Exonucleases: degrading DNA to deal with genome damage, cell death, inflammation and cancer

Although DNA degradation might seem an unwanted event, it is essential in many cellular processes that are key to maintaining genomic stability and cell and organism homeostasis. The capacity to cut out nucleotides one at a time from the end of a DNA chain is present in enzymes called exonucleases. Exonuclease activity might come from enzymes with multiple other functions or specialized enzymes only dedicated to this function. Exonucleases are involved in central pathways of cell biology such as DNA replication, repair, and death, as well as tuning the immune response. Of note, malfunctioning of these enzymes is associated with immune disorders and cancer. In this review, we will dissect the impact of DNA degradation on the DNA damage response and its links with inflammation and cancer

Single loss of a Trp53 allele triggers an increased oxidative, DNA damage and cytokine inflammatory responses through deregulation of IκBα expression

Dose of Trp53, the main keeper of genome stability, influences tumorigenesis; however, the causes underlying and driving tumorigenesis over time by the loss of a single p53 allele are still poorly characterized. Here, we found that single p53 allele loss specifically impacted the oxidative, DNA damage and inflammatory status of hematopoietic lineages. In particular, single Trp53 allele loss in mice triggered oxidative stress in peripheral blood granulocytes and spleenocytes, whereas lack of two Trp53 alleles produced enhanced oxidative stress in thymus cells, resulting in a higher incidence of lymphomas in the Trp53 knockout (KO) mice compared with hemizygous (HEM). In addition, single or complete loss of Trp53 alleles, as well as p53 downregulation, led to a differential increase in basal, LPS- and UVB-induced expression of a plethora of pro-inflammatory cytokine, such as interleukin-12 (Il-12a), TNFα (Tnfa) and interleukin (Il-23a) in bone marrow-derived macrophage cells (BMDMs) compared to WT cells. Interestingly, p53-dependent increased inflammatory gene expression correlated with deregulated expression of the NF-κB pathway inhibitor IκBα. Chromatin immunoprecipitation data revealed decreased p65 binding to Nfkbia in the absence of p53 and p53 binding to Nfkbia promoter, uncovering a novel crosstalk mechanism between p53 and NF-κB transcription factors. Overall, our data suggest that single Trp53 allele loss can drive a sustained inflammatory, DNA damage and oxidative stress response that, over time, facilitate and support carcinogenesis

Caspase-11 regulates the tumour suppressor function of STAT1 in a murine model of colitis-associated carcinogenesis

Murine inflammatory caspase-11 has an important role in intestinal epithelial inflammation and barrier function. Activation of the non-canonical inflammasome, mediated by caspase-11, serves as a regulatory pathway for the production of the proinflammatory cytokines IL-1β and IL-18, and has a key role in pyroptotic cell death. We have previously demonstrated a protective role for caspase-11 during dextran sulphate sodium (DSS)-induced colitis, however the importance of caspase-11 during colorectal tumour development remains unclear. Here, we show that Casp11−/− mice are highly susceptible to the azoxymethane (AOM)-DSS model of colitis-associated cancer (CAC), compared to their wild type (WT) littermates. We show that deficient IL-18 production occurs at initial inflammation stages of disease, and that IL-1β production is more significantly impaired in Casp11−/− colons during established CAC. We identify defective STAT1 activation in Casp11−/− colons during disease progression, and show that IL-1β signalling induces caspase-11 expression and STAT1 activation in primary murine macrophages and intestinal epithelial cells. These findings uncover an anti-tumour role for the caspase-11 and the non-canonical inflammasome during CAC, and suggest a critical role for caspase-11, linking IL-1β and STAT1 signalling pathways

Double deficiency of Trex2 and DNase1L2 nucleases leads to accumulation of DNA in lingual cornifying keratinocytes without activating inflammatory responses

The cornification of keratinocytes on the surface of skin and oral epithelia is associated with the degradation of nuclear DNA. The endonuclease DNase1L2 and the exonuclease Trex2 are expressed specifically in cornifying keratinocytes. Deletion of DNase1L2 causes retention of nuclear DNA in the tongue epithelium but not in the skin. Here we report that lack of Trex2 results in the accumulation of DNA fragments in the cytoplasm of cornifying lingual keratinocytes and co-deletion of DNase1L2 and Trex2 causes massive accumulation of DNA fragments throughout the cornified layers of the tongue epithelium. By contrast, cornification-associated DNA breakdown was not compromised in the epidermis. Aberrant retention of DNA in the tongue epithelium was associated neither with enhanced expression of DNA-driven response genes, such as Ifnb, Irf7 and Cxcl10, nor with inflammation. Of note, the expression of Tlr9, Aim2 and Tmem173, key DNA sensor genes, was markedly lower in keratinocytes and keratinocyte-built tissues than in macrophages and immune tissues, and DNA-driven response genes were not induced by introduction of DNA in keratinocytes. Altogether, our results indicate that DNase1L2 and Trex2 cooperate in the breakdown and degradation of DNA during cornification of lingual keratinocytes and aberrant DNA retention is tolerated in the oral epithelium

Multifaceted role of TREX2 in the skin defense against UV-induced skin carcinogenesis

TREX2 is a 3'-DNA exonuclease specifically expressed in keratinocytes. Here, we investigated the relevance and mechanisms of TREX2 in ultraviolet (UV)-induced skin carcinogenesis. TREX2 expression was up-regulated by chronic UV exposure whereas it was de-regulated or lost in human squamous cell carcinomas (SCCs). Moreover, we identified SNPs in the TREX2 gene that were more frequent in patients with head and neck SCCs than in healthy individuals. In mice, TREX2 deficiency led to enhanced susceptibility to UVB-induced skin carcinogenesis which was preceded by aberrant DNA damage removal and degradation as well as reduced inflammation. Specifically, TREX2 loss diminished the up-regulation of IL12 and IFNγ, key cytokines related to DNA repair and antitumor immunity. In UV-treated keratinocytes, TREX2 promoted DNA repair and passage to late apoptotic stages. Notably, TREX2 was recruited to low-density nuclear chromatin and micronuclei, where it interacted with phosphorylated H2AX histone, which is a critical player in both DNA repair and cell death. Altogether, our data provide new insights in the molecular mechanisms of TREX2 activity and establish cell autonomous and non-cell autonomous functions of TREX2 in the UVB-induced skin response

Paper de l’exonucleasa TREX2 en la biología i patologia de la pell

[cat] TREX2 (three-prime repair exonuclease 2) és una exonucleasa no processiva 3'-5', expressada específicament en la pell que pot estar implicada en el manteniment de la integritat del genoma editant els extrems 3' del DNA en diversos processos, des de la replicació a la reparació del dany passant per la degradació en l'apoptosi. L'estudi de TREX2 mitjançant immunohistofluorescència de mostres humanes de lesions precanceroses (queratosis actínica) i tumors derivats de queratinòcits, incloent cSCC (cutaneous squamous cell carcinomas) i HNSCCs (head and neck SCC), revela que en les carcinogènesis escamoses l'expressió de TREX2 pateix es troba alterada. La deleció de Trex2 en ratolins provoca un augment de la susceptibilitat de tumors induïts en pell provocats per radiació ultraviolada B (UVB). TREX2 promou l'eliminació del DNA danyat i l'apoptosi dels queratinòcits en les pells de ratolí irradiades amb UVB. Les lesions al DNA induïdes per UVB (CPD: cyclobutane pyrimidine_dimers), són més persistents en les pells dels ratolins Trex21- respecte els wt, paral• lelament també es dóna una menor activació de la caspasa 3, indicador d'apoptosi, en les pells Trex21-. Els resultat obtinguts en queratinòcits wt i deficients en TREX2 in vitxo reflexen una multifuncionalitat de la proteïna TREX2 intervenint en la reparació del dany al DNA en resposta a la radiació UV i promovent l'apoptosi de forma irreversible, essent aquesta última funció la que preval. TREX2 pot ser reclutada diferencialment a diferents localitzacions subnuclears en funció de l'estat de la cromatina, com per exemple els micronuclis i les regions de cromatina poc condensada. Els estudis de co-immunoprecipitació demostren que TREX2 interacciona amb yH2AX, així TREX2 pot accedir als extrems lliures del DNA interaccionant amb la proteïna H2AX fosforilada, preferentment en la cromatina destinada a ser degradada. L'estudi histopatològic de les pells de ratolins irradiats amb UVB, revela que els ratolins Trex21- són més resistents que els wt a la inflamació de la pell induïda per UVB. Addicionalment, en les pells irradiades repetidament del ratolí Trex2-/, l'augment en el nombre de capes cel• lulars en l'epidermis com a mesura de protecció de la pell contra la irradiació, és menor que en el wt. En resum, les nostres dades subratllen la rellevància de TREX2 a l'hora de promoure la resposta inflamatòria contra la UVB. Per altra part, a lesions psoriàsiques es dóna un fort increment i canvis en el patró d'expressió de TREX2 respecte la pell sana. La deficiència de TREX2 a ratolins disminueix el fenotip psoriàsic induït per l'Imiquimod. Concretament, TREX2 afavoreix la mort cel• lular i la resposta inflamatòria inicial. A més a més, TREX2 està implicada en la degradació del DNA durant la diferenciació terminal dels corneòcits, disminuint la paraqueratosi. Així, TREX2 té un paper actiu i rellevant en el desenvolupament de la patogènesi associada a la psoriasi.[eng] TREX2 is a 3 '-DNA exonuclease specifically expressed in keratinocytes that acts as tumor suppressor in skin carcinogenesis, but the importance in human pathologies like carcinogenesis and psoriasis and the underlying mechanisms remained unknown. Here, we investigated the relevance and molecular events of TREX2 in the DNA damage skin response. TREX2 expression was up-regulated by chronic exposure to ultraviolet (UV) radiation, whereas was de-regulated or lost in human squamous cell carcinomas (SCCs). TREX2 deficiency in mice enhanced the susceptibility to UVB-induced skin carcinogenesis, which was associated with alterations in DNA damage removal and degradation, cell death and inflammation, indicating multifaceted functions of TREX2. Notably, TREX2 deficiency diminished the up-regulation of key cytokines related to DNA repair and antitumor immunity. Interestingly, TREX2 was recruited to low-density chromatin regions in nuclei that were beginning to condensate chromatin and to micronuclei, interacting with phosphorylated H2AX histone (yH2AX). Altogether, our data establish cell and non-cell autonomous functions of TREX2 in the defense against UV-induced skin carcinogenesis. Moreover TREX2 is overexpressed in psoriasic lesions. To elucidate the importance of this exonuclease in the development of the disease, we induced a skin inflammation in mice by topical application of Imiquimod. The Trex24- mice were more resistant to the Imiquimod-induced skin inflammation than wt. Thus, clinical parameters like skin thickness, erythema and scaling were lower in the Trex24- mice compared to the wt, whereas parakeratosis was more abundant in the Trex24- skins. Altogether, our results show thatTREX2 contribute to the pathologic features of psoriasis by enhancing the keratinocyte apoptosis and the inflammation in response to Imiquimod application

Double deficiency of Trex2 and DNase1L2 nucleases leads to accumulation of DNA in lingual cornifying keratinocytes without activating inflammatory responses

The cornification of keratinocytes on the surface of skin and oral epithelia is associated with the degradation of nuclear DNA. The endonuclease DNase1L2 and the exonuclease Trex2 are expressed specifically in cornifying keratinocytes. Deletion of DNase1L2 causes retention of nuclear DNA in the tongue epithelium but not in the skin. Here we report that lack of Trex2 results in the accumulation of DNA fragments in the cytoplasm of cornifying lingual keratinocytes and co-deletion of DNase1L2 and Trex2 causes massive accumulation of DNA fragments throughout the cornified layers of the tongue epithelium. By contrast, cornification-associated DNA breakdown was not compromised in the epidermis. Aberrant retention of DNA in the tongue epithelium was associated neither with enhanced expression of DNA-driven response genes, such as Ifnb, Irf7 and Cxcl10, nor with inflammation. Of note, the expression of Tlr9, Aim2 and Tmem173, key DNA sensor genes, was markedly lower in keratinocytes and keratinocyte-built tissues than in macrophages and immune tissues, and DNA-driven response genes were not induced by introduction of DNA in keratinocytes. Altogether, our results indicate that DNase1L2 and Trex2 cooperate in the breakdown and degradation of DNA during cornification of lingual keratinocytes and aberrant DNA retention is tolerated in the oral epithelium

Multifaceted role of TREX2 in the skin defense against UV-induced skin carcinogenesis

TREX2 is a 3'-DNA exonuclease specifically expressed in keratinocytes. Here, we investigated the relevance and mechanisms of TREX2 in ultraviolet (UV)-induced skin carcinogenesis. TREX2 expression was up-regulated by chronic UV exposure whereas it was de-regulated or lost in human squamous cell carcinomas (SCCs). Moreover, we identified SNPs in the TREX2 gene that were more frequent in patients with head and neck SCCs than in healthy individuals. In mice, TREX2 deficiency led to enhanced susceptibility to UVB-induced skin carcinogenesis which was preceded by aberrant DNA damage removal and degradation as well as reduced inflammation. Specifically, TREX2 loss diminished the up-regulation of IL12 and IFNγ, key cytokines related to DNA repair and antitumor immunity. In UV-treated keratinocytes, TREX2 promoted DNA repair and passage to late apoptotic stages. Notably, TREX2 was recruited to low-density nuclear chromatin and micronuclei, where it interacted with phosphorylated H2AX histone, which is a critical player in both DNA repair and cell death. Altogether, our data provide new insights in the molecular mechanisms of TREX2 activity and establish cell autonomous and non-cell autonomous functions of TREX2 in the UVB-induced skin response