DNA vector-based RNAi approach for stable depletion of poly(ADP-ribose) polymerase-1

Poly(ADP-ribose) polymerase 1 escorts XPC to UV-induced DNA lesions during nucleotide excision repair

Xeroderma pigmentosum C (XPC) protein initiates the global genomic subpathway of nucleotide excision repair (GG-NER) for removal of UV-induced direct photolesions from genomic DNA. The XPC has an inherent capacity to identify and stabilize at the DNA lesion sites, and this function is facilitated in the genomic context by UV-damaged DNA-binding protein 2 (DDB2), which is part of a multiprotein UV-DDB ubiquitin ligase complex. The nuclear enzyme poly(ADP-ribose) polymerase 1 (PARP1) has been shown to facilitate the lesion recognition step of GG-NER via its interaction with DDB2 at the lesion site. Here, we show that PARP1 plays an additional DDB2-independent direct role in recruitment and stabilization of XPC at the UV-induced DNA lesions to promote GG-NER. It forms a stable complex with XPC in the nucleoplasm under steady-state conditions before irradiation and rapidly escorts it to the damaged DNA after UV irradiation in a DDB2-independent manner. The catalytic activity of PARP1 is not required for the initial complex formation with XPC in the nucleoplasm but it enhances the recruitment of XPC to the DNA lesion site after irradiation. Using purified proteins, we also show that the PARP1-XPC complex facilitates the handover of XPC to the UV-lesion site in the presence of the UV-DDB ligase complex. Thus, the lesion search function of XPC in the genomic context is controlled by XPC itself, DDB2, and PARP1. Our results reveal a paradigm that the known interaction of many proteins with PARP1 under steady-state conditions could have functional significance for these proteins.

An Infectious Disease-Associated Il12b Polymorphism Regulates IL-12/23 p40 Transcription Involving Poly(ADP-Ribose) Polymerase 1

IL-12 and IL-23 are important host defense factors produced by APCs against certain intracellular and extracellular pathogens. Their dysregulation has also been implicated in several autoimmune diseases. The nucleotide polymorphism in the promoter region of Il12b (rs41292470 consisting of the long or short allele) encoding the shared subunit of IL-12 and IL-23, p40, has been reported to associate with susceptibility to infectious diseases and autoimmune disorders. How these genetic variants impact Il12b expression at the molecular level was unclear. We established an Il12b promoter-luciferase reporter system containing the long or short allele driving the reporter gene expression and found that the long allele (infection-resistant) displayed ∼2-fold higher transcriptional activity than the short allele (infection-susceptible), associated with a selective and differential nuclear binding activity to the two alleles in activated macrophages. DNA pull-down assays coupled with mass spectrometry analyses identified the specific DNA binding activity as poly(ADP-ribose) polymerase 1 (PARP-1). Small hairpin RNA-mediated knockdown of the endogenous PARP-1 expression resulted in reduced p40 mRNA expression and Il12b promoter activity. Bone marrow-derived macrophages from PARP-1-deficient mice had decreased p40 expression at both mRNA and protein levels. Furthermore, selective PARP-1 inhibitors resulted in impaired production of IL-12p40 and IL-23 in bone-marrow derived macrophages and PBMCs. Chromatin immunoprecipitation assay revealed that PARP-1 could bind specifically to Il12b in LPS-stimulated macrophages. Our study opens the way for further elucidating the molecular mechanism whereby allele-specific immune responses to foreign and self-antigens mediated by IL-12/IL-23 are controlled in an individually variable manner.

Dmy initiates masculinity by altering Gsdf/Sox9a2/Rspo1 expression in medaka (Oryzias latipes)

Despite identification of several sex-determining genes in non-mammalian vertebrates, their detailed molecular cascades of sex determination/differentiation are not known. Here, we used a novel RNAi to characterise the molecular mechanism of Dmy (the sex-determining gene of medaka)-mediated masculinity in XY fish. Dmy knockdown (Dmy-KD) suppressed male pathway (Gsdf, Sox9a2, etc.) and favoured female cascade (Rspo1, etc.) in embryonic XY gonads, resulting in a fertile male-to-female sex-reversal. Gsdf, Sox9a2, and Rspo1 directly interacted with Dmy, and co-injection of Gsdf and Sox9a2 re-established masculinity in XY-Dmy-KD transgenics, insinuating that Dmy initiates masculinity by stimulating and suppressing Gsdf/Sox9a2 and Rspo1 expression, respectively. Gonadal expression of Wt1a starts prior to Dmy and didn’t change upon Dmy-KD. Furthermore, Wt1a stimulated the promoter activity of Dmy, suggesting Wt1a as a regulator of Dmy. These findings provide new insights into the role of vertebrate sex-determining genes associated with the molecular interplay between the male and female pathways.

Characterization of the interactions of PARP-1 with UV-damaged DNA in vivo and in vitro

The existing methodologies for studying robust responses of poly (ADP-ribose) polymerase-1 (PARP-1) to DNA damage with strand breaks are often not suitable for examining its subtle responses to altered DNA without strand breaks, such as UV-damaged DNA. Here we describe two novel assays with which we characterized the interaction of PARP-1 with UV-damaged DNA in vivo and in vitro. Using an in situ fractionation technique to selectively remove free PARP-1 while retaining the DNA-bound PARP-1, we demonstrate a direct recruitment of the endogenous or exogenous PARP-1 to the UV-lesion site in vivo after local irradiation. In addition, using the model oligonucleotides with single UV lesion surrounded by multiple restriction enzyme sites, we demonstrate in vitro that DDB2 and PARP-1 can simultaneously bind to UV-damaged DNA and that PARP-1 casts a bilateral asymmetric footprint from −12 to +9 nucleotides on either side of the UV-lesion. These techniques will permit characterization of different roles of PARP-1 in the repair of UV-damaged DNA and also allow the study of normal housekeeping roles of PARP-1 with undamaged DNA.

Role of poly(ADP-ribose) polymerase-1 in the removal of UV-induced DNA lesions by nucleotide excision repair

Among the earliest responses of mammalian cells to DNA damage is catalytic activation of a nuclear enzyme poly(ADP-ribose) polymerase-1 (PARP-1). Activated PARP-1 forms the polymers of ADP-ribose (pADPr or PAR) that posttranslationally modify its target proteins, such as PARP-1 and DNA repair-related proteins. Although this metabolism is known to be implicated in other repair pathways, here we show its role in the versatile nucleotide excision repair pathway (NER) that removes a variety of DNA damages including those induced by UV. We show that PARP inhibition or specific depletion of PARP-1 decreases the efficiency of removal of UV-induced DNA damage from human skin fibroblasts or mouse epidermis. Using NER-proficient and -deficient cells and in vitro PARP-1 assays, we show that damaged DNA-binding protein 2 (DDB2), a key lesion recognition protein of the global genomic subpathway of NER (GG-NER), associates with PARP-1 in the vicinity of UV-damaged chromatin, stimulates its catalytic activity, and is modified by pADPr. PARP inhibition abolishes UV-induced interaction of DDB2 with PARP-1 or xeroderma pigmentosum group C (XPC) and also decreases localization of XPC to UV-damaged DNA, which is a key step that leads to downstream events in GG-NER. Thus, PARP-1 collaborates with DDB2 to increase the efficiency of the lesion recognition step of GG-NER.

The Sound of Silence: RNAi in Poly (ADP-Ribose) Research

Poly(ADP-ribosyl)-ation is a nonprotein posttranslational modification of proteins and plays an integral part in cell physiology and pathology. The metabolism of poly(ADP-ribose) (PAR) is regulated by its synthesis by poly(ADP-ribose) polymerases (PARPs) and on the catabolic side by poly(ADP-ribose) glycohydrolase (PARG). PARPs convert NAD+ molecules into PAR chains that interact covalently or noncovalently with target proteins and thereby modify their structure and functions. PAR synthesis is activated when PARP1 and PARP2 bind to DNA breaks and these two enzymes account for almost all PAR formation after genotoxic stress. PARG cleaves PAR molecules into free PAR and finally ADP-ribose (ADPR) moieties, both acting as messengers in cellular stress signaling. In this review, we discuss the potential of RNAi to manipulate the levels of PARPs and PARG, and consequently those of PAR and ADPR, and compare the results with those obtained after genetic or chemical disruption.

Combined lentiviral and RNAi technologies for the delivery and permanent silencing of the hsp25 gene

Elevated heat shock protein 27 (Hsp27) expression has been found in a number of tumors, including breast, prostate, gastric, uterine, ovarian, head and neck, and tumor arising from the nervous system and urinary system, and determined to be a predictor of poor clinical outcome. Although the mechanism of action of Hsp27 has been well documented, there are currently no available inhibitors of Hsp27 in clinical trials. RNA interference (RNAi) has the potential to offer more specificity and flexibility than traditional drugs to silence gene expression. Not surprisingly, RNAi has become a major focus for biotechnology and pharmaceutical companies, which are now in the early stages of developing RNAi therapeutics, mostly based on short interfering RNA (siRNAs), to target viral infection, cancer, hypercholesterolemia, cardiovascular disease, macular degeneration, and neurodegenerative diseases. However, the critical issues associated with RNAi as a therapeutic are delivery, specificity, and stability of the RNAi reagents. To date, the delivery is currently considered the biggest hurdle, as the introduction of siRNAs systemically into body fluids can result in their degradation, off-target effects, and immune detection. In this chapter, we discuss a method of combined lentiviral and RNAi-based technology for the delivery and permanent silencing of the hsp25 gene.

Common fragile sites in colon cancer cell lines: role of mismatch repair, RAD51 and poly(ADP-ribose) polymerase-1

Common fragile sites (CFS) are specific chromosomal areas prone to form gaps and breaks when cells are exposed to stresses that affect DNA synthesis, such as exposure to aphidicolin (APC), an inhibitor of DNA polymerases. The APC-induced DNA damage is repaired primarily by homologous recombination (HR), and RAD51, one of the key players in HR, participates to CFS stability. Since another DNA repair pathway, the mismatch repair (MMR), is known to control HR, we examined the influence of both the MMR and HR DNA repair pathways on the extent of chromosomal damage and distribution of CFS provoked by APC and/or by RAD51 silencing in MMR-deficient and -proficient colon cancer cell lines (i.e., HCT-15 and HCT-15 transfected with hMSH6, or HCT-116 and HCT-116/3+6, in which a part of a chromosome 3 containing the wild-type hMLH1 allele was inserted). Here, we show that MMR-deficient cells are more sensitive to APC-induced chromosomal damage particularly at the CFS as compared to MMR-proficient cells, indicating an involvement of MMR in the control of CFS stability. The most expressed CFS is FRA16D in 16q23, an area containing the tumour suppressor gene WWOX often mutated in colon cancer. We also show that silencing of RAD51 provokes a higher number of breaks in MMR-proficient cells with respect to their MMR-deficient counterparts, likely as a consequence of the combined inhibitory effects of RAD51 silencing on HR and MMR-mediated suppression of HR. The RAD51 silencing causes a broader distribution of breaks at CFS than that observed with APC. Treatment with APC of RAD51-silenced cells further increases DNA breaks in MMR-proficient cells. The RNAi-mediated silencing of PARP-1 does not cause chromosomal breaks or affect the expression/distribution of CFS induced by APC. Our results indicate that MMR modulates colon cancer sensitivity to chromosomal breaks and CFS induced by APC and RAD51 silencing.

Approaches to detect PARP-1 activation in vivo, in situ, and in vitro

An accurate and sensitive detection of catalytic activation of poly(ADP-ribose) polymerase-1 (PARP-1) is required to be performed in a wide variety of samples because this activity plays a role in various cellular responses to DNA damage ranging from DNA repair to cell death, as well as in housekeeping functions, such as transcription. Since PARP-1 gene is expressed constitutively, its activation cannot be surmised from increased expression of its mRNA or protein, but by demonstrating the consequences of its catalytic -reaction which results in consumption of the substrate nicotinamide adenine dinucleotide (NAD(+)) and formation of three products, namely, polymer of ADP-ribose (pADPr or PAR), nicotinamide, and protons. Here, we describe various approaches commonly used in our laboratory for detection of PARP-1 activation in vivo (cells, tissues, and tumors), in situ, and in vitro via assessment of formation of pADPr, depletion of the substrate NAD, or formation of protons resulting in rapid and reversible intracellular acidification. It is important to note that although some other members of the PARP family can carry out the same catalytic reaction, many of these assays largely reflect PARP-1 activation in a vast majority of the experimental circumstances and more specifically in DNA damage responses. However, if required, PARP-1-specific action should be confirmed by use of PARP-1 knockout or RNAi-mediated knockdown approaches.

Persistence of different forms of transient RNAi during apoptosis in mammalian cells

Gene silencing by transient or stable RNA-interference (RNAi) is used for the study of apoptosis with an assumption that apoptotic events will not influence RNAi. However, we recently reported that stable RNAi, i.e., a permanent gene-knockdown mediated by shRNA-generating DNA vectors that are integrated in the genome, fails rapidly after induction of apoptosis due to caspase-3-mediated cleavage and inactivation of the endoribonuclease Dicer-1 that is required for conversion of shRNA to siRNA. Since apoptosis studies also increasingly employ transient RNAi models in which apoptosis is induced immediately after a gene is temporarily knocked down within a few days of transfection with RNAi-inducing agents, we examined the impact of apoptosis on various models of transient RNAi. We report here that unlike the stable RNAi, all forms of transient RNAi, whether Dicer-1-independent (by 21mer dsRNA) or Dicer-1-dependent (by 27mer dsRNA or shRNA-generating DNA vector), whether for an exogenous gene GFP or an endogenous gene poly(ADP-ribose) polymerase-1, do not fail for 2–3 days after onset of apoptosis. Our results reflect the differences in dynamics of achieving and maintaining RNAi during the early phase after transfection in the transient RNAi model and the late steady-state phase of gene-knockdown in stable RNAi model. Our results also sound a cautionary note that RNAi status should be frequently validated in the studies involving apoptosis and that while stable RNAi can be safely used for the study of early apoptotic events, transient RNAi is more suitable for the study of both early and late apoptotic events.

Inhibition of poly adenosine diphosphate-ribose polymerase decreases hepatocellular carcinoma growth by modulation of tumor-related gene expression

Hepatocellular carcinoma (HCC) is associated with a poor prognosis due to a lack of effective treatment options. In HCC a significant role is played by DNA damage and the inflammatory response. Poly (ADP-ribose) polymerase-1 (PARP-1) is an important protein that regulates both these mechanisms. The objective of this study was to examine the effect of pharmacology PARP-1 inhibition on the reduction of tumor volume of HCC xenograft and on the hepatocarcinogenesis induced by diethyl-nitrosamine (DEN). Pharmacologic PARP-1 inhibition with DPQ greatly reduces tumor xenograft volume with regard to a nontreated xenograft (394 mm(3) versus 2,942 mm(3), P < 0.05). This observation was paralleled by reductions in xenograft mitosis (P = 0.02) and tumor vasculogenesis (P = 0.007, confirmed by in vitro angiogenesis study), as well as by an increase in the number of apoptotic cells in DPQ-treated mice (P = 0.04). A substantial difference in key tumor-related gene expression (transformed 3T3 cell double minute 2 [MDM2], FLT1 [vascular endothelial growth factor receptor-1, VEGFR1], epidermal growth factor receptor [EPAS1]/hypoxia-inducible factor 2 [HIF2A], EGLN1 [PHD2], epidermal growth factor receptor [EGFR], MYC, JUND, SPP1 [OPN], hepatocyte growth factor [HGF]) was found between the control tumor xenografts and the PARP inhibitor-treated xenografts (data confirmed in HCC cell lines using PARP inhibitors and PARP-1 small interfering RNA [siRNA]). Furthermore, the results obtained in mice treated with DEN to induce hepatocarcinogenesis showed, after treatment with a PARP inhibitor (DPQ), a significant reduction both in preneoplastic foci and in the expression of preneoplastic markers and proinflammatory genes (Gstm3, Vegf, Spp1 [Opn], IL6, IL1b, and Tnf), bromodeoxyuridine incorporation, and NF-kappaB activation in the initial steps of carcinogenesis (P < 0.05).

CONCLUSION

This study shows that PARP inhibition is capable of controlling HCC growth and preventing tumor vasculogenesis by regulating the activation of different genes involved in tumor progression.

Global analysis of transcriptional regulation by poly(ADP-ribose) polymerase-1 and poly(ADP-ribose) glycohydrolase in MCF-7 human breast cancer cells

Poly(ADP-ribose) polymerase-1 (PARP-1) and poly(ADP-ribose) glycohydrolase (PARG) are enzymes that modify target proteins by the addition and removal, respectively, of ADP-ribose polymers. Although a role for PARP-1 in gene regulation has been well established, the role of PARG is less clear. To investigate how PARP-1 and PARG coordinately regulate global patterns of gene expression, we used short hairpin RNAs to stably knock down PARP-1 or PARG in MCF-7 cells followed by expression microarray analyses. Correlation analyses showed that the majority of genes affected by the knockdown of one factor were similarly affected by the knockdown of the other factor. The most robustly regulated common genes were enriched for stress-response and metabolic functions. In chromatin immunoprecipitation assays, PARP-1 and PARG localized to the promoters of positively and negatively regulated target genes. The levels of chromatin-bound PARG at a given promoter generally correlated with the levels of PARP-1 across the subset of promoters tested. For about half of the genes tested, the binding of PARP-1 at the promoter was dependent on the binding of PARG. Experiments using stable re-expression of short hairpin RNA-resistant catalytic mutants showed that PARP-1 and PARG enzymatic activities are required for some, but not all, target genes. Collectively, our results indicate that PARP-1 and PARG, nuclear enzymes with opposing enzymatic activities, localize to target promoters and act in a similar, rather than antagonistic, manner to regulate gene expression.

Inhibition of homologous recombination by treatment with BVDU (brivudin) or by RAD51 silencing increases chromosomal damage induced by bleomycin in mismatch repair-deficient tumour cells

Mismatch repair (MMR) has been shown to control homologous recombination (HR) by aborting strand exchange between divergent sequences. We previously demonstrated that MMR-deficient tumour cells are more resistant to chromosomal damage induced by bleomycin (BLM) during the G(2) phase, likely due to the lack of the MMR inhibitory effect on HR. Aim of this study was to investigate whether inhibition of HR by the nucleoside analogue BVDU [(E)-5(2-bromovinyl)-2'-deoxyuridine, brivudin], or silencing of genes involved in HR function, might affect sensitivity of MMR-deficient tumour cells to DNA damage induced by BLM in G(2). The results indicated that BVDU increased chromatid damage and DNA double strand breaks induced by BLM only in MMR-deficient MT-1, HL-60R, HCT116 cells, which are more resistant to BLM with respect to MMR-proficient TK-6, HL-60S and HCT116/3-6 lines. Silencing of RAD51, a key component of HR, increased sensitivity of MMR-deficient HCT-15 cells to BLM clastogenicity; in this case combined treatment with BVDU had no additional effect. Similarly, treatment with BVDU did not affect BLM clastogenicity in CAPAN-1 cells, characterized by a defective HR due to BRCA2 mutations. Conversely, BVDU increased chromatid breaks induced by BLM in HCT-15 cells transiently silenced for DNA-PK catalytic subunit, which plays a key role in non-homologous end joining. The BVDU-mediated increase of chromatid breaks in MMR-deficient cells did not depend on its previously reported inhibitory effect on poly(ADP-ribose) polymerase (PARP). In fact, it was observed also in cells stably silenced for PARP-1, which is responsible for most of cellular PARP activity. These data support the suggestion that the higher sensitivity of MMR-proficient versus MMR-deficient cells to BLM-induced chromatid breaks in the G(2) phase is a consequence of the inhibition of HR by MMR. In MMR-deficient cells, BVDU attenuates the repair of BLM-induced DSBs and this is likely to occur via inhibition of HR.

Abrogation of DNA vector-based RNAi during apoptosis in mammalian cells due to caspase-mediated cleavage and inactivation of Dicer-1

RNA interference (RNAi) is used as a reverse-genetic tool to examine functions of a gene in different cellular processes including apoptosis. As key cellular proteins are inactivated during apoptosis, and as RNAi requires cooperation of many cellular proteins, we examined whether DNA vector-based RNAi would continue to function during apoptosis. The short hairpin RNA transcribed from the DNA vector is processed by Dicer-1 to form small interfering RNA that is incorporated in the RNA-induced silencing complex (RISC) to guide a sequence-specific silencing of the target mRNA. We report here that DNA vector-based RNAi of three different genes, namely poly(ADP-ribose) polymerase-1, p14(ARF) and lamin A/C are abrogated during apoptosis. The failure of DNA vector-based RNAi was not at the level of Ago-2 or RISC-mediated step of RNAi but due to catalytic inactivation of Dicer-1 on specific cleavage at the STTD(1476) and CGVD(1538) sites within its RNase IIIa domain. Using multiple approaches, caspase-3 was identified as the major caspase responsible for the cleavage and inactivation of Dicer-1. As Dicer-1 is also the common endonuclease required for formation of microRNA (miRNA) in mammalian cells, we observed decreased levels of mature forms of miR-16, miR-21 and let-7a. Our results suggest a role for apoptotic cleavage and inactivation of Dicer-1 in controlling apoptotic events through altered availability of miRNA.

Stable depletion of poly (ADP-ribose) polymerase-1 reduces in vivo melanoma growth and increases chemosensitivity

Poly(ADP-ribose) polymerase (PARP)-1, which plays a key role in DNA repair, inflammation and transcription, has recently been shown to be involved in angiogenesis. The aim of this study was to investigate PARP-1 role in melanoma aggressiveness and chemoresistance in vivo using clones stably silenced for PARP-1 expression. Whilst the growth characteristics of PARP-1-deficient melanoma cells were comparable to those of PARP-1-proficient cells in vitro, their tumourigenic potential in vivo was significantly compromised. In fact, mice challenged intra-muscle with PARP-1-deficient cells showed a delayed development of measurable tumour nodules, which were also significantly reduced in size with respect to those of mice inoculated with PARP-1-proficient cells. Moreover, animals challenged intra-cranially with PARP-1-deficient cells, a model that mimics CNS localisation of melanoma, showed an increased survival. Immunohistochemical analyses of PARP-1-depleted melanoma grafts indicated a reduced expression of the angiogenesis marker PECAM-1/CD31 and of the pro-inflammatory mediators TNF-alpha and GITR. Notably, PARP-1-silenced melanoma was extremely sensitive to temozolomide, an anticancer agent used for the treatment of metastatic melanoma. These results provide novel evidence for a direct role of PARP-1 in tumour aggressiveness and chemoresistance.

Depletion of poly(ADP-ribose) polymerase-1 reduces host cell reactivation of a UV-damaged adenovirus-encoded reporter gene in human dermal fibroblasts

In response to ultraviolet radiation (UV), mammalian cells rapidly activate a nuclear enzyme poly(ADP-ribose) polymerase-1 (PARP), and we recently showed that one of the causes for PARP-activation is UV-induced direct DNA photolesions which are repaired by nucleotide excision repair process (NER). To determine whether PARP can play a role in NER, we stably depleted PARP in NER-proficient human skin fibroblasts GM637 by DNA vector-based RNAi. In these cells, we examined host cell reactivation (HCR) of UVB or UVC-irradiated recombinant adenovirus AdCA35lacZ, encoding a beta-galactosidase (beta-gal) reporter gene. The depletion of PARP decreased the HCR of UVB- or UVC-damaged reporter gene to a similar extent, indicating the role of PARP in NER. Moreover, PARP-depletion reduced the HCR capacity of the NER-competent GM637 cells to a level closer to that in the XP-C and CS-B cell lines, which are deficient in the lesion recognition steps of the global genome repair (GGR) and transcription-coupled repair (TCR) sub-pathways of NER, respectively. In order to identify the potential role of PARP in these two sub-pathways of NER from that of its known role in base excision repair (BER) of UVB-induced oxidant damage, we depleted PARP from XP-C and CS-B cells and examined HCR of the reporter gene damaged by UVB, UVC or photoactivated methylene blue, the latter causing predominantly 8-oxo-2'-deoxyguanosine damage that is repaired by BER. Interestingly, a decreased HCR due to PARP-depletion was observed in both the NER-deficient cell lines in response to virus damaged by these three agents, albeit with different kinetics from 12 to 44h after infection. The role of PARP in NER was highlighted by a decreased clonogenic survival of UV-irradiated NER-competent GM637 cells depleted of PARP. Our results, while confirming the role of PARP in base excision repair, suggest a novel role of PARP in both the GGR and TCR sub-pathways of NER.

Poly(ADP-ribose) polymerase 1 is inhibited by a histone H2A variant, MacroH2A, and contributes to silencing of the inactive X chromosome

Poly(ADP-ribose) polymerase 1 (PARP-1) is a nuclear enzyme that is involved in modulating chromatin structure, regulation of gene expression, and sensing DNA damage. Here, we report that PARP-1 enzymatic activity is inhibited by macroH2A, a vertebrate histone H2A variant that is enriched on facultative heterochromatin. MacroH2A family members have a large C-terminal non-histone domain (NHD) and H2A-like histone domain. MacroH2A1.2 and PARP-1 interact in vivo and in vitro via the NHD. The NHD of each macroH2A family member was sufficient to inhibit PARP-1 enzymatic activity in vitro. The NHD of macroH2A1.2 was a mixed inhibitor of PARP-1 catalytic activity, with affects on both catalytic activity and the substrate binding affinity of PARP-1. Depletion of PARP-1 by RNA interference caused reactivation of a reporter gene on the inactive X chromosome, demonstrating that PARP-1 participates in the maintenance of silencing. These results suggest that one function of macroH2A in gene silencing is to inhibit PARP-1 enzymatic activity, and this may affect PARP-1 association with chromatin.