p53 genes function to restrain mobile elements.

Throughout the animal kingdom, p53 genes govern stress response networks by specifying adaptive transcriptional responses. The human member of this gene family is mutated in most cancers, but precisely how p53 functions to mediate tumor suppression is not well understood. Using Drosophila and zebrafish models, we show that p53 restricts retrotransposon activity and genetically interacts with components of the piRNA (piwi-interacting RNA) pathway. Furthermore, transposon eruptions occurring in the p53(-) germline were incited by meiotic recombination, and transcripts produced from these mobile elements accumulated in the germ plasm. In gene complementation studies, normal human p53 alleles suppressed transposons, but mutant p53 alleles from cancer patients could not. Consistent with these observations, we also found patterns of unrestrained retrotransposons in p53-driven mouse and human cancers. Furthermore, p53 status correlated with repressive chromatin marks in the 5' sequence of a synthetic LINE-1 element. Together, these observations indicate that ancestral functions of p53 operate through conserved mechanisms to contain retrotransposons. Since human p53 mutants are disabled for this activity, our findings raise the possibility that p53 mitigates oncogenic disease in part by restricting transposon mobility.

p53 in the game of transposons.

Throughout the animal kingdom, p53 genes function to restrain mobile elements and recent observations indicate that transposons become derepressed in human cancers. Together, these emerging lines of evidence suggest that cancers driven by p53 mutations could represent "transpospoathies," i.e. disease states linked to eruptions of mobile elements. The transposopathy hypothesis predicts that p53 acts through conserved mechanisms to contain transposon movement, and in this way, prevents tumor formation. How transposon eruptions provoke neoplasias is not well understood but, from a broader perspective, this hypothesis also provides an attractive framework to explore unrestrained mobile elements as inciters of late-onset idiopathic disease. Also see the video abstract here.

Post-transcriptional regulation of wnt8a is essential to zebrafish axis development.

wnt8a Is essential for normal patterning during vertebrate embryonic development, and either gain or loss-of-function gene dysregulation results in severe axis malformations. The zebrafish wnt8a locus is structured such that transcripts may possess two regulatory 3' untranslated regions (UTRs), raising the possibility of post-transcriptional regulation as an important mode of wnt8a signaling control. To determine whether both UTRs contribute to post-transcriptional wnt8a gene regulation, each UTR (UTR1 and UTR2) was tested in transient and transgenic reporter assays. Both UTRs suppress EGFP reporter expression in cis, with UTR2 exhibiting a more pronounced effect. UTR2 contains a 6 base sequence necessary for UTR2 regulatory function that is complementary to the seed of the microRNA, miR-430. A target protector morpholino that overlaps the seed complement stabilizes both reporter mRNAs and wnt8a mRNAs, and produces phenotypic abnormalities consistent with wnt8a gain-of-function. In rescue assays, specific functions can be attributed to each of the two wnt8a proteins encoded by the locus. An interplay of wnt8a.1 and wnt8a.2 regulates neural and mesodermal patterning and morphogenesis as well as patterning between brain subdivisions. Thus, post-transcriptional control of wnt8a is essential to fine tune the balance of the signaling outputs of the complex wnt8a locus.

Distinct p53 isoforms code for opposing transcriptional outcomes.

p53 genes are conserved transcriptional activators that respond to stress. These proteins can also downregulate genes, but the mechanisms are not understood and are generally assumed to be indirect. Here, we investigate synthetic and native cis-regulatory elements in Drosophila to examine opposing features of p53-mediated transcriptional control in vivo. We show that transcriptional repression by p53 operates continuously through canonical DNA binding sites that confer p53-dependent transactivation at earlier developmental stages. p53 transrepression is correlated with local H3K9me3 chromatin marks and occurs without the need for stress or Chk2. In sufficiency tests, two p53 isoforms qualify as transrepressors and a third qualifies as a transcriptional activator. Targeted isoform-specific knockouts dissociate these opposing transcriptional activities, highlighting features that are dispensable for transactivation but critical for repression and for proper germ cell formation. Together, these results demonstrate that certain p53 isoforms function as constitutive tissue-specific repressors, raising important implications for tumor suppression by the human counterpart.

Silencing of LINE-1 retrotransposons is a selective dependency of myeloid leukemia.

Transposable elements or transposons are major players in genetic variability and genome evolution. Aberrant activation of long interspersed element-1 (LINE-1 or L1) retrotransposons is common in human cancers, yet their tumor-type-specific functions are poorly characterized. We identified MPHOSPH8/MPP8, a component of the human silencing hub (HUSH) complex, as an acute myeloid leukemia (AML)-selective dependency by epigenetic regulator-focused CRISPR screening. Although MPP8 is dispensable for steady-state hematopoiesis, MPP8 loss inhibits AML development by reactivating L1s to induce the DNA damage response and cell cycle exit. Activation of endogenous or ectopic L1s mimics the phenotype of MPP8 loss, whereas blocking retrotransposition abrogates MPP8-deficiency-induced phenotypes. Expression of AML oncogenic mutations promotes L1 suppression, and enhanced L1 silencing is associated with poor prognosis in human AML. Hence, while retrotransposons are commonly recognized for their cancer-promoting functions, we describe a tumor-suppressive role for L1 retrotransposons in myeloid leukemia.

Colorectal Neoplasia among Patients with and without Human Immunodeficiency Virus.

BACKGROUND: Increasing availability of highly active antiretroviral therapy (HAART) for human immunodeficiency virus (HIV) has led to prolonged survival and rising incidence of non-HIV-defining cancers among patients with HIV. Compared with the general population, risk of colorectal cancer may differ among those with HIV due to immunosuppression, oncogenic viral coinfections, and higher prevalence of risk factors. METHODS: We identified patients (age ≥50 years) diagnosed with HIV, prescribed HAART for ≥6 months, and receiving care in two large health care systems in Dallas, TX. Patients received a first colonoscopy between January 2009 and December 2017. We calculated a standardized prevalence ratio as the ratio of observed to expected number of advanced neoplasia (high-risk adenoma or colorectal cancer) using an age- and sex-matched cohort of patients without HIV (n = 10,250). RESULTS: Among patients with HIV (n = 839), about two thirds (60.1%) had normal findings at colonoscopy; 6.8% had hyperplastic polyps only, 20.4% had low-risk adenomas, 11.7% had high-risk adenomas, and 1.1% had colorectal cancer. Prevalence of advanced neoplasia was similar between patients with and without HIV, with a standardized prevalence ratio of 0.99 (95% confidence interval, 0.81-1.19). CONCLUSIONS: There was no difference in the prevalence of colorectal neoplasia between patients with and without HIV. IMPACT: Patients with HIV appear to have similar risk of colorectal neoplasia compared to those without HIV and can therefore follow average-risk colorectal cancer screening guidelines.

Drosophila p53 directs nonapoptotic programs in postmitotic tissue.

TP53 is the most frequently mutated gene in human cancers, and despite intensive research efforts, genome-scale studies of p53 function in whole animal models are rare. The need for such in vivo studies is underscored by recent challenges to established paradigms, indicating that unappreciated p53 functions contribute to cancer prevention. Here we leveraged the Drosophila system to interrogate p53 function in a postmitotic context. In the developing embryo, p53 robustly activates important apoptotic genes in response to radiation-induced DNA damage. We recently showed that a p53 enhancer (p53RErpr) near the cell death gene reaper forms chromatin contacts and enables p53 target activation across long genomic distances. Interestingly, we found that this canonical p53 apoptotic program fails to activate in adult heads. Moreover, this failure to exhibit apoptotic responses was not associated with altered chromatin contacts. Instead, we determined that p53 does not occupy the p53RErpr enhancer in this postmitotic tissue as it does in embryos. Through comparative RNA-seq and chromatin immunoprecipitation-seq studies of developing and postmitotic tissues, we further determined that p53 regulates distinct transcriptional programs in adult heads, including DNA repair, metabolism, and proteolysis genes. Strikingly, in the postmitotic context, p53-binding landscapes were poorly correlated with nearby transcriptional effects, raising the possibility that p53 enhancers could be generally acting through long distances.

Retrotransposons Mimic Germ Plasm Determinants to Promote Transgenerational Inheritance.

Retrotransposons are a pervasive class of mobile elements present in the genomes of virtually all forms of life [1, 2]. In metazoans, these are preferentially active in the germline, which, in turn, mounts defenses that restrain their activity [3, 4]. Here we report that certain classes of retrotransposons ensure transgenerational inheritance by invading presumptive germ cells before they are formed. Using sensitized Drosophila and zebrafish models, we found that diverse classes of retrotransposons migrate to the germ plasm, a specialized region of the oocyte that prefigures germ cells and specifies the germline of descendants in the fertilized egg. In Drosophila, we found evidence for a "stowaway" model, whereby Tahre retroelements traffic to the germ plasm by mimicking oskar RNAs and engaging the Staufen-dependent active transport machinery. Consistent with this, germ plasm determinants attracted retroelement RNAs even when these components were ectopically positioned in bipolar oocytes. Likewise, vertebrate retrotransposons similarly migrated to the germ plasm in zebrafish oocytes. Together, these results suggest that germ plasm targeting represents a fitness strategy adopted by some retrotransposons to ensure transgenerational propagation.

p53 activity is selectively licensed in the Drosophila stem cell compartment.

Oncogenic stress provokes tumor suppression by p53 but the extent to which this regulatory axis is conserved remains unknown. Using a biosensor to visualize p53 action, we find that Drosophila p53 is selectively active in gonadal stem cells after exposure to stressors that destabilize the genome. Similar p53 activity occurred in hyperplastic growths that were triggered either by the Ras(V12) oncoprotein or by failed differentiation programs. In a model of transient sterility, p53 was required for the recovery of fertility after stress, and entry into the cell cycle was delayed in p53(-) stem cells. Together, these observations establish that the stem cell compartment of the Drosophila germline is selectively licensed for stress-induced activation of the p53 regulatory network. Furthermore, the findings uncover ancestral links between p53 and aberrant proliferation that are independent of DNA breaks and predate evolution of the ARF/Mdm2 axis. DOI: http://dx.doi.org/10.7554/eLife.01530.001.