Irreversible cell cycle exit associated with senescence is mediated by constitutive MYC degradation.

Cells can irreversibly exit the cell cycle and become senescent to safeguard against uncontrolled proliferation. While the p53-p21 and p16-Rb pathways are thought to mediate senescence, they also mediate reversible cell cycle arrest (quiescence), raising the question of whether senescence is actually reversible or whether alternative mechanisms underly the irreversibility associated with senescence. Here, we show that senescence is irreversible and that commitment to and maintenance of senescence are mediated by irreversible MYC degradation. Senescent cells start dividing when a non-degradable MYC mutant is expressed, and quiescent cells convert to senescence when MYC is knocked down. In early oral carcinogenesis, epithelial cells exhibit MYC loss and become senescent as a safeguard against malignant transformation. Later stages of oral premalignant lesions exhibit elevated MYC levels and cellular dysplasia. Thus, irreversible cell cycle exit associated with senescence is mediated by constitutive MYC degradation, but bypassing this degradation may allow tumor cells to escape during cancer initiation.

A Knock-In Mouse Model of Thymoma With the GTF2I L424H Mutation.

INTRODUCTION: The pathogenesis of thymic epithelial tumors remains largely unknown. We previously identified GTF2I L424H as the most frequently recurrent mutation in thymic epithelial tumors. Nevertheless, the precise role of this mutation in tumorigenesis of thymic epithelial cells is unclear. METHODS: To investigate the role of GTF2I L424H mutation in thymic epithelial cells in vivo, we generated and characterized a mouse model in which the Gtf2i L424H mutation was conditionally knocked-in in the Foxn1+ thymic epithelial cells. Digital spatial profiling was performed on thymomas and normal thymic tissues with GeoMx-mouse whole transcriptome atlas. Immunohistochemistry staining was performed using both mouse tissues and human thymic epithelial tumors. RESULTS: We observed that the Gtf2i mutation impairs development of the thymic medulla and maturation of medullary thymic epithelial cells in young mice and causes tumor formation in the thymus of aged mice. Cell cycle-related pathways, such as E2F targets and MYC targets, are enriched in the tumor epithelial cells. Results of gene set variation assay analysis revealed that gene signatures of cortical thymic epithelial cells and thymic epithelial progenitor cells are also enriched in the thymomas of the knock-in mice, which mirrors the human counterparts in The Cancer Genome Atlas database. Immunohistochemistry results revealed similar expression pattern of epithelial cell markers between mouse and human thymomas. CONCLUSIONS: We have developed and characterized a novel thymoma mouse model. This study improves knowledge of the molecular drivers in thymic epithelial cells and provides a tool for further study of the biology of thymic epithelial tumors and for development of novel therapies.

Genetic susceptibility to hepatocellular carcinoma in chromosome 22q13.31, findings of a genome-wide association study.

BACKGROUND AND AIM: Chronic hepatitis C virus (HCV) infection, long-term alcohol use, cigarette smoking, and obesity are the major risk factors for hepatocellular carcinoma (HCC) in the United States, but the disease risk varies substantially among individuals with these factors, suggesting host susceptibility to and gene-environment interactions in HCC. To address genetic susceptibility to HCC, we conducted a genome-wide association study (GWAS). METHODS: Two case-control studies on HCC were conducted in the United States. DNA samples were genotyped using the Illumian microarray chip with over 710 000 single nucleotide polymorphisms (SNPs). We compared these SNPs between 705 HCC cases and 1455 population controls for their associations with HCC and verified our findings in additional studies. RESULTS: In this GWAS, we found that two SNPs were associated with HCC at P < 5E-8 and six SNPs at P < 5E-6 after adjusting for age, sex, and the top three principal components (PCs). Five of the SNPs in chromosome 22q13.31, three in PNPLA3 (rs2281135, rs2896019, and rs4823173) and two in SAMM50 (rs3761472, rs3827385), were replicated in a small US case-control study and a cohort study in Singapore. The associations remained significant after adjusting for body mass index and HCV infection. Meta-analysis of multiple datasets indicated that these SNPs were significantly associated with HCC. CONCLUSIONS: SNPs in PNPLA3 and SAMM50 are known risk loci for nonalcoholic fatty liver disease (NAFLD) and are suspected to be associated with HCC. Our GWAS demonstrated the associations of these SNPs with HCC in a US population. Biological mechanisms underlying the relationship remain to be elucidated.

Single-cell atlas of tumor cell evolution in response to therapy in hepatocellular carcinoma and intrahepatic cholangiocarcinoma.

BACKGROUND & AIMS: Intratumor molecular heterogeneity is a key feature of tumorigenesis and is linked to treatment failure and patient prognosis. Herein, we aimed to determine what drives tumor cell evolution by performing single-cell transcriptomic analysis. METHODS: We analyzed 46 hepatocellular carcinoma (HCC) and intrahepatic cholangiocarcinoma (iCCA) biopsies from 37 patients enrolled in interventional studies at the NIH Clinical Center, with 16 biopsies collected before and after treatment from 7 patients. We developed a novel machine learning-based consensus clustering approach to track cellular states of 57,000 malignant and non-malignant cells including tumor cell transcriptome-based functional clonality analysis. We determined tumor cell relationships using RNA velocity and reverse graph embedding. We also studied longitudinal samples from 4 patients to determine tumor cellular state and its evolution. We validated our findings in bulk transcriptomic data from 488 patients with HCC and 277 patients with iCCA. RESULTS: Using transcriptomic clusters as a surrogate for functional clonality, we observed an increase in tumor cell state heterogeneity which was tightly linked to patient prognosis. Furthermore, increased functional clonality was accompanied by a polarized immune cell landscape which included an increase in pre-exhausted T cells. We found that SPP1 expression was tightly associated with tumor cell evolution and microenvironmental reprogramming. Finally, we developed a user-friendly online interface as a knowledge base for a single-cell atlas of liver cancer. CONCLUSIONS: Our study offers insight into the collective behavior of tumor cell communities in liver cancer as well as potential drivers of tumor evolution in response to therapy. LAY SUMMARY: Intratumor molecular heterogeneity is a key feature of tumorigenesis that is linked to treatment failure and patient prognosis. In this study, we present a single-cell atlas of liver tumors from patients treated with immunotherapy and describe intratumoral cell states and their hierarchical relationship. We suggest osteopontin, encoded by the gene SPP1, as a candidate regulator of tumor evolution in response to treatment.

A Viral Exposure Signature Defines Early Onset of Hepatocellular Carcinoma.

Hepatocellular carcinoma (HCC) is an aggressive malignancy with its global incidence and mortality rate continuing to rise, although early detection and surveillance are suboptimal. We performed serological profiling of the viral infection history in 899 individuals from an NCI-UMD case-control study using a synthetic human virome, VirScan. We developed a viral exposure signature and validated the results in a longitudinal cohort with 173 at-risk patients who had long-term follow-up for HCC development. Our viral exposure signature significantly associated with HCC status among at-risk individuals in the validation cohort (area under the curve: 0.91 [95% CI 0.87-0.96] at baseline and 0.98 [95% CI 0.97-1] at diagnosis). The signature identified cancer patients prior to a clinical diagnosis and was superior to alpha-fetoprotein. In summary, we established a viral exposure signature that can predict HCC among at-risk patients prior to a clinical diagnosis, which may be useful in HCC surveillance.

Tumor Cell Biodiversity Drives Microenvironmental Reprogramming in Liver Cancer.

Cellular diversity in tumors is a key factor for therapeutic failures and lethal outcomes of solid malignancies. Here, we determined the single-cell transcriptomic landscape of liver cancer biospecimens from 19 patients. We found varying degrees of heterogeneity in malignant cells within and between tumors and diverse landscapes of tumor microenvironment (TME). Strikingly, tumors with higher transcriptomic diversity were associated with patient's worse overall survival. We found a link between hypoxia-dependent vascular endothelial growth factor expression in tumor diversity and TME polarization. Moreover, T cells from higher heterogeneous tumors showed lower cytolytic activities. Consistent results were found using bulk genomic and transcriptomic profiles of 765 liver tumors. Our results offer insight into the diverse ecosystem of liver cancer and its impact on patient prognosis.

Role of the mannose receptor (CD206) in innate immunity to ricin toxin.

The entry of ricin toxin into macrophages and certain other cell types in the spleen and liver results in toxin-induced inflammation, tissue damage and organ failure. It has been proposed that uptake of ricin into macrophages is facilitated by the mannose receptor (MR; CD206), a C-type lectin known to recognize the oligosaccharide side chains on ricin's A (RTA) and B (RTB) subunits. In this study, we confirmed that the MR does indeed promote ricin binding, uptake and killing of monocytes in vitro. To assess the role of MR in the pathogenesis of ricin in vivo, MR knockout (MR(-/-)) mice were challenged with the equivalent of 2.5× or 5× LD(50) of ricin by intraperitoneal injection. We found that MR(-/-) mice were significantly more susceptible to toxin-induced death than their age-matched, wild-type control counterparts. These data are consistent with a role for the MR in scavenging and degradation of ricin, not facilitating its uptake and toxicity in vivo.