SignificanceThe impact of DNA lesions on replication and mutagenesis is of high relevance for human health; however, the role of lesion-induced transcriptional mutagenesis (TM) in disease development is unknown. Here, the impact of O6-methylguanine–induced TM on p53 function as a tumor suppressor was investigated in human cells. Results showed that TM in 15% of the transcripts resulted in a reduced ability of p53 protein to transactivate genes that regulate cell-cycle arrest and induction of apoptosis. This resulted in the loss of functional cell-cycle checkpoints and in impaired activation of apoptosis, both canonical p53 tumor-suppressor functions. This work provides evidence that TM can induce phenotypic changes in mammalian cells that have important implications for its role in tumorigenesis.

RationaleBronchopulmonary Dysplasia (BPD) in preterm born infants is a risk factor for chronic airway obstruction in adulthood. Cytotoxic T-cells are implicated in chronic obstructive pulmonary disease (COPD), but their involvement in BPD is not known.ObjectivesTo characterise the distribution of airway T-cell subsets in adults with a history of BPD.MethodsYoung adults with former BPD (n=22; median age 19.6 years), age-matched adults born preterm (n=22), patients with allergic asthma born at term (n=22), and healthy control subjects born at term (n=24) underwent bronchoalveolar lavage (BAL). T-cell subsets in BAL were analysed using flow cytometry.ResultsThe total number of cells and the differential cell counts in BAL were similar among the study groups. The percentage of CD3+CD8+T-cells was higher (p=0.005) and the proportion of CD3+CD4+T-cells was reduced (p=0.01) in the BPD group, resulting in a lower CD4/CD8 ratio (p=0.007) compared to the healthy controls (median 2.2 versus 5.3). In BPD and preterm born study subjects, both CD3+CD4+T cells (rs=0.38, p=0.03) and CD4/CD8 ratio (rs=0.44, p=0.01) correlated positively with FEV1. Further, CD3+CD8+T-cells were negatively correlated with both FEV1 and FEV1/FVC (rs=−0.44, p=0.09 and rs=−0.41, p=0.01, respectively).ConclusionsYoung adults with former BPD have a T-cell subset pattern in the airways resembling features of COPD. Our findings are compatible with the hypothesis that CD3+CD8+T-cells are involved in mechanisms behind chronic airway obstruction in these patients.

Splicing fidelity is essential to the maintenance of cellular functions and viability, and mutations or natural variations in pre-mRNA sequences and consequent alteration of splicing have been implicated in the etiology and progression of numerous diseases. The extent to which transcriptional errors or lesion-induced transcriptional mutagenesis (TM) influences splicing fidelity is not currently known. To investigate this, we employed site-specific DNA lesions on the transcribed strand of a minigene splicing reporter in normal mammalian cells. These were the common mutagenic lesions O6-methylguanine (O6-meG) and 8-oxoguanine (8-oxoG). The minigene splicing reporters were derived from lamin A (LMNA) and proteolipid protein 1 (PLP1), both with known links to human diseases that result from deregulated splicing. In cells with active DNA repair, 1–4% misincorporation occurred opposite the lesions, which increased to 20–40% when repair was compromised. Furthermore, our results reveal that TM at a splice site significantly reduces in vivo splicing fidelity, thereby changing the relative expression of alternative splicing forms in mammalian cells. These findings suggest that splicing defects caused by transcriptional errors can potentially lead to phenotypic cellular changes and increased susceptibility to the development of disease.

The transcriptional error rate can be significantly increased by the presence of DNA lesions that instruct mis-insertion during transcription; a process referred to as transcriptional mutagenesis (TM) that can result in altered protein function. Herein, we determined the effect of O 6-methylguanine (O 6-meG) on transcription and subsequent transactivation activity of p53 in human lung H1299 cells. Levels of TM and effects on transactivation were determined genome wide by RNA-seq. Results showed that 47% of all p53 transcripts contained an uridine misincorporation opposite the lesion at 6 h post transfection, which was decreased to 18% at 24 h. TM at these levels reduced DNA binding activity of p53 to 21% and 80% compared to wild type p53, respectively. Gene expression data were analysed to identify differentially expressed genes due to TM of p53. We show a temporal repression of transactivation of > 100 high confidence p53 target genes including regulators of the cell cycle, DNA damage response and apoptosis. In addition, TM repressed the transcriptional downregulation by p53 of several negative regulators of proliferation and differentiation. Our work demonstrates that TM, even when restricting its effect to an individual transcription factor, has the potential to alter gene expression programs and diversify cellular phenotypes.

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