brothers but an
-tropomyosin mutation in only
one. Because haplotypes A and B together identify one parent (either
individual II-3 or II-4) and haplotype B is present in a relative
on the paternal side, we can further conclude that the chromosome with
haplotype A was inherited from the proband's father, individual
II-3.
| Discussion |
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-tropomyosin gene to an
unaffected child (III-4) and, on the same segment of chromosome 15, a
mutated copy to another son (III-2), who developed hypertrophic
cardiomyopathy. This observation demonstrates that a de novo
-tropomyosin gene mutation, Asp175Asn, occurred in individual III-2,
who has hypertrophic cardiomyopathy (Fig 1A
The Asp175Asn mutation in the
-tropomyosin gene of individual
III-2 was present in three different tissues: cardiac myocytes,
lymphocytes, and gametes. This suggests that the mutation arose either
in development of the gamete (ie, during spermatogenesis in the father,
II-3) or very early in embryonic development. We cannot determine
whether the proband's father (individual II-3) exhibited germ-line
mosaicism for this mutation; individual II-3 is deceased, and only the
proband and his unaffected brother inherited the relevant copy of the
-tropomyosin gene (haplotype A, Fig 1
). Individual III-2
did
transmit the mutation to his two sons (IV-1 and IV-3). Three examples
of de novo germ-line mutations in the ß-cardiac myosin heavy chain
gene have also been reported in individuals with apparent sporadic
hypertrophic cardiomyopathy.16 17 The finding of a de
novo
mutation in the germ line of proband III-3 again raises concerns for
individuals diagnosed with sporadic hypertrophic cardiomyopathy. These
individuals should be advised that they may transmit the disease to
their offspring.
Because mutations in many genes can cause hypertrophic cardiomyopathy
(ß-cardiac myosin heavy chain,2 cardiac troponin
T,1
-tropomyosin, and an unknown gene on chromosome
1118 ), linkage analysis is commonly the first step in
the genetic investigation of a newly ascertained family. In families of
sufficient size, this is an efficient way to target the search for a
mutation to the appropriate disease gene. However, in a pedigree with
an unrecognized instance of new mutation, linkage data would suggest
crossovers, leading to a false exclusion of the disease gene.
Recognition of the possibility of a de novo mutation will allow correct
interpretation of the linkage data in a kindred such as family DB.
Two of three known
-tropomyosin gene mutations involve independent
occurrences of the same nucleotide alteration. While this may reflect
an increased tendency to mutation at this particular residue (a G
A
transition at a CpG dinucleotide), an alternative explanation is that
the Asp 175 residue is of specific importance to
-tropomyosin
function in the heart. Analysis of the functional consequences of this
mutation may explain the cardiac-specific phenotype. The demonstration
that the Asp175Asn mutation has occurred independently in two families
with hypertrophic cardiomyopathy is analogous to the finding of
recurrent identical mutation in the ß-cardiac myosin heavy chain and
cardiac troponin T genes.3 19 Thus, the majority of
disease-causing mutations in individuals and families with hypertrophic
cardiomyopathy have arisen de novo rather than from a common ancestor.
This is consistent with the negative selective pressure expected for a
condition associated with premature mortality.
| Acknowledgments |
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Received January 3, 1995; accepted January 13, 1995.
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[Order article via Infotrieve]
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