Biology subjects

Viterbo, D.

Publications and source records attributed to Viterbo, D..

4 recordsLinked to original sources

bioRxiv2024

Imperfect hairpins formed by CTG trinucleotide repeats are heteroduplex DNA molecules in living cells

There are over fifty microsatellite expansion disorders, often neurodegenerative pathologies, due to large expansions of tandem repeat sequences. These microsatellites vary in motif composition, size and copy number in the genome. The ability of these repeats to form secondary structures in vitro is strongly suspected to trigger the expansion process and therefore, the pathology. However, there is no formal proof that similar structures are formed in vivo, in living and replicating cells. We are focused on the CTG trinucleotide repeat whose expansion is responsible for myotonic dystrophy type 1 (DM1 or Steinert disease), affecting 1 in 8,000 people worldwide. A series of llama nanobodies (VHH) were selected to bind to the secondary structure formed by a synthetic imperfect CTG hairpin. Yeast one-hybrid was used to identify VHH that were able to activate a reporter gene in vivo . One of them was shown to be very specific since it binds a CTG hairpin but not a CAG hairpin or a GC-rich sequence. This nanobody also bind to long CTG trinucleotide repeats carried by plasmidic or genomic DNAs, proportionally to the number of repeats, suggesting that longer repeats tend to form more frequently imperfect hairpins. Finally, the VHH was bound to a protein G column and used to selectively enrich structure-containing DNA molecules, that were further observed by electron microscopy. They exhibit different kinds of structures in which only one strand is folded into an imperfect hairpin. Our results unambiguously show that alternative DNA structures are transiently formed by CTG trinucleotide repeats in living bacterial and yeast cells and suggest that imperfect CTG hairpins mainly exist in vivo mainly as heteroduplex DNA molecules.

molecular biology↗

bioRxiv2023

TALEN-induced contraction of CTG trinucleotide repeats in myotonic dystrophy type 1 cells

Trinucleotide repeat expansions are the cause of two dozen neurodegenerative and developmental disorders. One of these, myotonic dystrophy type 1 (Steinert disease, or DM1) is due to the expansion of a CTG triplet in the 3 UTR of the DMPK gene. We used highly specific DNA endonucleases to induce a double-strand break in the repeat tract to contract it below pathological length. Expression of a TALE Nuclease (TALEN) in human DM1 cells induced moderate CTG repeat contractions in 27% of the clones analyzed. These clones exhibited large internal deletions within the TALEN, occurring by homologous recombination between internal TALE repeats, inactivating the nuclease, and explaining its reduced efficacy. Taking advantage of the degeneracy of the genetic code, we recoded the TALEN sequence, to decrease internal redundancy and optimize codon usage. The new recoded TALEN showed increased efficacy in DM1 cells, with 68% of clones exhibiting a moderate to large contraction of the CTG repeat tract. In contrast, Staphylococcus aureus Cas9 (SaCas9) was unable to contract the CTG repeat tract. In parallel, we completely sequenced to very high coverage the DM1 genome using the PacBio technology. Several clones in which the TALEN was induced were also totally sequenced. In some of them, length changes of other long CTG repeats were detected, possibly corresponding to off-target effects, all of them in introns or intergenic regions. Repeat contractions were never associated with recombination of flanking markers, suggesting that contractions most probably occur by an intra-allelic mechanism such as single-strand annealing. TALENs should now be considered as a promising gene therapy approach, not only for DM1 but also for many other microsatellite expansion disorders.

genomics↗

bioRxiv2022

Functional variability in adhesion and flocculation of yeast megasatellite genes

Megasatellites are large tandem repeats found in all fungal genomes but especially abundant in the opportunistic pathogen Candida glabrata. They are encoded in genes involved in cell-cell interactions, either between yeasts or between yeast and human cells. In the present work, we have been using an iterative genetic system to delete several C. glabrata megasatellite-containing genes and found that two of them were positively involved in adhesion to epithelial cells, whereas three genes controlled negatively adhesion. Two of the latter, CAGL0B05061g or CAGL0A04851g, are also negative regulators of yeast-to-yeast adhesion, making them central players in controlling C. glabrata adherence properties. Using a series of synthetic Saccharomyces cerevisiae strains in which the FLO1 megasatellite was replaced by other tandem repeats of similar length but different sequences, we showed that the capacity of a strain to flocculate in liquid culture was unrelated to its capacity to adhere to epithelial cells or to invade agar. Finally, in order to understand how megasatellites were initially created and subsequently expanded, an experimental evolution system was set up, in which modified yeast strains containing different megasatellite seeds were grown in bioreactors for more than 200 generations and selected for their ability to sediment at the bottom of the culture tube. Several flocculation-positive mutants were isolated. Functionally relevant mutations included general transcription factors as well as a 230 kb segmental duplication.

genetics↗

bioRxiv2019

Resection and repair of a Cas9 double-strand break at CTG trinucleotide repeats induces local and extensive chromosomal rearrangements

Microsatellites are short tandem repeats, ubiquitous in all eukaryotes and represent [~]2% of the human genome. Among them, trinucleotide repeats are responsible for more than two dozen neurological and developmental disorders. Targeting microsatellites with dedicated DNA endonucleases could become a viable option for patients affected with dramatic neurodegenerative disorders. Here, we used the Streptococcus pyogenes Cas9 to induce a double-strand break within the expanded CTG repeat involved in myotonic dystrophy type 1, integrated in a yeast chromosome. Repair of this double-strand break generated unexpected large chromosomal rearrangements around the repeat tract. These rearrangements depended on RAD52, DNL4 and SAE2, and both non-homologous end-joining and single-strand annealing pathways were involved. Resection and repair of the double-strand break (DSB) were totally abolished in a rad50{Delta} strain, whereas they were impaired in a sae2{Delta} mutant, only on the DSB end containing most of the repeat tract. This proved that Sae2 plays significant different roles in resecting a DSB end containing a repeated and structured sequence as compared to a non-repeated DSB end.\n\nIn addition, we also discovered that gene conversion was less efficient when the DSB could be repaired using a homologous template, suggesting that the trinucleotide repeat may interfer with gene conversion too. Altogether, these data show that SpCas9 is probably not a good choice when inducing a double-strand break at or near a microsatellite, especially in mammalian genomes that contain many more dispersed repeated elements than the yeast genome.

genetics↗