Altered expression and splicing of Ca(2+) metabolism genes in myotonic dystrophies DM1 and DM2

AIMS

Myotonic dystrophy types 1 and 2 (DM1 and DM2) are multisystem disorders caused by similar repeat expansion mutations, with similar yet distinct clinical features. Aberrant splicing of multiple effector genes, as well as dysregulation of transcription and translation, has been suggested to underlie different aspects of the complex phenotypes in DM1 and DM2. Ca(2+) plays a central role in both muscle contraction and control of gene expression, and recent expression profiling studies have indicated major perturbations of the Ca(2+) signalling pathways in DM. Here we have further investigated the expression of genes and proteins involved in Ca(2+) metabolism in DM patients, including Ca(2+) channels and Ca(2+) binding proteins.

METHODS

We used patient muscle biopsies to analyse mRNA expression and splicing of genes by microarray expression profiling and RT-PCR. We studied protein expression by immunohistochemistry and immunoblotting.

RESULTS

Most of the genes studied showed mRNA up-regulation in expression profiling. When analysed by immunohistochemistry the Ca(2+) release channel ryanodine receptor was reduced in DM1 and DM2, as was calsequestrin 2, a sarcoplasmic reticulum lumen Ca(2+) storage protein. Abnormal splicing of ATP2A1 was more pronounced in DM2 than DM1.

CONCLUSIONS

We observed abnormal mRNA and protein expression in DM affecting several proteins involved in Ca(2+) metabolism, with some differences between DM1 and DM2. Our protein expression studies are suggestive of a post-transcriptional defect(s) in the myotonic dystrophies.

Premutation allele pool in myotonic dystrophy type 2

BACKGROUND

The myotonic dystrophies (DM1, DM2) are the most common adult muscle diseases and are characterized by multisystem involvement. DM1 has been described in diverse populations, whereas DM2 seems to occur primarily in European Caucasians. Both are caused by the expression of expanded microsatellite repeats. In DM1, there is a reservoir of premutation alleles; however, there have been no reported premutation alleles for DM2. The (CCTG)(DM2) expansion is part of a complex polymorphic repeat tract of the form (TG)(n)(TCTG)(n)(CCTG)(n)(NCTG)(n)(CCTG)(n). Expansions are as large as 40 kb, with the expanded (CCTG)(n) motif uninterrupted. Reported normal alleles have up to (CCTG)(26) with one or more interruptions.

METHODS

To identify and characterize potential DM2 premutation alleles, we cloned and sequenced 43 alleles from 23 individuals. Uninterrupted alleles were identified, and their instability was confirmed by small-pool PCR. We determined the genotype of a nearby single nucleotide polymorphism (rs1871922) known to be in linkage disequilibrium with the DM2 mutation.

RESULTS

We identified three classes of large non-DM2 repeat alleles: 1) up to (CCTG)(24) with two interruptions, 2) up to (CCTG)(32) with up to four interruptions, and 3) uninterrupted (CCTG)(22-33). Large non-DM2 alleles were more common in African Americans than in European Caucasians. Uninterrupted alleles were significantly more unstable than interrupted alleles (p = 10(-4) to 10(-7)). Genotypes at rs1871922 were consistent with the hypothesis that all large alleles occur on the same haplotype as the DM2 expansion.

CONCLUSIONS

We conclude that unstable uninterrupted (CCTG)(22-33) alleles represent a premutation allele pool for DM2 full mutations.

New methods for molecular diagnosis and demonstration of the (CCTG)n mutation in myotonic dystrophy type 2 (DM2)

Myotonic dystrophy types 1 and 2 are autosomal dominant, multisystemic disorders with many similarities in their clinical manifestations. Myotonic dystrophy type 1 is caused by a (CTG)n expansion in the 3' untranslated region of the DMPK gene in 19q13.3 and myotonic dystrophy type 2 by a (CCTG)n expansion in intron 1 of ZNF9 in 3q21.3. However, the clinical diagnosis of myotonic dystrophy type 2 is more complex than that of myotonic dystrophy type 1, and conventional molecular genetic methods used for diagnosing myotonic dystrophy type 1 are insufficient for myotonic dystrophy type 2. Herein we describe two in situ hybridization protocols for the myotonic dystrophy type 2 mutation detection. Chromogenic in situ hybridization was used to detect both the genomic expansion and the mutant transcripts in muscle biopsy sections. Chromogenic in situ hybridization can be used in routine myotonic dystrophy type 2 diagnostics. Fluorescence in situ hybridization on extended DNA fibers was used to directly visualize the myotonic dystrophy type 2 mutation and to estimate the repeat expansion sizes.

Novel cardiac troponin T mutation as a cause of familial dilated cardiomyopathy

BACKGROUND

Familial dilated cardiomyopathy (FDCM) and hypertrophic cardiomyopathy (FHCM) are the 2 most common forms of primary cardiac muscle diseases. Studies indicate that mutations in sarcomeric proteins are responsible for FHCM and suggest that mutations in cytoskeletal proteins cause FDCM. Evidence is evolving, however, that such conclusions are premature.

METHODS AND RESULTS

A novel missense mutation in the cardiac troponin T gene was identified by direct sequencing and confirmed by endonuclease restriction analysis in a large family with FDCM that we had previously mapped to chromosome 1q32. The mutation substitutes tryptophan for a highly conserved amino acid, arginine, at amino acid residue 141 (Arg141Trp). The mutation occurs within the tropomyosin-binding domain of cardiac troponin T and alters the charge of the residue. This mutation cosegregates with the disease, being present in all 14 living affected individuals. The mutation was not found in 100 normal control subjects. Clinical features were congestive heart failure with premature deaths. The age of onset and severity of the disease are highly variable, with incomplete penetrance. Because 15 mutations in troponin T are known to cause FHCM, 219 probands with FHCM were screened, and none had the mutation.

CONCLUSIONS

Thus, the novel cardiac troponin T mutation Arg141Trp is responsible for FDCM in our family. Because several mutations in troponin T have already been recognized to be responsible for FHCM, it appears that the phenotype, whether it be hypertrophy or dilatation, is determined by the specific mutation rather than the gene.

Tissue Doppler imaging consistently detects myocardial abnormalities in patients with hypertrophic cardiomyopathy and provides a novel means for an early diagnosis before and independently of hypertrophy

BACKGROUND

Left ventricular hypertrophy (LVH), the clinical hallmark of familial hypertrophic cardiomyopathy (FHCM), is absent in a significant number of subjects with causal mutations. In transgenic rabbits that fully recapitulate the FHCM phenotype, reduced myocardial tissue Doppler (TD) velocities accurately identified the mutant rabbits, even in the absence of LVH. We tested whether humans with FHCM also consistently showed reduced myocardial TD velocities, irrespective of LVH.

METHODS AND RESULTS

We performed 2D and Doppler echocardiography and TD imaging in 30 subjects with FHCM, 13 subjects who were positive for various mutations but did not have LVH, and 30 age- and sex-matched controls (all adults; 77% women). LV wall thickness and mass were significantly greater in FHCM subjects (P<0.01 versus those without LVH and controls). There were no significant differences in 2D echocardiographic, mitral, and pulmonary venous flow indices between mutation-positives without LVH and controls. In contrast, systolic and early diastolic TD velocities were significantly lower in both mutation-positives without LVH and in FHCM patients than in controls (P<0.001). Reduced TD velocities had a sensitivity of 100% and a specificity of 93% for identifying mutation-positives without LVH.

CONCLUSIONS

Myocardial contraction and relaxation velocities, detected by TD imaging, are reduced in FHCM, including in those without LVH. Before and independently of LVH, TD imaging is an accurate and sensitive method for identifying subjects who are positive for FHCM mutations.

Genomic organization and isoform-specific tissue expression of human NAPOR (CUGBP2) as a candidate gene for familial arrhythmogenic right ventricular dysplasia

Neuroblastoma apoptosis-related RNA-binding protein (NAPOR; HGMW-approved symbol CUGBP2) is a newly discovered gene prominently induced during apoptosis, suggesting that it plays a role during apoptosis. We have found that it is encoded by a gene located on chromosome 10p13-p14 between Généthon markers D10S547 and D10S223, a region to which we have recently localized a gene responsible for arrhythmogenic right ventricular dysplasia (ARVD). To examine its possible role in the pathogenesis of ARVD, we determined the genomic organization of the human NAPOR gene including its exon-intron boundaries and the putative promoter sequence, which provide a plausible mechanism for its alternative mRNA splicing. We also demonstrated that three isoforms of the NAPOR transcript were differently expressed, with NAPOR-3 being nearly neuron specific while the other two forms were ubiquitously expressed. The expression of NAPOR is differentially regulated during development. Finally, we screened the members of the ARVD family for mutations and identified two DNA sequence variants in the protein-coding exons of NAPOR, neither of which was responsible for ARVD. While the function of NAPOR remains to be elucidated, our current characterization of the NAPOR gene will be valuable for further clinical and functional study.

Identification of a gene responsible for familial Wolff-Parkinson-White syndrome

BACKGROUND

The Wolff-Parkinson-White syndrome, with a prevalence in Western countries of 1.5 to 3.1 per 1000 persons, causes considerable morbidity and may cause sudden death. We identified two families in which the Wolff-Parkinson-White syndrome segregated as an autosomal dominant disorder.

METHODS

We studied 70 members of the two families (57 in Family 1 and 13 in Family 2). The subjects underwent 12-lead electrocardiography and two-dimensional echocardiography. Genotyping mapped the gene responsible to 7q34-q36, a locus previously identified to be responsible for an inherited form of Wolff-Parkinson-White syndrome. Candidate genes were identified, sequenced, and analyzed in normal and affected family members to identify the disease-causing gene.

RESULTS

A total of 31 members (23 from Family 1 and 8 from Family 2) had the Wolff-Parkinson-White syndrome. Affected members of both families had ventricular preexcitation with conduction abnormalities and cardiac hypertrophy. The maximal combined two-point lod score was 9.82 at a distance of 5 cM from marker D7S636, which confirmed the linkage of the gene in both families to 7q34-q36. Haplotype analysis indicated that there were no alleles in common in the two families at this locus, suggesting that the two families do not have a common founder. We identified a missense mutation in the gene that encodes the gamma2 regulatory subunit of AMP-activated protein kinase (PRKAG2). The mutation results in the substitution of glutamine for arginine at residue 302 in the protein.

CONCLUSIONS

The identification of this genetic defect has important implications for elucidating the pathogenesis of ventricular preexcitation. Further understanding of how this molecular defect leads to supraventricular arrhythmias could influence the development of specific therapies for other forms of supraventricular arrhythmia.

Hypertrophic cardiomyopathy caused by a novel alpha-tropomyosin mutation (V95A) is associated with mild cardiac phenotype, abnormal calcium binding to troponin, abnormal myosin cycling, and poor prognosis

BACKGROUND

We report hypertrophic cardiomyopathy (HCM) in a Spanish-American family caused by a novel alpha-tropomyosin (TPM1) mutation and examine the pathogenesis of the clinical disease by characterizing functional defects in the purified mutant protein.

METHODS AND RESULTS

HCM was linked to the TPM1 gene (logarithm of the odds [LOD] score 3.17). Sequencing and restriction digestion analysis demonstrated a TPM1 mutation V95A that cosegregated with HCM. The mutation has been associated with 13 deaths in 26 affected members (11 sudden deaths and 2 related to heart failure), with a cumulative survival rate of 73+/-10% at the age of 40 years. Left ventricular wall thickness (mean 16+/-6 mm) and disease penetrance (53%) were similar to those for the ss-myosin mutations L908V and G256E previously associated with a benign prognosis. Left ventricular hypertrophy was milder than with the ss-myosin mutation R403Q, but the prognosis was similarly poor. With the use of recombinant tropomyosins, we identified several functional alterations at the protein level. The mutation caused a 40% to 50% increase in calcium affinity in regulated thin filament-myosin subfragment-1 (S1) MgATPase assays, a 20% decrease in MgATPase rates in the presence of saturating calcium, a 5% decrease in unloaded shortening velocity in in vitro motility assays, and no change in cooperative myosin S1 binding to regulated thin filaments.

CONCLUSIONS

In contrast to other reported TPM1 mutations, V95A-associated HCM exhibits unusual features of mild phenotype but poor prognosis. Both myosin cycling and calcium binding to troponin are abnormal in the presence of the mutant tropomyosin. The genetic diagnosis afforded by this mutation will be valuable in the management of HCM.

Identification and characterization of a novel gene (C4orf5) located on human chromosome 4q with specific expression in cardiac and skeletal muscle

The loci of several genes responsible for arrhythmogenic right ventricular dysplasia (ARVD) have been mapped. Since ARVD involves the right ventricle, we sought candidate genes preferentially expressed in the right ventricle utilizing differential display polymerase chain reaction (PCR) on mRNA from the chambers of an adult human heart. PCR products were cloned, sequenced, and used to screen an adult heart cDNA library. A novel 1.3-kb cDNA (HGMW-approved symbol C4orf5) with an open reading frame of 795 bp was identified. A probe designed from the 3' untranslated region of the 1.3-kb cDNA was hybridized to the 1.3-kb transcript and an alternatively spliced 2.5-kb transcript in the heart and skeletal muscle RNA lanes on a multitissue Northern blot. Analysis of a 39-kb partial genomic sequence identified three intronic splice sites in the 1.3-kb transcript. The gene was mapped to human chromosome 4q26-q27. Computer-based analysis indicated that this gene is novel with no known function.

Human protein tyrosine phosphatase-like gene: expression profile, genomic structure, and mutation analysis in families with ARVD

The mouse protein tyrosine phosphatase-like gene (Ptpla) was recently cloned and data suggested that it plays a role in myogenesis and cardiogenesis. The human homologue (PTPLA) was mapped to chromosome 10p13-14, a region where we have mapped a locus responsible for arrhythmogenic right ventricular dysplasia (ARVD). As a positional candidate gene, we characterized PTPLA by determining its tissue expression, its genomic structure, and we also screened for mutations in the ARVD patients. Northern analysis demonstrated PTPLA is preferentially expressed in both adult and fetal heart. A much lower expression was detected in skeletal and smooth muscle tissues. Virtually no expression was observed in other tissues. The protein-encoding sequences of PTPLA consist of seven exons. A sequence variation (Lys64Gln) was found in all the affecteds in a large ARVD family. However, the same variant was also detected in normal control subjects (three alleles/100 chromosomes). Thus, the variant (Lys64Gln) is not responsible for ARVD in our family and is a benign polymorphism. Nevertheless, its tissue-specific expression in the developing and adult heart suggest PTPLA has a role in regulating cardiac development, differentiation, or other cellular events. The genomic structure and intragenic polymorphism of PTPLA should be useful for further clinical and genetic studies such as gene targeting of PTPLA.

Molecular cloning, expression analysis, and chromosome mapping of WDR6, a novel human WD-repeat gene

The WD-repeat proteins are found in all eukaryotes and play an important role in the regulation of a wide variety of cellular functions such as signal transduction, transcription, and proliferation. Here we report on the cloning and characterization of a novel human WD-repeat gene, WDR6, which encodes a protein of 1121 amino acids and contains 11 WD-repeat units. WDR6 is unique since its 11 WD repeats are clustered into two distinct groups separated by a putative transmembrane domain. The WDR6 gene was mapped to chromosome 15q21 by fluorescence in situ hybridization. Northern analysis demonstrated that WDR6 is ubiquitously expressed in human adult and fetal tissues. WDR6 is not homologous to any previously identified human WD-repeat genes including WDR1 through WDR5. However, it was found to have significant sequence similarity with Arabidopsis thaliana hypothetical protein T7B11.12, yeast putative elongation factor G, and probable membrane protein YPL183c. All of them have been defined as WD-repeat proteins. Therefore, WDR6 is a novel protein and probably belongs to a highly conserved subfamily of WD-repeat proteins in which T7B11.12 and YPL183c are its distantly related members.

The locus of a novel gene responsible for arrhythmogenic right-ventricular dysplasia characterized by early onset and high penetrance maps to chromosome 10p12-p14

Arrhythmogenic right-ventricular dysplasia (ARVD), a cardiomyopathy inherited as an autosomal-dominant disease, is characterized by fibro-fatty infiltration of the right-ventricular myocardium. Four loci for ARVD have been mapped in the Italian population, and recently the first locus was mapped in inhabitants of North America. None of the genes have been identified. We have now identified another North American family with early onset of ARVD and high penetrance. All of the children with the disease haplotype had pathological or clinical evidence of the disease at age <10 years. The family spans five generations, having 10 living and 2 dead affected individuals, with ARVD segregating as an autosomal-dominant disorder. Genetic linkage analysis excluded known loci, and a novel locus was identified on chromosome 10p12-p14. A peak two-point LOD score of 3.92 was obtained with marker D10S1664, at a recombination fraction of 0. Additional genotyping and haplotype analysis identified a shared region of 10.6 cM between marker D10S547 and D10S1653. Thus, a novel gene responsible for ARVD resides on the short arm of chromosome 10. This disease is intriguing, since it initiates exclusively in the right ventricle and exhibits pathological features of apoptosis. Chromosomal localization of the ARVD gene is the first step in identification of the genetic defect and the unraveling of the molecular basis responsible for the pathogenesis of the disease.

Desmin mutation responsible for idiopathic dilated cardiomyopathy

BACKGROUND

Idiopathic dilated cardiomyopathy, of which approximately 20% of cases are familial (FDCM), is a primary myocardial disorder characterized by ventricular dilatation and impaired systolic function. It is a common cause of heart failure and the need for cardiac transplantation. Although 6 chromosomal loci responsible for autosomal dominant FDCM have been mapped by linkage analysis, none of these genes have been identified. By use of the candidate-gene approach, actin was identified recently as being responsible for dilated cardiomyopathy. Considerable evidence suggests desmin, a muscle-specific intermediate filament, plays a significant role in cardiac growth and development.

METHODS AND RESULTS

To determine whether a defect of desmin induces dilated cardiomyopathy, 44 probands with FDCM underwent clinical evaluation and DNA analysis. Diagnostic criteria, detected by echocardiography, consisted of ventricular dimension of >/=2.7 cm/m(2) with an ejection fraction </=50% in the absence of other potential causes. After amplification by polymerase chain reaction, the exons of the desmin gene were sequenced. A missense desmin mutation, Ile451Met, which cosegregates with FDCM without clinically evident skeletal muscle abnormalities, was identified in a 4-generation family but was not detected in 460 unrelated healthy individuals.

CONCLUSIONS

A novel missense mutation of desmin, Ile451Met, was identified as the genetic cause of idiopathic dilated cardiomyopathy. This finding is of particular significance because this is the first mutation detected in the desmin tail domain, and the function of the desmin tail remains unknown. Because this mutation leads to a restricted cardiac phenotype in the family studied in the present report, it suggests that the tail of desmin plays an important functional role in cardiac tissue.

Mapping of the gene for a novel spinocerebellar ataxia with pure cerebellar signs and epilepsy

We investigated a family with a new type of autosomal dominant cerebellar ataxia (ADCA) in which pure cerebellar ataxia is often accompanied with epilepsy. No CAG repeat expansions were detected at the spinocerebellar ataxia (SCA) type 1, 2, 3, 6, or 7 locus, and SCAs 4 and 5 were excluded by linkage analysis. We found linkage between the disease locus and D22S274 (Zmax = 3.86 at theta = 0.00) and two other makers in 22q13-qter. Haplotype analysis of the crossover events and the multipoint linkage mapping localized the disease locus to an 8.8-cM region between D22S1177 and D22S1160.

Localization of a gene responsible for arrhythmogenic right ventricular dysplasia to chromosome 3p23

BACKGROUND

Arrhythmogenic right ventricular dysplasia (ARVD), a familial cardiomyopathy occurring with a prevalence of 1 in 5000, is characterized by replacement of myocytes with fatty and fibrous tissue. Clinical manifestations include structural and functional abnormalities of the right ventricle and arrhythmias, leading to a sudden death rate of 2.5% per year. Four loci have been mapped, but no gene has been identified as yet.

METHODS AND RESULTS

We identified a large family of >200 members with ARVD segregating as an autosomal dominant trait affecting 10 living individuals. The diagnosis of ARVD was based on international diagnostic criteria including history, physical examination, ECG, echocardiogram, right ventricular angiogram, endomyocardial biopsy, and 24-hour ambulatory ECG. Blood was collected for DNA from 149 family members. Analysis of 257 polymorphic microsatellite markers by genetic linkage excluded previously known loci for ARVD and identified a novel locus at 3p23. Analysis of an additional 20 markers further defined the region. A peak logarithm of the odds score of 6.91 was obtained with marker D3S3613 at theta=0% recombination. Haplotype analysis identified a shared region between markers D3S3610 and D3S3659 of 9. 3 cM.

CONCLUSIONS

A novel locus for ARVD has been mapped to 3p23 and the region narrowed to 9.3 cM. Identification of the gene will allow genetic screening and a specific diagnosis for a disease with protean nonspecific findings. It should also provide insight fundamental to understanding cardiac chamber-specific gene expression and/or the mechanism of myocyte apoptosis observed in this disease.

New theories. Causes of dilated cardiomyopathy

Dilated cardiomyopathy is a heterogeneous disease, both clinically and genetically. Two genes responsible for X-linked DCM have been identified. Five genetic loci responsible for X-linked DCM have been identified. Five genetic loci responsible for autosomal dominant DCM have also been mapped but no genes identified so far. New paradigms may be necessary in order to elucidate the etiology of primary dilated cardiomyopathy.

Mutations in the gene for cardiac myosin-binding protein C and late-onset familial hypertrophic cardiomyopathy

BACKGROUND

Mutations in the gene for cardiac myosin-binding protein C account for approximately 15 percent of cases of familial hypertrophic cardiomyopathy. The spectrum of disease-causing mutations and the associated clinical features of these gene defects are unknown.

METHODS

DNA sequences encoding cardiac myosin-binding protein C were determined in unrelated patients with familial hypertrophic cardiomyopathy. Mutations were found in 16 probands, who had 574 family members at risk of inheriting these defects. The genotypes of these family members were determined, and the clinical status of 212 family members with mutations in the gene for cardiac myosin-binding protein C was assessed.

RESULTS

Twelve novel mutations were identified in probands from 16 families. Four were missense mutations; eight defects (insertions, deletions, and splice mutations) were predicted to truncate cardiac myosin-binding protein C. The clinical expression of either missense or truncation mutations was similar to that observed for other genetic causes of hypertrophic cardiomyopathy, but the age at onset of the disease differed markedly. Only 58 percent of adults under the age of 50 years who had a mutation in the cardiac myosin-binding protein C gene (68 of 117 patients) had cardiac hypertrophy; disease penetrance remained incomplete through the age of 60 years. Survival was generally better than that observed among patients with hypertrophic cardiomyopathy caused by other mutations in the genes for sarcomere proteins. Most deaths due to cardiac causes in these families occurred suddenly.

CONCLUSIONS

The clinical expression of mutations in the gene for cardiac myosin-binding protein C is often delayed until middle age or old age. Delayed expression of cardiac hypertrophy and a favorable clinical course may hinder recognition of the heritable nature of mutations in the cardiac myosin-binding protein C gene. Clinical screening in adult life may be warranted for members of families characterized by hypertrophic cardiomyopathy.

Identification of a genetic locus for familial atrial fibrillation

BACKGROUND

Atrial fibrillation, the most common sustained cardiac-rhythm disturbance, affects over 2 million Americans and accounts for one third of all strokes in patients over 65 years of age. The molecular basis for atrial fibrillation is unknown, and palliative therapy is used to control the ventricular rate and prevent systemic emboli. We identified a family of 26 members of whom 10 had atrial fibrillation which segregated as an autosomal dominant disease. We subsequently identified two additional families in which the disease was linked to the same locus.

METHODS

We screened the human genome with 300 polymorphic dinucleotide-repeat markers using an unconventional strategy of pooling the DNA samples into two groups (affected and unaffected), which reduced the sample size by approximately 90 percent, before performing linkage analysis to map the locus. This made it possible to identify potential loci within a few weeks.

RESULTS

The lod scores for markers D10S569 and D10S607, located at 10q22-q24, were 3.60 in Family 1. The disease locus in Families 2 and 3 was also linked to the same markers, with lod scores of 6.02 and 5.35 for markers D10S569 and D10S607, respectively, when data on all three families were combined. Haplotype analysis of the three families showed that the locus was between D10S1694 and D10S1786, an interval of 11.3 centimorgans.

CONCLUSIONS

Identification of the gene for familial atrial fibrillation will help to elucidate the molecular basis of the disease and provide insights into acquired forms. The strategy of pooling DNA samples for analysis is more time and cost effective than conventional screening and should accelerate the process of gene mapping in the future.

Polymorphic trinucleotide repeat in the MEF2A gene at 15q26 is not expanded in familial cardiomyopathies

A trinucleotide repeat polymorphism in the MEF2A gene is described. MEF2A is expressed early in cardiac muscle development; thus the possibility of linkage between this polymorphism and familial cardiomyopathies was investigated in three families not linked to genes coding for known sarcomeric proteins. MEF2A was excluded as a candidate for dilated cardiomyopathy (DCM)(LOD of -9.03) and hypertrophic cardiomyopathy (HCM)(LODs of -5.43 and -2.44) in these families. Because expansion of triplet repeats has been shown to be responsible for several inherited diseases, 121 unrelated HCM probands and 28 unrelated DCM probands were examined for evidence of expansion of this repeat. No expansion of this trinucleotide repeat was seen in any of the 149 cardiomyopathy probands.

Familial hypertrophic cardiomyopathy: diagnostic and therapeutic implications of recent genetic studies

Familial hypertrophic cardiomyopathy is the first primary cardiomyopathy to have yielded to the techniques of modern molecular genetics. In the past few years, four genes responsible for this disease have been identified, all of which code for sarcomeric structural proteins. In addition, structure-function analysis and genotype-phenotype correlation studies have shed significant light on the molecular basis of this disease. It is hoped that within the next few years the application of molecular genetic tools will not only facilitate the diagnosis of hypertrophic cardiomyopathy but will also provide prognostic and therapeutic stratification for more definitive therapy.

Localization of a gene responsible for familial dilated cardiomyopathy to chromosome 1q32

BACKGROUND

Dilated cardiomyopathy, characterized by ventricular dilatation and decreased systolic contraction, is twofold to threefold more common as a cause of heart failure than hypertrophic cardiomyopathy and costs several billion dollars annually. The idiopathic form occurring early in life, with a 75% mortality in 5 years, is a common reason for transplantation. It is estimated that at least 20% of cases are familial.

METHODS AND RESULTS

A family of 46 members spanning four generations underwent history and physical examinations, echocardiographic analysis, and blood sampling for genotyping. Diagnostic criteria, detected by echocardiography, consisted of ventricular dimension of > or = 2.7 cm/m2 with an ejection fraction < or = 50% in the absence of other potential causes. DNA from all members was analyzed by polymerase chain reaction for amplification of short tandem-repeat polymorphic markers located every 10 cM throughout the human genome. Assuming a penetrance of 90%, linkage analysis was performed to map the responsible chromosomal locus. Linkage analysis, after 412 markers were analyzed, indicated the locus to be on chromosome 1q32, with a peak multipoint logarithm of the odds score at D1S414 of 6.37.

CONCLUSIONS

The locus identified in this study for familial dilated cardiomyopathy, 1q32, is rich in candidate genes, such as MEF-2, renin, and helix loop helix DNA binding protein MYF-4. Identification of the genetic defect could provide insight into the molecular basis for the cardiac dilatory response in both familial and acquired disorders.

An informative panel of somatic cell hybrids for physical mapping on human chromosome 19q

A panel of 22 somatic cell hybrids divides the q arm of human chromosome 19 into 22 ordered subregions. The panel was characterized with respect to 41 genetic markers. In most cases, a single fragment of chromosome 19 was present in each hybrid. In two cell lines the presence of multiple fragments of the chromosome was demonstrated by segregation of these fragments in subclones. On the basis of the results of marker analysis in this panel, the most likely order of the markers tested is MANB-D19S7-PEPD-D19S9-GPI-C/EBP-TGFB1++ +-(CYP2A,BCKDHA,CGM2,NCA)-PSG1-(D19S8, XRCC1)-(ATP1A3,D19S19)-(D19S37,APOC2)-C KM-ERCC2-ERCC1-(D19S116,D19S117)- (D19S118,D19S119, D19S63,p36.1,D19S112,D19S62,D19S51,D19S54, D19S55)-pW39-D19S6-(D19S50,TNNT1)-D19S2 2-(HRC,CGB,FTL,PRKCG)-qter. This gene order is generally consistent with published physical and genetic mapping orders, although some discrepancies exist. By means of a mapping function that relates the frequency of cosegregation of markers to the distance between them, estimates were made of the sizes, in megabases, of the 19q subregions. The relative physical distances between reference markers were compared with published genetic distances for 19q. Excellent correlation was observed, suggesting that the physical distances calculated by this method are predictive of genetic distances in this region of the genome and, therefore, are just as useful in estimating relative positions of markers.

Hypertrophic cardiomyopathy mutation is expressed in messenger RNA of skeletal as well as cardiac muscle

BACKGROUND

The beta-myosin heavy chain (beta-MHC) gene has been identified as a major locus for familial hypertrophic cardiomyopathy (FHCM). We recently showed that one of the common mutations associated with FHCM is expressed in the cardiac muscle messenger RNA (mRNA) of an affected individual. Since beta-MHC is a major sarcomeric protein of cardiac and skeletal muscle, studies were performed to determine whether the mutation is also expressed in skeletal muscle.

METHODS AND RESULTS

Biopsies were obtained of skeletal muscle (biceps brachii) from a proband with FHCM known to have the missense mutation in exon 13 of the beta-MHC gene. RNA was extracted from skeletal muscle and lymphocytes by the RNAzol method. First-strand complementary DNA was synthesized by reverse transcription using an antisense primer to exon 16. Polymerase chain reaction (PCR) was performed using primers to exons 12 and 14 to amplify the segment encompassing exon 13. The PCR products were digested with Ddel restriction endonuclease. Undigested PCR product in the control and the proband was 321 base-pairs (bp). Ddel digestion of the PCR product from normal skeletal and lymphocytes showed two DNA fragments of 181 and 140 bp as expected, whereas digestion of the PCR product from the proband's skeletal muscle and lymphocytes showed four DNA fragments of 181, 149, 140, and 32 bp due to the mutation in exon 13. This indicates that the mutation in affected individuals is also expressed in the mRNA of skeletal muscle and lymphocytes.

CONCLUSIONS

To our knowledge, this is the first documentation of a beta-MHC gene mutation expressed in skeletal muscle. This finding is provocative. Does it impair skeletal muscle function? If so, how? If not, why not? Is the impairment, or lack of it, a clue to the molecular defect of cardiac muscle? Furthermore, skeletal muscle provides a readily accessible source of mRNA for expression studies and for purification of the beta-MHC protein, which is probably essential to future investigation designed to unravel the molecular basis of this disorder.

The CCAAT/enhancer binding protein (C/EBP alpha) gene (CEBPA) maps to human chromosome 19q13.1 and the related nuclear factor NF-IL6 (C/EBP beta) gene (CEBPB) maps to human chromosome 20q13.1

The CEBPA gene encoding CCAAT/enhancer binding protein (C/EBP alpha) has been mapped to human chromosome 19 and the CEBPB (formerly TCF5) gene encoding NF-IL6 (C/EBP beta) to human chromosome 20 by Southern blot analysis of Chinese hamster x human and mouse x human somatic cell hybrids. CEBPA has been further mapped to 19q13.1 between the loci GPI and TGFB using human x hamster somatic cell hybrids containing restricted fragments of human chromosome 19. This position was confirmed by fluorescence in situ hybridization. Furthermore, CEBPB has been mapped to 20q13.1 by fluorescence in situ hybridization.

A new polymorphic probe which defines the region of chromosome 19 containing the myotonic dystrophy locus

The region of human chromosome 19 which includes the myotonic dystrophy locus (DM) has recently been redefined by the tight linkage between it and the gene for muscle-specific creatine kinase (CKMM), which lies just proximal to DM. Utilizing human/hamster hybrid cell lines containing defined breakpoints within this region, we have assigned a number of new probes close to DM. Two of these probes, p134B and p134C, were isolated from a single cosmid clone (D19S51) and detect the same BglI RFLP; p134C detects an additional RFLP with the enzyme PstI. Analysis of these probes in the Centre d'Etude du Polymorphisme Humain families demonstrates tight linkage with a number of markers known to be proximal to DM. A two-point lod score of 6.34 at theta = .025 demonstrates the linkage of this probe to DM. Analysis of a DM individual previously shown to be recombinant for other tightly linked markers indicates that p134C is distal to DM. This result indicates that both the new probe and the existing group of proximal probes including CKMM and ERCC1 probably flank DM and define the genetic interval into which this mutation maps.

Molecular cloning, expression, and chromosome 19 localization of a human Ro/SS-A autoantigen

Ro/SS-A antibodies are found in a number of human autoimmune disorders including Sjogren's syndrome and several systemic lupus erythematosus-related disorders. These heterogeneous autoantibodies are known to recognize several distinct cellular antigens. With synthetic oligonucleotides corresponding to amino acid sequence information we have isolated a full-length cDNA clone which encodes a human Ro ribonucleoprotein autoantigen. The 1,890-base pair clone contains an open reading frame that encodes a 417-amino acid, 48-kD polypeptide that migrates aberrantly at 60 kD by SDS-PAGE. Rabbit antibodies raised against this protein's recently described amino-terminal epitope react with a previously identified 52-kD human Ro protein and immunoprecipitate the human cytoplasmic RNAs. Ultraviolet light cross-linking studies suggest that this Ro protein binds each of the four major human cytoplasmic RNAs. The deduced amino acid sequence is 63% homologous to an Onchocerca volvulus antigen. Southern filter hybridization analysis indicates that this gene is not highly polymorphic and exists as a single copy in the human genome. Chromosomal localization studies place this gene on the short arm of chromosome 19 near the gene encoding the low density lipoprotein receptor.

Complementation of repair gene mutations on the hemizygous chromosome 9 in CHO: a third repair gene on human chromosome 19

A human DNA repair gene, ERCC2 (Excision Repair Cross Complementing 2), was assigned to human chromosome 19 using hybrid clone panels in two different procedures. One set of cell hybrids was constructed by selecting for functional complementation of the DNA repair defect in mutant CHO UV5 after fusion with human lymphocytes. In the second analysis, DNAs from an independent hybrid panel were digested with restriction enzymes and analyzed by Southern blot hybridization using DNA probes for the three DNA repair genes that are located on human chromosome 19: ERCC1, ERCC2, and X-Ray Repair Cross Complementing 1 (XRCC1). The results from hybrids retaining different portions of this chromosome showed that ERCC2 is distal to XRCC1 and in the same region of the chromosome 19 long arm (q13.2-q13.3) as ERCC1, but on different MluI macrorestriction fragments. Similar experiments using a hybrid clone panel containing segregating Chinese hamster chromosomes revealed the hamster homologs of the three repair genes to be part of a highly conserved linkage group on Chinese hamster chromosome number 9. The known hemizygosity of hamster chromosome 9 in CHO cells can account for the high frequency at which genetically recessive mutations are recovered in these three genes in CHO cells. Thus, the conservation of linkage of the repair genes explains the seemingly disproportionate number of repair genes identified on human chromosome 19.

Human creatine kinase genes on chromosomes 15 and 19, and proximity of the gene for the muscle form to the genes for apolipoprotein C2 and excision repair

The human chromosomal assignments of genes of the creatine kinase (CK) family--loci for brain (CKBB), muscle (CKMM), and mitochondrial (CKMT) forms--were studied by Southern filter hybridization analysis of DNAs isolated from a human x rodent somatic cell hybrid clone panel. Probes for the 3'-noncoding sequences of human CKBB and CKMM hybridized concordantly only to DNAs from somatic cell hybrids containing chromosomes 14 and 19, respectively. Thus the earlier assignment of the gene coding for the CKBB isozyme to chromosome 14 was confirmed by molecular means, as was the provisional assignment of CKMM to the long arm of chromosome 19. A probe containing canine sequences for CKMM cross-hybridized with human sequences on chromosomes 14 and 19, a result consistent with the assignments of CKBB and CKMM. A probe containing human sequences for CKMT enabled the provisional assignment of CKMT to human chromosome 15. Independent hybrids with portions of the long arm of chromosome 19 missing indicated the order of genes on the long arm of chromosome 19 as being cen-GPI-(TGFB, CYP1)-[CKMM, (APOC2-ERCC1)]-(CGB, FTL). The unexpectedly more distal location of APOC2 among the genes on the long arm--and APOC2's close association with CKMM--is discussed with respect to the close linkage relationship of APOC2 to myotonic muscular dystrophy.