A small unstructured nucleic acid disrupts a trinucleotide repeat hairpin

Dynamic DNA Energy Landscapes and Substrate Complexity in Triplet Repeat Expansion and DNA Repair

DNA repeat domains implicated in DNA expansion diseases exhibit complex conformational and energy landscapes that impact biological outcomes. These landscapes include ensembles of entropically driven positional interchanges between isoenergetic, isomeric looped states referred to as rollamers. Here, we present evidence for the position-dependent impact on repeat DNA energy landscapes of an oxidative lesion (8oxodG) and of an abasic site analogue (tetrahydrofuran, F), the universal intermediate in base excision repair (BER). We demonstrate that these lesions modulate repeat bulge loop distributions within the wider dynamic rollamer triplet repeat landscapes. We showed that the presence of a lesion disrupts the energy degeneracy of the rollameric positional isomers. This lesion-induced disruption leads to the redistribution of loop isomers within the repeat loop rollamer ensemble, favoring those rollameric isomers where the lesion is positioned to be energetically least disruptive. These dynamic ensembles create a highly complex energy/conformational landscape of potential BER enzyme substrates to select for processing or to inhibit processing. We discuss the implications of such lesion-induced alterations in repeat DNA energy landscapes in the context of potential BER repair outcomes, thereby providing a biophysical basis for the intriguing in vivo observation of a linkage between pathogenic triplet repeat expansion and DNA repair.

Current molecular insight to reveal the dynamics of CAG repeating units in spinocerebellar ataxia

Spinocerebellar ataxia (SCA) is a heterogeneous genetic disorder with overlapping clinical phenotypes arising from the degeneration of purkinje cells and other regions of the brain. There are approximately 36 different subtypes of SCA, but SCA 1, 2, 3, 6 and 7 are most prevalent in the Indian population. Many findings suggested that cerebellar Purkinje cells region may be a uniquely vulnerable neuronal cell type, and more susceptible to a wider variety of genetic or cellular problems than other neuron types. In this review we emphasized mainly five common subtypes of SCA (1, 2, 3, 6 and 7) their pathophysiology, therapeutics, drugs studies and the technical challenges in the field of molecular genetic diagnosis.

Genetic characterization of Spinocerebellar ataxia 1 in a South Indian cohort

Background

Spinocerebellar ataxia type 1 (SCA1) is a late onset autosomal dominant cerebellar ataxia, caused by CAG triplet repeat expansion in the ATXN1 gene. The frequency of SCA1 occurrence is more in Southern India than in other regions as observed from hospital-based studies. However there are no reports on variability of CAG repeat expansion, phenotype-genotype association and founder mutations in a homogenous population from India.

Methods

Genomic DNA isolated from buccal mouthwash of the individuals in the cohort was used for PCR-based diagnosis of SCA1. Subsequently SNP’s found within the ATXN1 loci were identified by Taqman allelic discrimination assays. Significance testing of the genotype-phenotype associations was calculated by Kruskal-Wallis ANOVA test with post-hoc Dunnett’s test and Pearson’s correlation coefficient.

Results

By genetic analysis of an affected population in Southern India we identified 21 pre-symptomatic individuals including four that were well past the average age of disease onset of 44 years, 16 symptomatic and 63 normal individuals. All pre-symptomatic cases harbor “pure” expansions of greater than 40 CAGs. Genotyping to test for the presence of two previously identified SNPs showed a founder effect of the same repeat carrying allele as in the general Indian population. We show that SCA1 disease onset is significantly delayed when transmission of the disease is maternal.

Conclusions

Our finding of early disease onset in individuals with a paternally inherited allele could serve as valuable information for clinicians towards early detection of SCA1 in patients with affected fathers. Identification of older pre-symptomatic individuals (n = 4) in our cohort among individuals with a shared genetic and environmental background, suggests that second site genetic or epigenetic modifiers might significantly affect SCA1 disease progression. Moreover, such undetected SCA1 cases could underscore the true prevalence of SCA1 in India.

Quencher-free molecular beacons as probes for oligonucleotides containing CAG repeat sequences

We have identified quencher-free molecular beacons that allow the sensitive probing of CAG repeat oligonucleotides, including mRNA fragments of trinucleotide repeat diseases, with significant increases in fluorescence intensity mediated by disruption of the stacking of their (Py)U units.

Impact of bulge loop size on DNA triplet repeat domains: Implications for DNA repair and expansion

Repetitive DNA sequences exhibit complex structural and energy landscapes, populated by metastable, noncanonical states, that favor expansion and deletion events correlated with disease phenotypes. To probe the origins of such genotype-phenotype linkages, we report the impact of sequence and repeat number on properties of (CNG) repeat bulge loops. We find the stability of duplexes with a repeat bulge loop is controlled by two opposing effects; a loop junction-dependent destabilization of the underlying double helix, and a self-structure dependent stabilization of the repeat bulge loop. For small bulge loops, destabilization of the underlying double helix overwhelms any favorable contribution from loop self-structure. As bulge loop size increases, the stabilizing loop structure contribution dominates. The role of sequence on repeat loop stability can be understood in terms of its impact on the opposing influences of junction formation and loop structure. The nature of the bulge loop affects the thermodynamics of these two contributions differently, resulting in unique differences in repeat size-dependent minima in the overall enthalpy, entropy, and free energy changes. Our results define factors that control repeat bulge loop formation; knowledge required to understand how this helix imperfection is linked to DNA expansion, deletion, and disease phenotypes.

Oligodeoxynucleotide binding to (CTG) · (CAG) microsatellite repeats inhibits replication fork stalling, hairpin formation, and genome instability

(CTG)(n) · (CAG)(n) trinucleotide repeat (TNR) expansion in the 3' untranslated region of the dystrophia myotonica protein kinase (DMPK) gene causes myotonic dystrophy type 1. However, a direct link between TNR instability, the formation of noncanonical (CTG)(n) · (CAG)(n) structures, and replication stress has not been demonstrated. In a human cell model, we found that (CTG)(45) · (CAG)(45) causes local replication fork stalling, DNA hairpin formation, and TNR instability. Oligodeoxynucleotides (ODNs) complementary to the (CTG)(45) · (CAG)(45) lagging-strand template eliminated DNA hairpin formation on leading- and lagging-strand templates and relieved fork stalling. Prolonged cell culture, emetine inhibition of lagging-strand synthesis, or slowing of DNA synthesis by low-dose aphidicolin induced (CTG)(45) · (CAG)(45) expansions and contractions. ODNs targeting the lagging-strand template blocked the time-dependent or emetine-induced instability but did not eliminate aphidicolin-induced instability. These results show directly that TNR replication stalling, replication stress, hairpin formation, and instability are mechanistically linked in vivo.