Human mitochondrial topoisomerase I.

Tension generated in the circular mitochondrial genome during replication and transcription points to the need for mtDNA topoisomerase activity. Here we report a 601-aa polypeptide highly homologous to nuclear topoisomerase I. The N-terminal domain of this novel topoisomerase contains a mitochondrial localization sequence and lacks a nuclear localization signal. Therefore, we refer to this polypeptide as top1mt. The pattern of top1mt expression matches the requirement for high mitochondrial activity in specific tissues. top1mt is a type IB topoisomerase that requires divalent metal (Ca(2+) or Mg(2+)) and alkaline pH for optimum activity. The TOP1mt gene is highly homologous to the nuclear TOP1 gene and consists of 14 exons. It is localized on human chromosome 8q24.3.

Fluorescence in situ hybridization analysis of the replication properties of the myotonic dystrophy protein kinase (DMPK) gene region.

Myotonic dystrophy (DM) is caused by an expansion of a CTG repeat sequence in the 3' noncoding region of a protein kinase gene (DMPK) at 19q13.3. We used in situ hybridization to analyse the replication timing of the genomic region containing DMPK in fibroblasts and myoblasts from controls and myotonic dystrophy patients. In this method the relative proportion of singlet to doublet hybridization signals is used to infer the relative time of replication of specific loci or regions. Our results show that in cells from normal individuals approximately 65% of signals appear as doublets, indicating early replication. In DM patients with a number of CTG repeats ranging from about 600-1800 we observed a significant increase of singlet-doublets compared to the background level. These results suggest the existence of replication alternations and/or structural differences between the normal and mutant alleles induced by the presence of the DM mutation.

Characterization of large CTG repeat expansions in myotonic dystrophy alleles using PCR.

The CTG trinucleotide repeat expansions that are associated with myotonic dystrophy can be up to several thousand repeat units in length. We have developed a PCR protocol that has the potential to amplify mutant alleles with very large numbers of CTG repeats. The amplification uses the rTth DNA polymerase, XL system for long PCR targets together with primers which do not closely flank the repeat region and partial substitution of 7-deaza-dGTP for dGTP. Alleles containing up to approximately 800 CTG repeats were detected directly in agarose gels stained with ethidium bromide. Larger CTG repeat expansions required Southern blot transfer and detection with a repeat sequence probe; using this method, alleles containing up to approximately 2700 CTG repeats were detected. The PCR-based method described here was comparable to previous Southern blots of EcoRI-restriction digested genomic DNA in both the approximate size and heterogeneity of mutant alleles detected, but provided more precise sizes of the CTG repeat expansions than the restriction digest approach. This PCR protocol could potentially simplify current mutation detection protocols in the molecular diagnosis of myotonic dystrophy, and facilitate molecular studies of the disease.

Characteristics of intergenerational contractions of the CTG repeat in myotonic dystrophy.

In myotonic dystrophy (DM), the size of a CTG repeat in the DM kinase gene generally increases in successive generations with clinical evidence of anticipation. However, there have also been cases with an intergenerational contraction of the repeat. We examined 1,489 DM parent-offspring pairs, of which 95 (6.4%) showed such contractions in peripheral blood leukocytes (PBL). In 56 of the 95 pairs, clinical data allowed an analysis of their anticipation status. It is surprising that anticipation occurred in 27 (48%) of these 56 pairs, while none clearly showed a later onset of DM in the symptomatic offspring. The contraction occurred in 76 (10%) of 753 paternal transmissions and in 19 (3%) of 736 maternal transmissions. Anticipation was observed more frequently in maternal (85%) than in paternal (37%) transmissions (P < .001). The parental repeat size correlated with the size of intergenerational contraction (r2 = .50, P << .001), and the slope of linear regression was steeper in paternal (-.62) than in maternal (-.30) transmissions (P << .001). Sixteen DM parents had multiple DM offspring with the CTG repeat contractions. This frequency was higher than the frequency expected from the probability of the repeat contractions (6.4%) and the size of DM sib population (1.54 DM offspring per DM parent, in 968 DM parents). We conclude that (1) intergenerational contractions of the CTG repeat in leukocyte DNA frequently accompanies apparent anticipation, especially when DM is maternally transmitted, and (2) the paternal origin of the repeat and the presence of the repeat contraction in a sibling increase the probability of the CTG repeat contraction.

High resolution genetic analysis suggests one ancestral predisposing haplotype for the origin of the myotonic dystrophy mutation.

The mutation causing myotonic dystrophy (DM) has been identified as an amplification of an unstable trinucleotide (CTG)n repeat in over 99% of the global DM population. It is in complete linkage disequilibrium with an Alu element polymorphism within the DM kinase gene, suggesting that DM is a consequence of one or few ancestral mutations. A recent analysis utilizing this polymorphism as well as a flanking dinucleotide marker, suggested that similar to Fragile X syndrome, DM exhibited a founder effect (Imbert et al., 1993 Nature Genet. 4, 72-76). In contrast, the low reproductive fitness of individuals with congenital DM (the endpoint of genetic anticipation in myotonic dystrophy) suggests a higher rate of new mutations. We present a high resolution genetic analysis of the DM locus using PCR based assays of nine polymorphisms, spanning a physical distance of 30 kb, within and immediately flanking the DM kinase gene. The persistent complete allelic association of the DM mutation with all these polymorphisms provides further support to previous observations and suggests more strongly that the DM mutation occurred on the background of a particular haplotype in which the (CTG)n repeat became inherently unstable and therefore predisposed to amplification.

Intergenerational stability of the myotonic dystrophy protomutation.

The amplification of the CTG trinucleotide repeat in myotonic dystrophy (DM) correlates with increasingly severe phenotypes. We designate its minimal amplification the 'protomutation' since it is the mutation itself at an early stage of intergenerational evolution and is associated with very mild clinical signs. From the study of 536 DM mutation carriers (from 158 affected families), a total of 60 DM-parent/DM-offspring pairings were identified in which the parent had the protomutation. We found a strong correlation between the protomutation length and the amplification observed in the next generation. We also observed the stable transmission of the protomutation through successive generations. This stability may explain the maintenance in the population of this autosomal dominant disease despite the low reproductive fitness of severe DM phenotypes.