Disease-associated CAG{middle dot}CTG triplet repeats expand rapidly in non-dividing mouse cells, but cell cycle arrest is insufficient to drive expansion

Abstract

Genetically unstable expanded CAG·CTG trinucleotide repeats are causal in a number of human disorders, including Huntington disease and myotonic dystrophy type 1. It is still widely assumed that DNA polymerase slippage during replication plays an important role in the accumulation of expansions. Nevertheless, somatic mosaicism correlates poorly with the proliferative capacity of the tissue and rates of cell turnover, suggesting that expansions can occur in the absence of replication. We monitored CAG·CTG repeat instability in transgenic mouse cells arrested by chemical or genetic manipulation of the cell cycle and generated unequivocal evidence for the continuous accumulation of repeat expansions in non-dividing cells. Importantly, the rates of expansion in non-dividing cells were at least as high as those of proliferating cells. These data are consistent with a major role for cell division-independent expansion in generating somatic mosaicism in vivo. Although expansions can accrue in non-dividing cells, we also show that cell cycle arrest is not sufficient to drive instability, implicating other factors as the key regulators of tissue-specific instability. Our data reveal that de novo expansion events are not limited to S-phase and further support a cell division-independent mutational pathway.

Age-, tissue- and length-dependent bidirectional somatic CAG•CTG repeat instability in an allelic series of R6/2 Huntington disease mice

“…However, analysis of broader patterns of tissue specificity does not support an association with DNA mismatch repair protein levels (Mason et al, 2014) but instead highlights pathways regulating exit from the cell cycle (Lee et al, 2010). Such an observation is consistent with the lack of association of tissue-specific patterns of instability with cell proliferation rates in vivo (Lia et al, 1998;Fortune et al, 2000;Kennedy and Shelbourne, 2000;Shelbourne et al, 2007;Gonitel et al, 2008) or in vitro (Gomes-Pereira et al, 2001;Gomes-Pereira et al, 2014). As observed here in the R6/2 mice with smaller expansions and previously in HD knock-in mice (Lee et al, 2011), a broader spread of expanded alleles is observed in striatal cells, despite a similar mean level of expansion in liver.…”

Semi-automated quantification of C9orf72 expansion size reveals inverse correlation between hexanucleotide repeat number and disease duration in frontotemporal degeneration

“…The low degree of repeat instability in cerebellum may be related to the post-mitotic nature of neurons which is particularly relevant for the cerebellum which exhibits the highest neuronal density of any tissue or brain region. Notably, CAG-CTG trinucleotide repeats have been demonstrated to expand in non-dividing mouse cells, suggesting that cell division-independent expansion can also play a role in repeat instability [16]. …”

Repeat instability during DNA repair: Insights from model systems

“…Since even expansion-prone somatic cells that are not post-mitotic proliferate very slowly, non-replicative expansion mechanisms are likely to explain many somatic expansions. Supporting this idea, inhibition of cell proliferation using a number of different approaches does not reduce expansions in cultured transgenic DM1 mouse cells (Gomes-Pereira et al ., 2014). However, there may be replication-coupled somatic expansions in some tissues.…”