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M S Sedra

Publications and source records attributed to M S Sedra.

14 recordsLinked to original sources
PubMed2001

Hybrids monosomal for human chromosome 5 reveal the presence of a spinal muscular atrophy (SMA) carrier with two SMN1 copies on one chromosome.

We have analyzed the survival motor neuron gene (SMN1) dosage in 100 parents of children with homozygous SMN1 deletions. Of these parents, 96 (96%) demonstrated the expected one-copy SMN1 carrier genotype. However, four parents (4%) were observed to have a normal two-copy SMN1 dosage. The presence of two intact SMN1 genes in the parent of an affected child indicates either the occurrence of a de novo mutation event or a situation in which one chromosome has two copies of SMN1, whereas the other is null. We have separated individual chromosomes from two of these parents with two-copy SMN1 dosage by somatic cell hybridization and have employed a modified quantitative dosage assay to provide direct evidence that one parent is a two-copy/ zero-copy SMN1 carrier, whereas the other parent had an affected child as the result of a de novo mutation. These findings are important for assessing the recurrence risk of parents of children with spinal muscular atrophy and for providing accurate family counseling.

Autoradiography↗
PubMed1997

Dystrophin expression in a Duchenne muscular dystrophy patient with a frame shift deletion.

The exon 45 deletion is a common dystrophin gene deletion. Although this is an out-of-frame deletion, which should not allow for protein synthesis, it has been observed in mildly affected patients. We describe a patient with an exon 45 deletion who produced protein, but still had a severe Duchenne muscular dystrophy phenotype. RT-PCR analysis and cDNA sequencing from the muscle biopsy sample revealed that the exon 45 deletion induced exon skipping of exon 44, which resulted in an in-frame deletion and the production of dystrophin. A conformational change in dystrophin induced by the deletion is proposed as being responsible for the severe phenotype in the patient. We feel that the variable clinical phenotype observed in patients with the exon 45 deletion is not due to exon splicing but may be the result of other environmental or genetic factors, or both.

Base Sequence↗
PubMed1996

A novel splice site mutation in a Becker muscular dystrophy patient.

A Becker muscular dystrophy patient was found to have a single base substitution at the 5' end of intron 54. This single base substitution disrupts the invariant GT dinucleotide within the 5' donor splice site and was shown to cause an out of frame deletion of exon 54 during mRNA processing. This is predicted to produce a truncated dystrophin protein which is more consistent with a DMD phenotype. However, small quantities of normal mRNA are also transcribed and these are sufficient to produce a reduced amount of normal molecular weight dystrophin and give rise to a milder BMD phenotype. This indicates that a single base substitution at an invariant dinucleotide of the splice site consensus sequence may still allow read through of the message and allow the production of some normal protein. This shows that there are a greater number of possible intronic mutations that can lead to a mild phenotype and it also underlines the importance of performing cDNA analysis when screening for small gene alterations in the BMD patient population.

Base Sequence↗
PubMed1995

Germline mosaicism at the fragile X locus.

We have identified a fragile X syndrome pedigree where the disorder is associated with a molecular deletion. The deletion was present in the DNA of 2 sons but was absent in the mother's somatic cell (lymphocyte) DNA. The results are consistent with the deletion arising as a postzygotic event in the mother, who therefore is germinally mosaic. This finding has important implications for counseling fragile X families with deletion mutations.

Female↗
PubMed1995

Rapid DNA haplotyping using a multiplex heteroduplex approach: application to Duchenne muscular dystrophy carrier testing.

A new strategy has been developed for rapid haplotype analysis based on an initial multiplex amplification of several polymorphic sites, followed by heteroduplex detection. Heteroduplexes formed between two different alleles are detected because they migrate differently than the corresponding homoduplexes in Hydrolink-MDE gel. This simple, rapid method does not depend on specific sequences such as restriction enzyme sites or CA boxes and does not require the use of isotope. This approach has been tested using commonly occurring polymorphisms spanning the dystrophin gene as a model. We describe the use of the method to assign the carrier status of females in Duchenne muscular dystrophy (DMD) pedigrees. The method may be used for other genetic diseases when mutations are unknown or there are few dinucleotide markers in the gene proximity, and for the identification of haplotype backgrounds of mutant alleles.

Base Sequence↗
PubMed1995

Spectrum of small mutations in the dystrophin coding region.

Duchenne and Becker muscular dystrophies (DMD and BMD) are caused by defects in the dystrophin gene. About two-thirds of the affected patients have large deletions or duplications, which occur in the 5' and central portion of the gene. The nondeletion/duplication cases are most likely the result of smaller mutations that cannot be identified by current diagnostic screening strategies. We screened approximately 80% of the dystrophin coding sequence for small mutations in 158 patients without deletions or duplications and identified 29 mutations. The study indicates that many of the DMD and the majority of the BMD small mutations lie in noncoding regions of the gene. All of the mutations identified were unique to single patients, and most of the mutations resulted in protein truncation. We did not find a clustering of small mutations similar to the deletion distribution but found > 40% of the small mutations 3' of exon 55. The extent of protein truncation caused by the 3' mutations did not determine the phenotype, since even the exon 76 nonsense mutation resulted in the severe DMD phenotype. Our study confirms that the dystrophin gene is subject to a high rate of mutation in CpG sequences. As a consequence of not finding any hotspots or prevalent small mutations, we conclude that it is presently not possible to perform direct carrier and prenatal diagnostics for many families without deletions or duplications.

Base Sequence↗
PubMed1995

A molecular protocol for diagnosing myotonic dystrophy.

Myotonic dystrophy (DM) is an autosomal dominant genetic disease caused by an unstable CTG repeat sequence in the 3' untranslated region of the myotonin protein kinase gene. The CTG repeat is present 5-30 times in the normal population, whereas DM patients have CTG expansions of 50 to several thousand repeats. The age of onset of the disorder and the severity of the phenotype is roughly correlated with the size of the CTG expansion. We developed a molecular protocol for the diagnosis of DM based on an initial polymerase chain reaction screen to detect normal-sized alleles and small expansions, followed by an improved Southern protocol to detect larger expansions.

Adult↗
PubMed1994

Heteroduplex analysis of the dystrophin gene: application to point mutation and carrier detection.

Approximately one-third of the Duchenne muscular dystrophy patients have undefined mutations in the dystrophin gene. For carrier and prenatal studies in families without detectable mutations, the indirect restriction fragment length polymorphism linkage approach is used. Using a multiplex amplification and heteroduplex analysis of dystrophin exons, we identified nonsense mutations in two DMD patients. Although the nonsense mutations are predicted to severely truncate the dystrophin protein, both patients presented with mild clinical courses of the disease. As a result of identifying the mutation in the affected boys, direct carrier studies by heteroduplex analysis were extended to other relatives. We conclude that the technique is not only ideal for mutation detection but is also useful for diagnostic testing.

Base Sequence↗
PubMed1993

Detection of an exon 53 polymorphism in the dystrophin gene.

We utilized a heteroduplex method to screen for small mutations in Duchenne muscular dystrophy patients who did not have deletions or duplications. A dystrophin exon 53 heteroduplex band was identified in 14.4% of the affected patients. Direct sequencing of the amplified product from DNA producing the heteroduplex revealed the presence of a polymorphism in the coding region. The codon for asparagine was converted from AAT to AAC.

Asparagine↗
PubMed1993

Exon 44 nonsense mutation in two-Duchenne muscular dystrophy brothers detected by heteroduplex analysis.

Utilizing a heteroduplex method, we screened the dystrophin exon 43-45 region for point mutations, including small deletions and insertions. The method depends upon the formation of a heteroduplex between wild-type and mutant DNA PCR products. DNA specimens from one hundred and four DMD patients without detected deletions or duplications were multiplexed amplified for exons 43, 44, and 45. The PCR products were mixed with the PCR products from nonaffected controls, electrophoresed, and examined for the presence of altered mobility heteroduplex bands. An exon 44 nonsense mutation in two DMD brothers and a common intron 44 polymorphism were identified using this approach. Although the exon 44-45 region is a hotspot for deletion breakpoints, it does not appear to be prone to point mutations. The technique is extremely useful for screening several exons simultaneously and it allowed us to screen a large number of patients.

Base Sequence↗
PubMed1993

A missense mutation in the dystrophin gene in a Duchenne muscular dystrophy patient.

About two thirds of Duchenne muscular dystrophy (DMD) patients have either gene deletions or duplications. The other DMD cases are most likely the result of point mutations that cannot be easily identified by current strategies. Utilizing a heteroduplex technique and direct sequencing of amplified products, we screened our nondeletion/duplication DMD population for point mutations. We now describe what we believe to be the first dystrophin missense mutation in a DMD patient. The mutation results in the substitution of an evolutionarily conserved leucine to arginine in the actin-binding domain. The patient makes a dystrophin protein which is properly localized and is present at a higher level than is observed in DMD patients. This suggests that an intact actin-binding domain is necessary for protein stability and essential for function.

Amino Acid Sequence↗
PubMed1993

Identification of two point mutations and a one base deletion in exon 19 of the dystrophin gene by heteroduplex formation.

Two thirds of the Duchenne muscular dystrophy population have either gene deletions or duplications. The nondeletion/duplication cases are most likely the result of point mutations or small deletions and duplications that cannot be easily identified by current strategies. The major obstacle in identifying small mutations is due to the large size of the dystrophin gene. We selectively screened 5 DMD exons containing CpG dinucleotides in 110 DMD patients without detectable deletions or duplications. Nonsenses mutations are frequently due to a C- to -T transition within a CG dinucleotide pair. To screen for the nonsense mutations, we used the heteroduplex method. Utilizing this approach, we identified 2 different nonsense mutations and a single base deletion all occurring in exon 19. This is the first report of a clustering of small mutations in the dystrophin gene.

Base Sequence↗