DXS28 (C7) maps centromeric to DXS68 (L1-4) and DXS67 (B24) by deletion analysis

A population-based study of dystrophin mutations in Canada

INTRODUCTION

We carried out a population-based study of dystrophin mutations in patients followed by members of the Canadian Paediatric Neuromuscular Group (CPNG) over a ten-year period.

OBJECTIVES

We aimed to describe the changes in diagnostic testing for dystrophinopathy and to determine the frequency of dystrophin mutations from 2000 to 2009.

METHODS

De-identified data containing the clinical phenotypes, diagnostic methods, and mutational reports from dystrophinopathy patients followed by CPNG centres from January 2000 to December 2009 were analyzed using descriptive statistics.

RESULTS

773 patients had a confirmed diagnosis of dystrophinopathy based on genetic testing (97%), muscle biopsy (2%), or family history (1%). 573 (74%) had complete deletion/duplication analysis of all 79 exons or whole gene sequencing, resulting in 366 (64%) deletions, 64 (11%) duplications, and 143 (25%) point mutations. The percentage of patients who were diagnosed using currently accepted genetic testing methods varied across Canada, with a mean of 63% (SD 23). 246 (43%) mutations involved exons 45 to 53. The top ten deletions (n=147, 26%) were exons 45-47, 45-48, 45, 45-50, 45-55, 51, 45-49, 45-52, 49-50, and 46-47. 169 (29%) mutations involved exons 2 to 20. The most common duplications (n=29, 5.1%) were exons 2, 2-7, 2-17, 3-7, 8-11, 10, 10-11, and 12.

CONCLUSION

This is the most comprehensive report of dystrophin mutations in Canada. Consensus guidelines regarding the diagnostic approach to dystrophinopathy will hopefully reduce the geographical variation in mutation detection rates in the coming decade.

Isolation of a yeast artificial chromosome contig spanning the X chromosomal translocation breakpoint in a patient with Rett syndrome

Rett syndrome is a neurodevelopmental disorder observed exclusively in females. A de novo X;3 translocation was detected in a patient (TH) with Rett syndrome. The X chromosomal breakpoint maps to Xp21.3 between the distal end of the Duchenne muscular dystrophy (DMD) gene and the DXS28 (C7) locus. To determine if this translocation caused the Rett syndrome in this patient, our efforts focused on mapping and cloning of the X chromosomal breakpoint in this patient. Toward these goals, we generated a set of radiation-reduced hybrid cell lines for the short arm of the X chromosome to use as a source for region-specific markers. Using Alu-PCR, 13 new DNA markers were isolated from a radiation-reduced hybrid, which retained both DMD and DXS28. These markers were localized within Xp21 using DNA from males with various interstitial deletions in this region. Two new markers, K23-2p and K23b-1, were found to be closer flanking markers to the X chromosomal breakpoint than DMD and DXS28. Long range restriction mapping using K23-2p and K23b-1 determined that the maximum distance between them was 800 kb. Several of the new markers were developed into sequence tagged-sites and were used to isolate yeast artificial chromosome (YAC) clones. A total of 22 YAC clones was isolated and characterized; these YACs were then developed into 3 large contigs in the Xp21.3 region. This effort resulted in the cloning of the region containing the X chromosomal translocation breakpoint of the Rett syndrome patient in a 170-kb YAC clone.

A case of myopathy associated with a dystrophin gene deletion and abnormal glycogen storage

A 30-year-old man with no family history of muscle disease presented with a progressive proximal myopathy and calf hypertrophy characteristic of Becker muscular dystrophy. A deletion of exons 45 to 48 in the dystrophin gene was confirmed by Southern blotting and multiplex polymerase chain reaction. However, muscle biopsy showed massive accumulation of glycogen, although no significant abnormality of glycolytic pathway enzymes could be demonstrated. This patient therefore has a previously undescribed myopathy associated with both Becker muscular dystrophy and a glycogen storage disorder of unknown aetiology.

Identification of new markers in Xp21 between DXS28 (C7) and DMD

Characterization of Xp21 distal to Duchenne muscular dystrophy (DMD) in the region containing the genes for adrenal hypoplasia congenita (AHC) and glycerol kinase deficiency (GKD) has been limited due to a paucity of probes. Two probes were localized between DXS28 (C7) and AHC, the yeast artificial chromosome insert YHX39 (DXS727) and the polymorphic phage clone QST59 (DXS319). A genomic clone, FT1 (DXS726), 3' to DMD, was also characterized. Portions of the three probes were sequenced and primer pairs were generated to amplify a sequence-tagged site within each probe. Amplification of DNA from patients confirmed the deletion results obtained by Southern blot analysis, and these three sequence-tagged sites were successfully combined for triplex PCR. In addition to facilitating molecular genetic diagnosis in Xp21, these probes can be used to identify additional YACs and other probes to further increase the genomic information and diagnostic capabilities in this region.

Fluorescence in situ hybridization establishes the order cen-DXS28(C7)-DXS67(B24)-DXS68(L1)-tel in human chromosome Xp21.3

We report here on the order of three DNA markers, C7, B24, and L1, based on the arrangement of their fluorescently labeled hybridization sites in interphase cell nuclei. The three markers map distal to the Duchenne muscular dystrophy (DMD), glycerol kinase deficiency (GKD), and adrenal hypoplasia (AHC) loci on human chromosome Xp21.3. Their order has been a matter of controversy. In interphase chromatin, B24 maps between C7 and L1. We estimate from interphase distance that C7 and L1 are 300-500 kb apart. When the three markers are hybridized to interphase cells of Nijmegen1, a patient with DMD, GKD, and AHC, only C7 appears to be deleted, rather than both C7 and L1, as had been reported elsewhere. C7 is also the only one of the three markers deleted in several other DMD patients studied by others. The deletion results indicate that C7 is the most proximal of the three markers and allow the trio of ordered probes to be oriented on the chromosome: cen-C7(DXS28)-B24(DXS67)-L1(DXS68)-tel.

Are cysteine-rich and COOH-terminal domains of dystrophin critical for sarcolemmal localization?

It has been hypothesized that the tight localization of dystrophin at the muscle membrane is carried out by its cysteine-rich and/or carboxyl domains. We report the results of biochemical and immunocytochemical investigations of dystrophin in muscle from a 1-yr-old patient with a large deletion that removes the distal part of the dystrophin gene, thus spanning the exons coding for the cysteine-rich and the carboxy-terminal domains, and extends beyond the glycerol kinase and congenital adrenal hypoplasia genes. Immunological analysis of muscle dystrophin shows that the deletion results in the production of a truncated, but stable, polypeptide correctly localized at the sarcolemma. These data indicate that neither the cysteine-rich domain, nor the carboxyl domain, are necessary for the appearance of normal dystrophin sarcolemmal localization.

Porin interaction with hexokinase and glycerol kinase: metabolic microcompartmentation at the outer mitochondrial membrane

Porin is the pore-forming protein involved in the movement of adenine nucleotides across the outer mitochondrial membrane (OMM). Hexokinase and glycerol kinase interact with porin on the outer surface of the OMM in a manner which provides these enzymes with preferred access to the ATP generated in the mitochondrion. We review recent evidence which permits refinement of our knowledge of these proteins and their interactions at the OMM. The involvement of this system in metabolic microcompartmentation is discussed, as well as possible pathological consequences of its disruption in malignancy and genetic deficiencies of hexokinase, glycerol kinase, and porin.

Physical mapping distal to the DMD locus

We report a new locus, designated JC-1, which maps between the gene responsible for adrenal hypoplasia (AHC) and the gene that encodes glycerol kinase (GK) in Xp21.2-21.3. The probe identifying this locus was obtained by cloning the distal sequence of a junction fragment from a Duchenne muscular dystrophy (DMD) patient with a large deletion. Pulsed-field gel electrophoresis analysis shows that a region of at least 4 Mb separates the 3' end of the dystrophin gene and the closest distal marker to AHC, DXS28. This region of the human genome contains few genes whose deletion results in a clinical phenotype. JC-1 is a useful probe from which to initiate strategies directed at cloning the AHC and GK loci.