Radiation-reduced hybrids for the myotonic dystrophy locus

Distinct pathological signatures in human cellular models of myotonic dystrophy subtypes

Myotonic dystrophy (DM) is the most common autosomal dominant muscular dystrophy and encompasses both skeletal muscle and cardiac complications. DM is nucleotide repeat expansion disorder in which type 1 (DM1) is due to a trinucleotide repeat expansion on chromosome 19 and type 2 (DM2) arises from a tetranucleotide repeat expansion on chromosome 3. Developing representative models of DM in animals has been challenging due to instability of nucleotide repeat expansions, especially for DM2, which is characterized by nucleotide repeat expansions often greater than 5,000 copies. To investigate mechanisms of human DM, we generated cellular models of DM1 and DM2. We used regulated MyoD expression to reprogram urine-derived cells into myotubes. In this myogenic cell model, we found impaired dystrophin expression, in the presence of muscleblind-like 1 (MBNL1) foci, and aberrant splicing in DM1 but not in DM2 cells. We generated induced pluripotent stem cells (iPSC) from healthy controls and DM1 and DM2 subjects, and we differentiated these into cardiomyocytes. DM1 and DM2 cells displayed an increase in RNA foci concomitant with cellular differentiation. iPSC-derived cardiomyocytes from DM1 but not DM2 had aberrant splicing of known target genes and MBNL sequestration. High-resolution imaging revealed tight association between MBNL clusters and RNA foci in DM1. Ca2+ transients differed between DM1- and DM2 iPSC-derived cardiomyocytes, and each differed from healthy control cells. RNA-sequencing from DM1- and DM2 iPSC-derived cardiomyocytes revealed distinct misregulation of gene expression, as well as differential aberrant splicing patterns. Together, these data support that DM1 and DM2, despite some shared clinical and molecular features, have distinct pathological signatures.

Molecular cloning of a novel splice variant of human type IVA (PDE-IVA) cyclic AMP phosphodiesterase and localization of the gene to the p13.2-q12 region of human chromosome 19 [corrected]

We have isolated from a human T-cell Jurkat cDNA library a novel human cDNA (2EL) that is closely related to the human type-IV PDE splice variant family 'A' (PDE-IVA) cDNA characterized previously by us [Sullivan, Egerton, Shakur, Marquardsen and Houslay (1994) Cell. Signalling 6, 793-812]; (h6.1, PDE-IVA/h6.1; HSPDE4A7). (PDE stands for cyclic nucleotide phosphodiesterase). The novel cDNA 2EL (PDE-IVA/2EL; HSPDE4A8) contains two regions of unique sequence not found in PDE-IVA/h6.1. These are a distinct 5'-end and a 34 bp insert which occurs within a domain thought to encode the type-IV PDE catalytic site and which can be expected to result in premature truncation of any expressed protein. HSPDE4A8 appeared to be catalytically inactive. Isolation and characterization of a human genomic cosmid clone revealed that 2EL and h6.1 represent alternative splice variants of the human PDE-IVA gene. Using a unique sequence found at the 5'-end of the 2EL cDNA, a probe was generated which was used to screen the DNA of human-hamster hybrids. This located the human gene for PDE-IVA to human chromosome 19. Through both the analysis of genomic DNAs from a human-hamster somatic cell hybrid panel and also using fluorescent in situ hybridization, it was shown that the human PDE-IVA gene is located on human chromosome 19, between p13.2 [corrected] and q12. This region on chromosome 19 has been shown to be related to genetic diseases such as the autosomal dominant cerebrovascular disease CADASIL, susceptibility to late-onset Alzheimer's disease and changes seen in benign pituitary and thyroid adenomas.

Irradiation and fusion gene transfer

Irradiation and fusion gene transfer (IFGT) is a technique that spans the gap between the limitations of molecular methods and somatic-cell genetics, allowing the separation of DNA fragments between 0.25 and 30 Mb in size. In conjunction with genetic linkage analysis and physical mapping techniques, IFGT provides a very useful addition to methods for cloning disease loci, and mapping chromosomes and entire genomes.

Physical map of small cell lung cancer deletion region on short arm of human chromosome 3 (3p13-22) based on radiation fusion hybrid analysis

Deletion of DNA sequences from various regions of the short arm of human chromosome 3 (3p13-14, 3p21, and 3p25) has been observed during the development of a variety of solid tumors, including lung and renal cell carcinomas. In this study we have used a set of radiation fusion hybrids to generate a physical map of chromosome 3p to orient the search for putative tumor suppressor genes. Eighty-six human-hamster radiation fusion hybrids were screened on Southern blots for the retention of 55 human chromosome 3p DNA markers. The high marker density enabled us to identify a set of successively overlapping chromosome fragments in the 3p13-22 area guided by eight markers with previously known order. Twenty-four map intervals were suggested using breakpoints determined by partial fragment overlaps. The final order between the markers derived is consistent with previous information about localizations for 26 of the markers to three larger cytogenetic intervals.

Genomic organization and transcriptional units at the myotonic dystrophy locus

The genomic structure and apparently complete coding sequence of the myotonic dystrophy protein kinase gene have been determined. The gene contains 15 exons distributed over about 13 kb of genomic DNA. It codes for a protein of 624 amino acids with an N-terminal domain highly homologous to cAMP-dependent serine-threonine protein kinases, an intermediate domain with a high alpha-helical content and weak similarity to various filamentous proteins, and a hydrophobic C-terminal segment. Located in close proximity is a second gene, coding for a transcript of about 3 kb, that is homologous to the gene DMR-N9 in the corresponding mouse locus, but has no homologies to other known genes or proteins. Strong expression of the latter gene in brain suggests that it may have a role in the development of mental symptoms in severe cases of the disease.

Dual Alu polymerase chain reaction primers and conditions for isolation of human chromosome painting probes from hybrid cells

A method for rapid and efficient production of chromosome- and chromosome-region specific probes for fluorescent in situ hybridization (FISH) detectable by simple fluorescent microscopy is described. The procedure is based on simultaneous use of two inter-Alu-polymerase chain reaction (PCR) primers for extraction of highly heterogeneous human DNA from interspecific somatic cell hybrids containing the chromosome regions of interest. Probes so produced do not hybridize to centromeric sequences and simultaneously band the target chromosomes, making them useful for unambiguous identification of chromosomal elements and breakpoints associated with cancer.

Characterization of a YAC and cosmid contig containing markers tightly linked to the myotonic dystrophy locus on chromosome 19

Myotonic dystrophy (DM) is caused by a defect in an unknown gene that maps to 19q13.3, flanked by the tightly linked markers ERCC1 on the proximal side and D19S51 on the distal side. We report the isolation and characterization of overlapping YAC and cosmid clones around D19S51 for the construction of a physical map around this locus. The resulting contig contains the markers D19S51 and D19S62 (another new marker tightly linked to the DM locus) and the distal breakpoint of a radiation hybrid cell line used in the physical mapping of the DM region. We have compared the restriction maps of the YACs and cosmids with that of the genome to investigate the fidelity of these clones.

Molecular basis of myotonic dystrophy: expansion of a trinucleotide (CTG) repeat at the 3' end of a transcript encoding a protein kinase family member

Using positional cloning strategies, we have identified a CTG triplet repeat that undergoes expansion in myotonic dystrophy patients. This sequence is highly variable in the normal population. PCR analysis of the interval containing this repeat indicates that unaffected individuals have been 5 and 27 copies. Myotonic dystrophy patients who are minimally affected have at least 50 repeats, while more severely affected patients have expansion of the repeat containing segment up to several kilobase pairs. The CTG repeat is transcribed and is located in the 3' untranslated region of an mRNA that is expressed in tissues affected by myotonic dystrophy. This mRNA encodes a polypeptide that is a member of the protein kinase family.

Myotonic dystrophy: another case of too many repeats?

Myotonic dystrophy (DM) is an adult form of muscular dystrophy affecting about 1 in 8,000 individuals in most populations. Although common symptoms include progressive muscle weakness and stiffness, it is characterised by a heterogeneous clinical picture. Despite this variation in both the nature and severity of the symptoms seen in affected individuals, DM is genetically homogeneous, segregating as a single locus on the proximal long arm of human chromosome 19. As the biochemical abnormality underlying the disease was unknown, a reverse genetics (or positional cloning) strategy for identifying the gene responsible was adopted. The resulting collaborative effort culminated in the detection of the molecular mutation event and the gene within which it lies: the expansion of a trinucleotide repeat (CTG) at the 3' end of a gene encoding a member of the cyclic AMP-dependent protein kinase family. This has diagnostic implications since an easy, reliable and predictive test can now be offered to individuals with a family history of DM. These findings are also a prerequisite for further studies concerning the biochemical and physiological aetiology of DM and possible therapeutic strategies. In addition, the striking similarity between findings at the DNA level in DM and those in fragile X syndrome and spinal and bulbar muscular atrophy suggests that the mechanism leading to the increase in copy number of trinucleotide repeats at particular loci may be responsible for a number of other genetic diseases.