The Fecal Position: Tracking Progressive DNA Repeat Expansion via Stool DNA Extraction

Friedreich Ataxia is a progressive DNA repeat expansion disease. Examining DNA repeat expansion in mouse models require sacrificing the mouse and taking samples of organs. The obvious non-lethal targets, such as ears, tails, and blood do not have levels of repeat expansion comparable to internal organs. However, recent publications suggest that stool may be a suitable non-lethal candidate for tracking repeat expansion over time. Stool DNA may allow researchers to monitor the effect of interventions aimed at slowing DNA repeat expansion over time without harming the mouse. A commercial kit was first used to extract DNA from feces without success. Stool DNA was isolated using a “home-made” approach based on older methodology. The presence of mouse DNA within the bacterial background DNA was first confirmed using mouse beta actin PCR primers. This was followed by two rounds of nested PCR with PCR primers specific for the expanded GAA●TTC tract in the frataxin transgene carried by the Friedrich Ataxia model mouse. Stool DNA poses a unique challenge due to the degradation of its components, high lipid content, and high level of bacterial DNA contamination. However, this method of DNA extraction was nearly 100% successful. We compare ear samples taken at three weeks to stool taken later to prove the utility of this approach for repeat expansion models

Transposon Tn7 Preferentially Inserts into GAA•TTC Triplet Repeats under Conditions Conducive to Y•R•Y Triplex Formation

BACKGROUND: Expansion of an unstable GAA*TTC repeat in the first intron of the FXN gene causes Friedreich ataxia by reducing frataxin expression. Structure formation by the repeat has been implicated in both frataxin repression and GAA*TTC instability. The GAA*TTC sequence is capable of adopting multiple non-B DNA structures including Y*R*Y and R*R*Y triplexes. Lower pH promotes the formation of Y*R*Y triplexes by GAA*TTC. Here we used the bacterial transposon Tn7 as an in vitro tool to probe whether GAA*TTC repeats can attract a well-characterized recombinase. METHODOLOGY/PRINCIPAL FINDINGS: Tn7 showed a pH-dependent preference for insertion into uninterrupted regions of a Friedreich ataxia patient-derived repeat, inserting 48, 39 and 14 percent of the time at pH 7, pH 8 and pH 9, respectively. Moreover, Tn7 also showed orientation and region specific insertion within the repeat at pH 7 and pH 8, but not at pH 9. In contrast, transposon Tn5 showed no strong preference for or against the repeat during in vitro transposition at any pH tested. Y*R*Y triplex formation was reduced in predictable ways by transposon interruption of the GAA*TTC repeat. However, transposon interruptions in the GAA*TTC repeats did not increase the in vitro transcription efficiency of the templates. CONCLUSIONS/SIGNIFICANCE: We have demonstrated that transposon Tn7 will recognize structures that form spontaneously in GAA*TTC repeats and insert in a specific orientation within the repeat. The conditions used for in vitro transposition span the physiologically relevant range suggesting that long GAA*TTC repeats can form triplex structures in vivo, attracting enzymes involved in DNA repair, recombination and chromatin modification

Mismatch in Mice and Men

Friedreich ataxia is a progressive degenerative neuromuscular disease that is caused by the expansion of a repetitive region of DNA, composed of three nucleotide repeats (GAA•TTC). Expansion of the DNA occurs throughout the lifespan of the patient and has been linked to the activity of specific DNA mismatch repair proteins. Disease onset occurs when the expansion increases in size beyond a certain threshold, silencing the gene and causing progressive ataxia, diabetes mellitus, and cardiomyopathy. These symptoms are linked to an increased repeat number observed within the heart, pancreas, and brain relative to other tissues within an individual. Friedreich ataxia is a fatal disease, most patients die of heart failure due to cardiomyopathy. Transgenic mice are commonly used as a model to study disease. Friedreich ataxia repeat expansion has been shown to be highest in human cardiac tissue. In contrast, our lab has observed that no expansion occurs in the heart of Friedreich ataxia model mice. The purpose of this experiment was to perform a Western Blot assay on the cardiac tissue of transgenic mice to detect the presence of specific mismatch repair proteins. The goal was to compare this to the expansion positive tissue of the same mouse and normal human cardiac tissue. Primary antibodies specific to the human mismatch repair protein subunits available in the lab were used following protein extraction from the target tissues with the intention of cross-reactivity between species. Unfortunately, the human-specific primary antibodies did not cross react with the mouse mismatch repair protein subunits. Acquisition of mouse specific primary antibodies was restrained by financial and temporal limitations. Next steps in the investigation of tissue specific mismatch repair proteins present within mice include western blot analysis using mousespecific, preferably polyclonal primary antibodies to ensure visualization of the proteins present is achieved

A persistent RNA·DNA hybrid formed by transcription of the Friedreich ataxia triplet repeat in live bacteria, and by T7 RNAP in vitro

Expansion of an unstable GAA·TTC repeat in the first intron of the FXN gene causes Friedreich ataxia by reducing frataxin expression. Deficiency of frataxin, an essential mitochondrial protein, leads to progressive neurodegeneration and cardiomyopathy. The degree of frataxin reduction correlates with GAA·TTC tract length, but the mechanism of reduction remains controversial. Here we show that transcription causes extensive RNA·DNA hybrid formation on GAA·TTC templates in bacteria as well as in defined transcription reactions using T7 RNA polymerase in vitro. RNA·DNA hybrids can also form to a lesser extent on smaller, so-called ‘pre-mutation’ size GAA·TTC repeats, that do not cause disease, but are prone to expansion. During in vitro transcription of longer repeats, T7 RNA polymerase arrests in the promoter distal end of the GAA·TTC tract and an extensive RNA·DNA hybrid is tightly linked to this arrest. RNA·DNA hybrid formation appears to be an intrinsic property of transcription through long GAA·TTC tracts. RNA·DNA hybrids have a potential role in GAA·TTC tract instability and in the mechanism underlying reduced frataxin mRNA levels in Friedreich Ataxia

Progressive GAA·TTC Repeat Expansion in Human Cell Lines

Trinucleotide repeat expansion is the genetic basis for a sizeable group of inherited neurological and neuromuscular disorders. Friedreich ataxia (FRDA) is a relentlessly progressive neurodegenerative disorder caused by GAA·TTC repeat expansion in the first intron of the FXN gene. The expanded repeat reduces FXN mRNA expression and the length of the repeat tract is proportional to disease severity. Somatic expansion of the GAA·TTC repeat sequence in disease-relevant tissues is thought to contribute to the progression of disease severity during patient aging. Previous models of GAA·TTC instability have not been able to produce substantial levels of expansion within an experimentally useful time frame, which has limited our understanding of the molecular basis for this expansion. Here, we present a novel model for studying GAA·TTC expansion in human cells. In our model system, uninterrupted GAA·TTC repeat sequences display high levels of genomic instability, with an overall tendency towards progressive expansion. Using this model, we characterize the relationship between repeat length and expansion. We identify the interval between 88 and 176 repeats as being an important length threshold where expansion rates dramatically increase. We show that expansion levels are affected by both the purity and orientation of the repeat tract within the genomic context. We further demonstrate that GAA·TTC expansion in our model is independent of cell division. Using unique reporter constructs, we identify transcription through the repeat tract as a major contributor to GAA·TTC expansion. Our findings provide novel insight into the mechanisms responsible for GAA·TTC expansion in human cells

Somatic CAG Expansion in Huntington\u27s Disease Is Dependent on the MLH3 Endonuclease Domain, Which Can Be Excluded via Splice Redirection

Somatic expansion of the CAG repeat tract that causes Huntington\u27s disease (HD) is thought to contribute to the rate of disease pathogenesis. Therefore, factors influencing repeat expansion are potential therapeutic targets. Genes in the DNA mismatch repair pathway are critical drivers of somatic expansion in HD mouse models. Here, we have tested, using genetic and pharmacological approaches, the role of the endonuclease domain of the mismatch repair protein MLH3 in somatic CAG expansion in HD mice and patient cells. A point mutation in the MLH3 endonuclease domain completely eliminated CAG expansion in the brain and peripheral tissues of a HD knock-in mouse model (HttQ111). To test whether the MLH3 endonuclease could be manipulated pharmacologically, we delivered splice switching oligonucleotides in mice to redirect Mlh3 splicing to exclude the endonuclease domain. Splice redirection to an isoform lacking the endonuclease domain was associated with reduced CAG expansion. Finally, CAG expansion in HD patient-derived primary fibroblasts was also significantly reduced by redirecting MLH3 splicing to the endogenous endonuclease domain-lacking isoform. These data indicate the potential of targeting the MLH3 endonuclease domain to slow somatic CAG repeat expansion in HD, a therapeutic strategy that may be applicable across multiple repeat expansion disorders