Constitutive and regulated modes of splicing produce six major myotonic dystrophy protein kinase (DMPK) isoforms with distinct properties

Myotonic dystrophy (DM) is the most prevalent inherited neuromuscular disease in adults. The genetic defect is a CTG triplet repeat expansion in the 3'-untranslated region of the myotonic dystrophy protein kinase ( DMPK ) gene, consisting of 15 exons. Using a transgenic DMPK-overexpressor mouse model, we demonstrate here that the endogenous mouse DMPK gene and the human DMPK transgene produce six major alternatively spliced mRNAs which have almost identical cell type-dependent distribution frequencies and expression patterns. Use of a cryptic 5' splice site in exon 8, which results in absence or presence of 15 nucleotides specifying a VSGGG peptide motif, and/or use of a cryptic 3' splice site in exon 14, which leads to a frameshift in the mRNA reading frame, occur as independent stochastic events in all tissues examined. In contrast, the excision of exons 13/14 that causes a frameshift and creates a C-terminally truncated protein is clearly cell type dependent and occurs predominantly in smooth muscle. We generated all six full-length mouse cDNAs that result from combinations of these three major splicing events and show that their transfection into cells in culture leads to production of four different approximately 74 kDa full-length (heart-, skeletal muscle- or brain-specific) and two C-terminally truncated approximately 68 kDa (smooth muscle-specific) isoforms. Information on DMPK mRNA and protein isoform expression patterns will be useful for recognizing differential effects of (CTG)(n)expansion in DM manifestation.

Abnormal myotonic dystrophy protein kinase levels produce only mild myopathy in mice

Myotonic dystrophy (DM) is commonly associated with CTG repeat expansions within the gene for DM-protein kinase (DMPK). The effect of altered expression levels of DMPK, which is ubiquitously expressed in all muscle cell lineages during development, was examined by disrupting the endogenous Dmpk gene and overexpressing a normal human DMPK transgene in mice. Nullizygous (-/-) mice showed only inconsistent and minor size changes in head and neck muscle fibres at older age, animals with the highest DMPK transgene expression showed hypertrophic cardiomyopathy and enhanced neonatal mortality. However, both models lack other frequent DM symptoms including the fibre-type dependent atrophy, myotonia, cataract and male-infertility. These results strengthen the contention that simple loss- or gain-of-expression of DMPK is not the only crucial requirement for development of the disease.

Structural organization and developmental expression pattern of the mouse WD-repeat gene DMR-N9 immediately upstream of the myotonic dystrophy locus

The diverse biological consequences of size-expansion of the unstable (CTG)n repeat in the myotonic dystrophy protein kinase (DM-PK) gene at chromosome region 19q13.3, are still poorly understood. Abnormal (CTG)n length may affect either DM-PK mRNA fate or function, or alternatively, compromise gene transcription by distortion of chromatin configuration. In the latter model involvement of neighbouring genes in DM upon extreme expansion of the repeat cannot be discarded as a possibility and should be studied further. Here we report on the elucidation of the complete genomic structure and expression pattern of the mouse DMR-N9 gene (called 59 gene in humans), which is at 1.1 kbp upstream of the DM-PK gene. This gene contains five exons spanning 7 kbp and codes for a protein of 650 amino acids. Two regions of the predicted protein show significant homology to WD repeats, highly conserved amino acid sequences found in a family of proteins engaged in signal transduction or cell regulatory functions. The start site of transcription has been determined and we have identified putative transcription factor binding sequences in a 400 bp putative promoter area immediately upstream of the transcribed unit. Northern blotting analysis and RNA in situ hybridization revealed ubiquitous low expression in all tissues of the mouse embryo and enhanced expression in adult brain and testis. The onset of transcription is phased early in mouse embryogenesis, before or at day 9.5 of gestation. From day 14.5 onwards DMR-N9 mRNAs were detected in all neural tissues, especially in the telencephalon and mesencephalon. Later, mRNA presence is evident in distinct tubules of the mature testis, restricted to secondary spermatocytes of stages VIII to XII of the spermatogenic proliferation cycle. We conclude that the DMR-N9 gene is a candidate for being involved in the manifestation of mental and testicular symptoms in severe cases of DM.

Gonosomal mosaicism in myotonic dystrophy patients: involvement of mitotic events in (CTG)n repeat variation and selection against extreme expansion in sperm

Myotonic dystrophy (DM) is caused by abnormal expansion of a polymorphic (CTG)n repeat, located in the DM protein kinase gene. We determined the (CTG)n repeat lengths in a broad range of tissue DNAs from patients with mild, classical, or congenital manifestation of DM. Differences in the repeat length were seen in somatic tissues from single DM individuals and twins. Repeats appeared to expand to a similar extent in tissues originating from the same embryonal origin. In most male patients carrying intermediate- or small-sized expansions in blood, the repeat lengths covered a markedly wider range in sperm. In contrast, male patients with large allele expansions in blood (> 700 CTGs) had similar or smaller repeats in sperm, when detectable. Sperm alleles with > 1,000 CTGs were not seen. We conclude that DM patients can be considered gonosomal mosaics, i.e., combined somatic and germ-line tissue mosaics. Most remarkably, we observed multiple cases where the length distributions of intermediate- or small-sized alleles in fathers' sperm were significantly different from that in their offspring's blood. Our combined findings indicate that intergenerational length changes in the unstable CTG repeat are most likely to occur during early embryonic mitotic divisions in both somatic and germ-line tissue formation. Both the initial CTG length, the overall number of cell divisions involved in tissue formation, and perhaps a specific selection process in spermatogenesis may influence the dynamics of this process. A model explaining mitotic instability and sex-dependent segregation phenomena in DM manifestation is discussed.

A long-range restriction map of the human chromosome 19q13 region: close physical linkage between CKMM and the ERCC1 and ERCC2 genes

We report on the physical ordering of genes in a relatively small area of chromosome 19, segment q13, containing the locus for myotonic dystrophy (DM), the most frequent heritable muscular dystrophy of adulthood in man. DNAs from somatic cell hybrids with der 19q products that carry a breakpoint across the muscle-specific creatine kinase (CKMM) gene were analyzed by Southern blotting using probes for CKMM, APOC2, and the repair genes ERCC1 and ERCC2. Results were combined with data from CHEF and field inversion-gel-electrophoresis separation of large-sized DNA restriction fragments to establish a map localizing both DNA-repair genes and the CKMM gene within the same 250 kb of DNA, the order being cen-CKMM-ERCC2-ERCC1-ter, with APOC2 being at more than 260 kb proximal to CKMM. Transcriptional start sites of the CKMM and DNA-repair genes are all on the telomeric side of the genes. Our results provide a framework for the construction of a larger physical map of the area, which will facilitate the search for the DM gene.

An MspI RFLP detected by the human glandular kallikrein gene (hGK) on chromosome 19q

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EcoRI RFLP at 19q13.1 identified by the anonymous DNA sequence p58B18 [D19S32]

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MspI RFLP at 19q12-q13.1 identified by the anonymous DNA sequence p30B18 [D19S31]

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TaqI RFLP at 19q13.1 identified by the anonymous DNA sequence p5B18 [D19S28]

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MspI RFLP at 19q12-q13.1 identified by the anonymous DNA sequence p20B18 [D19S30]

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A locus at 19cen-19q13.2 (D19S15) containing three RFLPs linked to myotonic dystrophy (DM) is recognized by probe pJSB6

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pHW60, an anonymous single copy clone from the proximal portion of the long arm of chromosome 19 (HGM8 assignment no. D19S13)

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