Common fragile sites (CFSs) are particularly vulnerable regions of the genome that become visible as breaks, gaps, or constrictions on metaphase chromosomes when cells are under replicative stress. Impairment in DNA replication, late replication timing, enrichment of A/T nucleotides that tend to form secondary structures, the paucity of active or inducible replication origins, the generation of R-loops, and the collision between replication and transcription machineries on particularly long genes are some of the reported characteristics of CFSs that may contribute to their tissue-specific fragility. Here, we validated the induction of two CFSs previously found in the human fetal lung fibroblast line, Medical Research Council cell strain 5 (MRC-5), in another cell line derived from the same fetal tissue, Institute for Medical Research-90 cells (IMR-90). After induction of CFSs through aphidicolin, we confirmed the expression of the CFS 1p31.1 on chromosome 1 and CFS 3q13.3 on chromosome 3 in both fetal lines. Interestingly, these sites were found to not be fragile in lymphocytes, suggesting a role for epigenetic or transcriptional programs for this tissue specificity. Both these sites contained late-replicating genes NEGR1 (neuronal growth regulator 1) at 1p31.1 and LSAMP (limbic system-associated membrane protein) at 3q13.3, which are much longer, 0.880 and 1.4 Mb, respectively, than the average gene length. Given the established connection between long genes and CFS, we compiled information from the literature on all previously identified CFSs expressed in fibroblasts and lymphocytes in response to aphidicolin, including the size of the genes contained in each fragile region. Our comprehensive analysis confirmed that the genes found within CFSs are longer than the average human gene; interestingly, the two longest genes in the human genome are found within CFSs: Contactin Associated Protein 2 gene (CNTNAP2) in a lymphocytes’ CFS, and Duchenne muscular dystrophy gene (DMD) in a CFS expressed in both lymphocytes and fibroblasts. This indicates that the presence of very long genes is a unifying feature of all CFSs. We also obtained replication profiles of the 1p31.1 and 3q13.3 sites under both perturbed and unperturbed conditions using a combination of fluorescent in situ hybridization (FISH) and immunofluorescence against bromodeoxyuridine (BrdU) on interphase nuclei. Our analysis of the replication dynamics of these CFSs showed that, compared to lymphocytes where these regions are non-fragile, fibroblasts display incomplete replication of the fragile alleles, even in the absence of exogenous replication stress. Our data point to the existence of intrinsic features, in addition to the presence of long genes, which affect DNA replication of the CFSs in fibroblasts, thus promoting chromosomal instability in a tissue-specific manner.

Although clinical antitumor activity of Tumor Treating Fields (TTFields) has been reported in malignant pleural mesothelioma (MPM) patients, the mechanisms behind the different selectivity displayed by the various MPM histotypes to this physical therapy has not been elucidated yet. Taking advantage of the development of well characterized human MPM cell lines derived from pleural effusion and/or lavages of patients’ thoracic cavity, we investigated the biological effects of TTFields against these cells, representative of epithelioid, biphasic, and sarcomatoid histotypes. Growth inhibition and cell cycle perturbations caused by TTFields were investigated side by side with RNA-Seq analyses at different exposure times to identify pathways involved in cell response to treatment. We observed significant differences of response to TTFields among the cell lines. Cell cycle analysis revealed that the most sensitive cells (epithelioid CD473) were blocked in G2M phase followed by formation of polyploid cells. The least sensitive cells (sarcomatoid CD60) were only slightly affected by TTFields with a general delay in all cell cycle phases. Apoptosis was present in all samples, but while epithelioid cell death was already observed during the first 24 h of treatment, sarcomatoid cells needed longer times before they engaged apoptotic pathways. RNA-Seq experiments demonstrated that TTFields induced a transcriptional response already detectable at early time points (8 h). The number of differentially expressed genes was higher in CD473 than in CD60 cells, involving several pathways, such as those pertinent to cell cycle checkpoints, DNA repair, and histone modifications. Our data provide further support to the notion that the antitumor effects of TTFields are not simply related to a non-specific reaction to a physical stimulus, but are dependent on the biological background of the cells and the particular sensitivity to TTFields observed in epithelioid MPM cells is associated with a higher transcriptional activity than that observed in sarcomatoid models.

The reported clinical activity of lurbinectedin in Small Cell Lung Cancer (SCLC) prompted us to investigate the selective mechanism of this drug in SCLC cell lines. Since lurbinectedin binds the DNA minor groove modifying DNA structure, we hypothesized that it can interfere with transcription regulation. Using SHP-77 SCLC cell line, we investigated the effect of lurbinectedin on transcriptomic profile by microarray approach, at drug concentration that reduced the cell growth rate by 50%. We found only 89 differently expressed genes (DEG), 31 up- regulated and 58 down-regulated, after 6 hours of treatment whereas there was a dramatic change in gene expression profiles after 24 hours (4529 DEGs of which 1982 up-regulated and 2547 down-regulated) and 48 hours (5457 DEGs of which 2368 up-regulated and 3089 down-regulated) of exposure. Among the genes that were clearly down-regulated at both 24 and 48 hours, there was ASCL1, a gene that encodes a transcription factor required for proper development of neuronal and pulmonary neuroendocrine cells. ASCL1 regulates stemness and cell cycle progression and it is increased in high grade neuroendocrine tumours including SCLC. Western blot analysis confirmed strong reduction of the ASCL1 protein level in SHP-77 cell line. From literature data we retrieved the ChIP-Seq target genes of ASCL1 derived from different SCLC cell lines and compared them to the list of genes at basal condition in SHP-77 cell line. Then we computed Pearson correlation between the expression of ASCL1 and these targets and filtered only strong positive correlations. Through this approach, we finally selected 213 target genes of ASCL1 many of which were coherently inhibited after 24 and 48 hours of treatment. These genes were mainly involved in MAPK and Wnt signalling pathways, both crucial for SCLC. The mechanism of transcription modulation driven by ASCL1 after lurbinectedin treatment is under investigation by ChIP-Seq technology. Moreover, work is in progress to extend our findings on others SCLC cell lines and in sub-lines in which ASCL1 was silenced. We tested whether lurbinectedin shared a similar effect on transcriptomic profile as other known drugs: by dividing up and down regulated genes and testing their correlation to the drug-related gene profiles provided by the tool Enrichr, we found that lurbinectedin has a high correlation to the topoisomerase I inhibitor class. This finding deserves further experimental verifications also in view of potential clinical interest in the combination of lurbinectedin and topoisomerase I inhibitors. Citation Format: Laura Mannarino, Ilaria Craparotta, Federica Mirimao, Nicolò Panini, Monica Lupi, Giulia Protti, Roberta Frapolli, Sergio Marchini, Maurizio D'Incalci. Lurbinectedin down-regulates ASCL1 transcription factor in Small Cell Lung Cancer (SCLC) [abstract]. In: Proceedings of the AACR-NCI-EORTC International Conference on Molecular Targets and Cancer Therapeutics; 2019 Oct 26-30; Boston, MA. Philadelphia (PA): AACR; Mol Cancer Ther 2019;18(12 Suppl):Abstract nr LB-B13. doi:10.1158/1535-7163.TARG-19-LB-B13