Common fragile site instability in normal cells: Lessons and perspectives

Abstract

Mechanisms and events related to common fragile site (CFS) instability are well known in cancer cells. Here, we argue that normal cells remain an important experimental model to address questions related to CFS instability in the absence of alterations in cell cycle and DNA damage repair pathways, which are common features acquired in cancer. Furthermore, a major gap of knowledge concerns the stability of CFSs during gametogenesis. CFS instability in meiotic or postmeiotic stages of the germ cell line could generate chromosome deletions or large rearrangements. This in turn can lead to the functional loss of the several CFS‐associated genes with tumor suppressor function. Our hypothesis is that such mutations can potentially result in genetic predisposition to develop cancer. Indirect evidence for CFS instability in human germ cells has been provided by genomic investigations in family pedigrees associated with genetic disease. The issue of CFS instability in the germ cell line should represent one of the future efforts, and may take advantage of the existence of sequence and functional conservation of CFSs between rodents and humans.

Meiotic Chromosome Contacts as a Plausible Prelude for Robertsonian Translocations

“…We revealed differences in mitotic cell divisions for fibroblast and chondrocyte cultures. Cell culture often demonstrates unstable karyotype structures and distinct abnormality types and frequencies [67,68]. This assumption was our rationale for comparing data on chromosome numbers and behaviors using bone marrow slides, and two different cell culture approaches.…”

The epicenter of chromosomal fragility of Fra14A2, the mouse ortholog of human FRA3B common fragile site, is largely attached to the nuclear matrix in lymphocytes but not in other cell types that do not express such a fragility

“…Such NHOS correlates with the fact that Fra14A2 is not a CFS in proliferating hepatocytes, that although usually quiescent display a most remarkable proliferating capacity after partial hepatectomy in vivo . In contrast, the same genomic region is largely attached or at least embedded within the NM in naive B lymphocytes (Figure C) that usually express the Fra14A2 CFS when subjected to replication stress . Moreover, the exon 5 that coincides with the main core of fragility seems to be bound to (or at least to interact with) the NM in a very special fashion, since the target N sequence that flags the 5′ end of exon 5 cannot be directly amplified on NM‐bound templates (Figure C) and so it is necessary to eliminate the NM so that the corresponding target DNA becomes amplifiable (Figure B).…”

“…Common fragile sites (CFSs) are chromosomal regions susceptible to present constrictions, discontinuities, breaks or re‐arrangements in metaphase, condensed chromosomes from cells subjected to replication stress . Such CFSs were recognized as recurrent locations for double‐stranded DNA breaks in cultured peripheral blood lymphocytes from most sampled healthy humans, and so they were considered as a structural feature of normal chromosomes even though they may exhibit differential frequency of expression in a population . Moreover, disruption of DNA replication by agents that reduce the progression of the replication fork (such as aphidicolin that inhibits DNA polymerases) induces the expression of fragility; therefore, the CFSs are considered regions that are intrinsically difficult to copy during DNA replication .…”

“…Common fragile sites (CFSs) are chromosomal regions susceptible to present constrictions, discontinuities, breaks or re‐arrangements in metaphase, condensed chromosomes from cells subjected to replication stress . Such CFSs were recognized as recurrent locations for double‐stranded DNA breaks in cultured peripheral blood lymphocytes from most sampled healthy humans, and so they were considered as a structural feature of normal chromosomes even though they may exhibit differential frequency of expression in a population .…”

Sites of chromosomal instability in the context of nuclear architecture and function

“…Every cell is exposed to up to thousands of single lesions from exogenous and endogenous sources to mostly challenge the integrity of the genetic material within one generation when affecting somatic cells [4,5]. However, when germ cells are affected, mutations can be passed on to subsequent generations, thereby leading to a number of about 70 de novo mutations per diploid human genome per generation with a rate of 0.35 deleterious amino acid mutations per diploid genome [6] and might predispose to cancer development [7]. Hence, mutations accumulate in the course of cellular division that will be positive, negative or neutral for the affected organism.…”