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Aligning pigmentation and repair: a holistic approach for UV protection dynamics

Periodic Reporting for period 2 - UVdynamicsProtection (Aligning pigmentation and repair: a holistic approach for UV protection dynamics)

Reporting period: 2018-10-01 to 2020-03-31

The human body takes different measures in order to protect itself against the results of UV exposure and its accompanied hazards, such as skin cancer. Despite extensive studies regarding the molecular regulation of the two main UV protection mechanisms, namely, the DNA repair system and the pigmentation system, a comprehensive theory that simultaneously accounts for the two systems is still missing. Hence, the ground-breaking goal of this proposal is to elucidate, for the first time, the dynamic control used to schedule and synchronize the UV protection subsystems.
Since these two systems serve the same physiological purpose, but on different time scales (DNA repair takes minutes, while pigmentation lasts hours to days), I propose to take the novel approach of focusing on their timing as an opportunity to uncover their regulation. As a first step, we exposed human and mouse skin to UV and found that UV exposure at 48hr intervals resulted in higher skin pigmentation than did exposure at 24hr intervals, even after controlling for total UV dosage. Furthermore, we found that the expression level of the melanocyte central regulator, MITF, exhibits damped oscillatory behaviour during this 48hr interval. I therefore hypothesize that the dynamic behaviour of the central regulator dictates the UV–response timing of the two protection systems. In the proposed research, I will take a holistic approach and address this issue from three complementary perspectives: (1) transcriptional dynamics, (2) temporal effects on cellular output, and (3) DNA repair after UV. This will be achieved by utilizing and developing new experimental and analytical tools that will allow us to correlate the temporal behaviours of a wide set of molecular markers. Reaching our goals will provide a breakthrough in our understanding of skin protection from UV and the underlying mechanisms that control it. These findings may offer exciting new avenues for future skin cancer prevention. UV exposure is a major risk factor for all common cutaneous malignancies. Skin cancer incidence increases by ~3% per year worldwide. Only in melanoma, the most lethal and treatment-resistant of human skin cancers, there are 132,000 new cases per year. New treatment and prevention approaches are needed for all skin cancers. Here I build a strong framework to uncover, for the first time, the dynamic regulatory mechanisms of skin protection against UVB.
Skin sun exposure induces two protection programs, stress responses and pigmentation, the former within minutes and the latter only hours afterwards. Although serving the same physiological purpose, it is not known if and how these programs are coordinated. Here we report that UVB exposure every-other-day induces significantly more skin pigmentation than the higher frequency of daily exposure, without an associated increase in stress responses. Using mathematical modelling and empirical studies we show that the melanocyte master regulator, MITF, serves to synchronize stress responses and pigmentation and furthermore, functions as a UV-protection timer via damped oscillatory dynamics, thereby conferring a tradeoff between the two programs. MITF oscillations are controlled by multiple negative regulatory loops, one at the transcriptional level involving HIF1a and another post-transcriptional loop involving microRNA-148a. Additionally, we found that miRNA biogenesis depends on DNA methylation. When the regions flanking the miRNA coding sequence are highly methylated, the miRNAs are more highly expressed, have greater sequence conservation, and are more likely to drive cancer-related phenotypes than miRNAs encoded by unmethylated loci. These findings support trait linkage between the two skin protection programs, which we speculate arose during furless skin evolution to minimize skin damage.
We have completed the first aim of our proposal and identify the negative regulatory feedback loops that result in damped oscillation of MITF expression. We have massively progress in the second aim and elucidated the effect of MITF transcriptional dynamics on temporal gene expression and cellular output. We are progressing in our 3rd aim in order to reveal the effect of the transcriptional dynamics on UV-damage DNA repair. Our works were published in Malcov et al Mol Cell 2018 and Glaich et at Nature Communications 2019, our work was selected for the cover of Mol Cell 2018. We have submitted additional work recently and few more major discoveries are about to come.
We have generated useful high throughput data for the scientific community, including full transcriptome data of 10 time points upon UV stimulation at the skin (GEO server: GSE114764) and full miRNAs profiling upon DNA methylation manipulation (GEO server: GSE124879). Moreover, we have discovered a gender depended response to UV exposure in terms of DNA damage repair and systemic effect. We aim to further investigate these point and publish several article in the near future.