Faculty Opinions recommendation of Replication of a common fragile site, FRA3B, occurs late in S phase and is delayed further upon induction: implications for the mechanism of fragile site induction.

ABSTRACT Human DNA polymerase η (Pol η) modulates susceptibility to skin cancer by promoting translesion DNA synthesis (TLS) past sunlight-induced cyclobutane pyrimidine dimers. Despite its well-established role in TLS synthesis, the role of Pol η in maintaining genome stability in the absence of external DNA damage has not been well explored. We show here that short hairpin RNA-mediated depletion of Pol η from undamaged human cells affects cell cycle progression and the rate of cell proliferation and results in increased spontaneous chromosome breaks and common fragile site expression with the activation of ATM-mediated DNA damage checkpoint signaling. These phenotypes were also observed in association with modified replication factory dynamics during S phase. In contrast to that seen in Pol η-depleted cells, none of these cellular or karyotypic defects were observed in cells depleted for Pol ι, the closest relative of Pol η. Our results identify a new role for Pol η in maintaining genomic stability during unperturbed S phase and challenge the idea that the sole functional role of Pol η in human cells is in TLS DNA damage tolerance and/or repair pathways following exogenous DNA damage.

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Replication Delay along FRA7H, a Common Fragile Site on Human Chromosome 7, Leads to Chromosomal Instability

Molecular and Cellular Biology ◽

10.1128/mcb.20.12.4420-4427.2000 ◽

2000 ◽

Vol 20 (12) ◽

pp. 4420-4427 ◽

Cited By ~ 69

Author(s):

Asaf Hellman ◽

Ayelet Rahat ◽

Stephen W. Scherer ◽

Ariel Darvasi ◽

Lap-Chee Tsui ◽

...

Keyword(s):

Chromosomal Instability ◽

Fragile Site ◽

S Phase ◽

Fragile Sites ◽

Common Fragile Site ◽

Replication Pattern ◽

Replication Time ◽

The Common ◽

Human Chromosome 7

ABSTRACT Common fragile sites are specific chromosomal loci that show gaps, breaks, or rearrangements in metaphase chromosomes under conditions that interfere with DNA replication. The mechanism underlying the chromosomal instability at fragile sites was hypothesized to associate with late replication time. Here, we aimed to investigate the replication pattern of the common fragile site FRA7H, encompassing 160 kb on the long arm of human chromosome 7. Using in situ hybridization on interphase nuclei, we revealed that the replication of this region is initiated relatively early, before 30% of S phase is completed. However, a high fraction (∼35%) of S-phase nuclei showed allelic asynchrony, indicating that the replication of FRA7H is accomplished at different times in S phase. This allelic asynchrony is not the result of a specific replication time of each FRA7H allele. Analysis of the replication pattern of adjacent clones along FRA7H by using cell population and two-color fluorescent in situ hybridization analyses showed significant differences in the replication of adjacent clones, under normal growth condition and upon aphidicolin treatment. This pattern significantly differed from that of two nonfragile regions which showed a coordinated replication under both conditions. These results indicate that aphidicolin is enhancing an already existing difference in the replication time along the FRA7H region. Based on our replication analysis of FRA7H and on previous analysis of the common fragile site FRA3B, we suggest that delayed replication is underlying the fragility at aphidicolin-induced common fragile sites.

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