A novel Alu-like element rearranged in the dystrophin gene causes a splicing mutation in a family with X-linked dilated cardiomyopathy

Mutations in the dystrophin gene are associated with sporadic dilated cardiomyopathy

Dilated cardiomyopathy (DCM) is the major indication for heart transplantation. Approximately 30% of all DCM is thought to be inherited, while 70% is sporadic. Mutations in the dystrophin gene have been associated with the uncommon X-linked form of DCM. We hypothesized that missense mutations and other less severe mutations of the dystrophin gene might predispose to the common form of sporadic DCM. To test this hypothesis, 22kb of genomic dystrophin DNA was scanned with DOVAM-S in each of the 22 patients with sporadic DCM, including all 79 coding sequences and splice junctions, as well as six alternative exon 1 dystrophin isoforms (484kb, total). Three putative new mutations (IVS5+1 G>T, K18N, and F3228L) and seven polymorphisms were identified. The splice site mutation IVS5+1 is predicted to cause skipping of exon 5, which is within a region containing an actin binding site. The missense mutations occur at amino acids that display substantial evolutionary conservation. Screening of 236 control individuals failed to identify these three mutations. The three patients with putative mutations had CK-MM (creatine kinase, skeletal muscle) levels greater than 250 units while the 14 patients without mutations for which CK-MM were available had values ranging from 20 to 200. The first comprehensive mutation scanning of the exons and splice junctions of the dystrophin gene in patients with sporadic DCM presents the evidence that point mutations are associated with sporadic DCM without clinical evidence of skeletal myopathy. It may be prudent to measure CK-MM in all patients with dilated cardiomyopathy to identify candidates at high risk for dystrophin mutations.

Site-specific DNA hypomethylation permits expression of the IRBP gene

Interphotoreceptor retinoid binding protein (IRBP), a putative component of the visual cycle, is expressed selectively in the retina and pineal gland. This study examined whether site-specific DNA hypomethylation plays a role in this expression regulation. Southern blotting of HpaII and MspI digests of DNA from various bovine and murine tissues (whole brain, retina, pineal gland, superior colliculus, cortex, thymus, habenular nucleus, cornea, liver, tail, and kidney) revealed that specific CpG dinucleotides in the IRBP gene promoter are hypomethylated in DNA from retinal photoreceptor cells and pineal gland compared to DNA from other tissues. These sites are methylated in DNA from non-photoreceptor retinal cells. Exogenous methylation of these sites diminished DNA:protein binding in electrophoretic mobility shift assays. HpaII methylation of chloramphenicol acetyltransferase reporter constructs suppressed IRBP but not SV40 promoter activity in transiently transfected primary cultures of embryonic chick retinal cells. These data indicate that specific cytosines in the bovine and murine IRBP promoters are unmethylated in photoreceptive cells but methylated in other tissues. This differential DNA methylation may modulate IRBP gene expression since exogenous methylation of the murine sites suppresses reporter gene transcription, apparently by inhibiting DNA:protein binding events.

Genetic abnormalities responsible for dilated cardiomyopathy

Dilated cardiomyopathy (DCM), a disorder in which left ventricular dilation and dysfunction leads to congestive heart failure, is inherited in over 30% of cases. The underlying genetic mechanisms are slowly being unraveled, with multiple genes recently identified as causing DCM in some patients. The genes identified to date appear to encode proteins that either support the cytoskeleton or interact with the cytoskeleton. When mutated, these proteins destabilize the cardiomyocyte membrane or cytoskeleton via mechanical instability or force transduction causing poor cardiac systolic function and compensatory dilation. Once the entire group of genes causing DCM (genetic heterogeneity) are identified, improvements in diagnosis and treatment are expected.

Prevalence and characteristics of dystrophin defects in adult male patients with dilated cardiomyopathy

OBJECTIVES

To assess the prevalence of dystrophin defects in dilated cardiomyopathy (DCM) in male patients and to formulate investigation strategies for their identification.

BACKGROUND

Dystrophin defects presenting with predominant or exclusive cardiac involvement may be clinically indistinguishable from "idiopathic" DCM. Diagnosis may be missed, unless specifically investigated.

METHODS

Clinical and biochemical evaluation, right ventricular endomyocardial biopsy (EMB), light and electron microscopic and immunohistochemical studies of biopsy samples, six multiplex and two single polymerase chain reactions for 38 exons and automated sequencing of exon 9 and muscle promoter-exon 1 were undertaken in 201 consecutive male patients presenting with DCM, with (n = 14) and without (n = 187) increased serum creatine phosphokinase (sCPK).

RESULTS

Dystrophin defects were identified in 13 of the 201 patients (6.5%, age 16-50). Family history was positive in four patients. Serum CPK levels were increased in 11 of 13 patients. Light microscopy examination of EMB was uninformative; ultrastructural study showed multiple membrane defects. Dystrophin immunostain was abnormal. Eight patients, all older than 20, had deletions affecting midrod domain, normal or mildly increased CPK and better outcome than the five remaining cases all younger than 20, with more than five-fold increase of sCPK. Two of these latter had proximal and rod-domain deletions. Sisters of two patients were diagnosed as noncarriers with microsatellite analysis.

CONCLUSIONS

Although the overall prevalence of dystrophin defects in our consecutive DCM male series is low (6.5%), immunohistochemical and molecular studies are essential to identify protein and gene defects; screening studies are justified to define prevalence, clinical profile and genotype-phenotype correlation.

Association of nonsense mutation of dystrophin gene with disruption of sarcoglycan complex in X-linked dilated cardiomyopathy

BACKGROUND

In a systematic analysis of inherited forms of cardiomyopathy, we previously identified a family with X-linked dilated cardiomyopathy characterised by a mutation in the rod region of dystrophin. We have now attempted to eludicate the genetic mechanism involved in this disease, as well as the role of dystrophin-associated glycoproteins.

METHODS

The affected dystrophin epitope, which lacks binding to the dys-1 antibody, was analysed by single-strand conformation polymorphism analysis, reverse-transcription PCR, and DNA sequencing. Effects on dystrophin-associated glycoproteins were studied by immunohistochemistry and western blotting.

FINDINGS

A translation-termination mutation (C4148T) in exon 29 of the dystrophin gene was found in all affected family members. Alternative splicing rescued the reading frame and led to the expression of a dystrophin molecule lacking 50 aminoacids both in cardiac and skeletal muscle. Immunohistochemical analysis of the dystrophin-associated proteins revealed a reduction of beta-sarcoglycan and delta-sarcoglycan in the sarcolemma of cardiac muscle but not skeletal muscle tissue. However, western blotting revealed similar amounts of sarcoglycan subunits in both tissues.

INTERPRETATION

The molecular mechanism of this subtype of X-linked cardiomyopathy may be explained by a conformational change in exon-29-deleted dystrophin, resulting in disruption of the sarcoglycan assembly in heart muscle but not skeletal muscle.

An alternatively spliced surfactant protein B mRNA in normal human lung: disease implication

We identified an alternatively-spliced surfactant protein B (SP-B) mRNA from normal human lung with a 12 nt deletion at the beginning of exon 8. This deletion causes a loss of four amino acids in the SP-B precursor protein. Sequence comparison of the 3' splice sites reveals only one difference in the frequency of U/C in the 11 predominantly-pyrimidine nucleotide tract, 73% for the normal and 45% for the alternatively-spliced SP-B mRNA (77-99% for the consensus sequence). Analysis of SP-B mRNA in lung indicates that the abundance of the alternatively-spliced form is very low and varies among individuals. Although the relative abundance of the deletion form of SP-B mRNA remains constant among normal lungs, it is found with relatively higher abundance in the lungs of some individuals with diseases such as congenital alveolar proteinosis, respiratory distress syndrome, bronchopulmonary dysplasia, alveolar capillary dysplasia and hypophosphatasia. This observation points to the possibility that the alternative splicing is a potential regulatory mechanism of SP-B and may play a role in the pathogenesis of disease under certain circumstances.

X-linked dilated cardiomyopathy and the dystrophin gene

X-linked dilated cardiomyopathy (XLDC) represents a well known genetic disease, allelic to Duchenne and Becker muscular dystrophies and caused by dystrophin gene mutations. XLDC is a rare disease and only few families have been fully characterised. In several of them, the dystrophin mutations show a different pattern of expression in cardiac compared to skeletal muscle. In the families with the most severe cardiac phenotype, the cardiac muscle is usually unable to produce dystrophin, due to a specific effect that the mutation(s) have on the gene transcription in this tissue. The skeletal muscle escapes the dystrophic changes by maintaining dystrophin synthesis via exon skipping or alternative splicing that the heart is not able to put in place. In this paper we have reviewed the families with X-linked dilated cardiomyopathy reported so far; in addition we provided novel transcription data on two families we previously described. The aim of this review is to attempt a genotype-phenotype correlation and speculate on common pathogenic mechanisms underlying this disease.