Biology subjects

Barndt, S.

Publications and source records attributed to Barndt, S..

3 recordsLinked to original sources

bioRxiv2025

DNA extrusion size determines pathway choice during CAG repeat expansion

DNA triplet repeat expansion is the mutational cause of neurodegenerative disorders such as Huntingtons disease, myotonic dystrophy type 1, and fragile-X related disorders. There is a general consensus that recognition of extrahelical extrusions or hairpin-loop structures (formed by strand slippage) by the DNA mismatch repair protein MutS{beta} leads to repeat expansion by a mutagenic repair process. By contrast, the FAN1 nuclease prevents triplet repeat expansion, the molecular basis of which was explained by our recent finding that FAN1 nuclease cleaves and initiates removal of extrahelical extrusions. We have proposed that competition for extrusion binding between FAN1 and MutS{beta} governs the outcome of the opposing effects of these two pathways. Here we show that extrusions containing 2-3 triplet repeats are recognized and processed by both FAN1 and MutS{beta} pathways. However, a single triplet extrusion escapes FAN1 cleavage and is exclusively processed by MutS{beta}-dependent MMR, leading to repeat expansion. Thus, the size of the extrahelical extrusions formed by strand slippage events affects the ultimate fate of the repeat elements, and controls the bias between repeat expansion or stability. These findings provide new insights into the role of DNA structural dynamics in establishing pathway choice in DNA repair.

biochemistry↗

bioRxiv2024

A novel isoform of Tensin1 promotes actin filament assembly for efficient erythroblast enucleation

Mammalian red blood cells are generated via a terminal erythroid differentiation pathway culminating in cell polarization and enucleation. Actin filament polymerization is critical for enucleation, but the molecular regulatory mechanisms remain poorly understood. We utilized publicly available RNA-seq and proteomics datasets to mine for actin-binding proteins and actin- nucleation factors differentially expressed during human erythroid differentiation and discovered that a focal adhesion protein--Tensin-1--dramatically increases in expression late in differentiation. Remarkably, we found that differentiating human CD34+ cells express a novel truncated form of Tensin-1 (eTNS1; Mr [~]125 kDa) missing the N-terminal half of the protein, due to an internal mRNA translation start site resulting in a unique exon 1. eTNS1 localized to the cytoplasm during terminal erythroid differentiation, with no apparent membrane association or focal adhesion formation. Knocking out eTNS1 had no effect on assembly of the spectrin membrane skeleton but led to impaired enucleation and absent or mis-localized actin filament foci in enucleating erythroblasts. We conclude that eTNS1 is a novel regulator of actin filament assembly during human erythroid terminal differentiation required for efficient enucleation.

cell biology↗

bioRxiv2024

Structural and molecular basis of FAN1 defects in promoting Huntington's disease

FAN1 is a DNA dependent nuclease whose proper function is essential for maintaining human health. For example, a genetic variant in FAN1, Arg507 to His hastens onset of Huntingtons disease, a repeat expansion disorder for which there is no cure. How the Arg507His mutation affects FAN1 structure and enzymatic function is unknown. Using cryo-EM and biochemistry, we have discovered that FAN1 arginine 507 is critical for its interaction with PCNA, and mutation of Arg507 to His attenuates assembly of the FAN1-PCNA on a disease-relevant extrahelical DNA extrusions formed within DNA repeats. This mutation concomitantly abolishes PCNA-FAN1-dependent cleavage of such extrusions, underscoring the importance of PCNA to the genome stabilizing function of FAN1. These results unravel the molecular basis for a specific mutation in FAN1 that dramatically hastens the onset of Huntingtons disease.

biochemistry↗