Abstract 36: Synthetic lethality across normal tissues is strongly associated with cancer risk, onset, and tumor suppressor specificity
Authors: Nair Nishanth Ulhas , Cheng Kuoyuan , Lee Joo Sang , Ruppin Eytan 
Published: September 2020

Abstract
The tissue-specificity of cancer and cancer risk is a fundamental open research question. Beyond advancing our understanding of carcinogenesis, elucidating the factors underlying cancer risk may also contribute to cancer prevention. Recent studies have shown that the variation in tissue cancer risk can be explained by the number of tissue stem cell divisions occurring during the lifetime and by abnormal levels of DNA methylation. While cancer risk is likely not determined by a single factor, no other factor has been reported since to account for this fundamental variation.
Here we show that cancer synthetic lethality (SL) is another strong contributor of cancer risk in human tissues. SL is a well-known type of genetic interaction where cell death occurs under the combined inactivation of two paired SL genes but not either of them alone. Targeting SLs has been recognized as a highly valuable approach for cancer treatment. We hypothesized that since down-regulated cancer SL (cSL) gene pairs reduce the viability of cancer cells, they may impede the malignant transformation of normal cells, thus modulating cancer risk. Utilizing several recently published large-scale cancer SL networks, we systematically quantified the cancer SL load (defined as the fraction of down-regulated cancer SL gene pairs) in numerous normal and cancer tissues from the TCGA and GTEx datasets. Our key findings are:
1. The cSL loads are lower in cancers vs their matched normal tissues. Furthermore, we find that the cSL load decreases progressively as cancers develop from normal tissues through multiple stages of pre-malignant lesions. These results testify that high cSL load is detrimental to cancer cells, acting as a barrier to cancer development. That is, as normal cells undergo malignant transformation, they need to reactivate at least some of the down-regulated cSL genes for the emerging cancer cells to survive.
2. In accordance with these observations, we find that cSL load in normal tissues is strongly inversely correlated with their lifetime cancer risk.
3. We find that cSL load is the first identified predictor of cancer onset time across different normal tissues - higher cSL load in the younger population is associated with later onset of cancers in that tissue.
4. cSL are important contributors of the tissue/cancer-type specificity of numerous tumor suppressor genes (like BRCA1) - that is, the activity state of cSL partners of quite a few tumor suppressor genes predicts the specific tissues in which they are known to drive cancer.
Taken together, these results show that synthetic lethality load in normal tissues is a novel important biological contributor of cancer risk in humans. While synthetic lethality has been attracting tremendous attention as a way to identify cancer vulnerabilities and target them, this is the first time that its role in mediating cancer development is uncovered.
Citation Format: Nishanth Ulhas Nair, Kuoyuan Cheng, Joo Sang Lee, Eytan Ruppin. Synthetic lethality across normal tissues is strongly associated with cancer risk, onset, and tumor suppressor specificity [abstract]. In: Proceedings of the Annual Meeting of the American Association for Cancer Research 2020; 2020 Apr 27-28 and Jun 22-24. Philadelphia (PA): AACR; Cancer Res 2020;80(16 Suppl):Abstract nr 36.