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Inhibition of transcription caused by DNA damage-impaired RNA polymerase II (Pol II) elongation conceals a local increase in de novo transcription, slowly progressing from Transcription Start Sites (TSSs) to gene ends. Although associated with accelerated repair of Pol II-encountered lesions and limited mutagenesis, it is still unclear how this mechanism is maintained during genotoxic stress-recovery. Here we uncover a widespread gain in chromatin accessibility and preservation of the active H3K27ac mark after UV-irradiation. The concomitant increase in Pol II escape from promoter-proximal pause (PPP) sites of most active genes, PROMPTs and enhancer RNAs favors unrestrained initiation, as evidenced by the synthesis of nascent RNAs including start RNAs. Accordingly, drug-inhibition of PPP-release replenishes levels of pre-initiating Pol II at TSSs after UV. Our data show that such continuous engagement of Pol II molecules ensures maximal transcription-driven repair throughout expressed genes and regulatory loci. Importantly, revealing this unanticipated regulatory layer of UV-response provides physiological relevant traction to the emerging concept that Pol II initiation rate is determined by pause-release dynamics., A transcription-driven cellular response is activated upon UV stress. Here the authors reveal mechanistic insights into the regulatory process affecting transcription and chromatin dynamics, showing how maintaining Pol II firing safeguards the integrity of cells’ transcriptome

Cockayne syndrome (CS) is a DNA repair syndrome characterized by a broad spectrum of clinical manifestations such as neurodegeneration, premature aging, developmental impairment, photosensitivity and other symptoms. Mutations in Cockayne syndrome protein B (CSB) are present in the vast majority of CS patients and in other DNA repair-related pathologies. In the literature, the role of CSB in different DNA repair pathways has been highlighted, however, new CSB functions have been identified in DNA transcription, mitochondrial biology, telomere maintenance and p53 regulation. Herein, we present an overview of identified structural elements and processes that impact on CSB activity and its post-translational modifications, known to balance the different roles of the protein not only during normal conditions but most importantly in stress situations. Moreover, since CSB has been found to be overexpressed in a number of different tumors, its role in cancer is presented and possible therapeutic targeting is discussed.

Specific capture of chromatin fractions with distinct and well-defined features has emerged as both challenging and a key strategy towards a comprehensive understanding of genome biology. In this context, we developed aniFOUND (accelerated native isolation of factors on unscheduled nascent DNA), an antibody-free method, which can label, capture, map and characterise nascent chromatin fragments that are synthesized in response to specific cues outside S-phase. We used the ‘unscheduled’ DNA synthesis (UDS) that takes place during the repair of UV-induced DNA lesions and coupled the captured chromatin to high-throughput analytical technologies. By mass-spectrometry we identified several factors with no previously known role in UVC-DNA damage response (DDR) as well as known DDR proteins. We experimentally validated the repair-dependent recruitment of the chromatin remodeller RSF1 and the cohesin-loader NIPBL at sites of UVC-induced photolesions. Developing aniFOUND-seq, a protocol for mapping UDS activity with high resolution, allowed us to monitor the landscape of UVC repair-synthesis events genome wide. We further resolved repair efficacy of the rather unexplored repeated genome, in particular rDNA and telomeres. In summary, aniFOUND delineates the proteome composition and genomic landscape of chromatin loci with specific features by integrating state-of-the-art ‘omics’ technologies to promote a comprehensive view of their function.

Inhibition of RNA synthesis caused by DNA damage-impaired RNA polymerase II (Pol II) elongation is found to conceal a local increase in de novo transcription, slowly progressing from Transcription Start Sites (TSSs) to gene ends. Although associated with accelerated repair of Pol II-encountered lesions and limited mutagenesis, it is still unclear how this mechanism is maintained during recovery from genotoxic stress. Here we uncover a surprising widespread gain in chromatin accessibility and preservation of the active histone mark H3K27ac after UV-irradiation. We show that the concomitant increase in Pol II release from promoter-proximal pause (PPP) sites of most active genes, PROMoter uPstream Transcripts (PROMPTs) and enhancer RNAs (eRNAs) favors unrestrained initiation, as demonstrated by the synthesis of short nascent RNAs, including TSS-associated RNAs (start-RNAs). In accordance, drug-inhibition of the transition into elongation replenished the post-UV reduced levels of pre-initiating pol II at TSSs. Continuous engagement of new Pol II thus ensures maximal transcription-driven DNA repair of active genes and non-coding regulatory loci. Together, our results reveal an unanticipated layer regulating the UV-triggered transcriptional-response and provide physiologically relevant traction to the emerging concept that transcription initiation rate is determined by pol II pause-release dynamics.

Complex molecular responses preserve gene expression accuracy and genome integrity in the face of environmental perturbations. Here we report that, in response to UV irradiation, RNA polymerase II (RNAPII) molecules are dynamically and synchronously released from promoter-proximal regions into elongation to promote uniform and accelerated surveillance of the whole transcribed genome. The maximised influx of de novo released RNAPII correlates with increased damage-sensing, as confirmed by RNAPII progressive accumulation at dipyrimidine sites and by the average slow-down of elongation rates in gene bodies. In turn, this transcription elongation ‘safe’ mode guarantees efficient DNA repair regardless of damage location, gene size and transcription level. Accordingly, we detect low and homogenous rates of mutational signatures associated with UV exposure or cigarette smoke across all active genes. Our study reveals a novel advantage for transcription regulation at the promoter-proximal level and provides unanticipated insights into how active transcription shapes the mutagenic landscape of cancer genomes., Precise orchestration of gene expression regulation upon DNA damage is essential for genome integrity. Here the authors identify a novel widespread stress-triggered defence mechanism that promotes rapid transcription-driven genomic surveillance thus limiting mutagenesis and shaping cancer genomes.

DNA damage poses a constant threat to genome integrity taking a variety of shapes and arising by normal cellular metabolism or environmental insults. Human syndromes, characterized by increased cancer pre-disposition or early onset of age-related pathology and developmental abnormalities, often result from defective DNA damage responses and compromised genome integrity. Over the last decades intensive research worldwide has made important contributions to our understanding of the molecular mechanisms underlying genomic instability and has substantiated the importance of DNA repair in cancer prevention in the general population. In this chapter, we discuss Nucleotide Excision Repair pathway, the causative role of its components in disease-related pathology and recent technological achievements that decipher mutational landscapes and may facilitate pathological classification and personalized therapy.

DNA damage poses a constant threat to genome integrity taking a variety of shapes and arising by normal cellular metabolism or environmental insults. Human syndromes, characterized by increased cancer pre-disposition or early onset of age-related pathology and developmental abnormalities, often result from defective DNA damage responses and compromised genome integrity. Over the last decades intensive research worldwide has made important contributions to our understanding of the molecular mechanisms underlying genomic instability and has substantiated the importance of DNA repair in cancer prevention in the general population. In this chapter, we discuss Nucleotide Excision Repair pathway, the causative role of its components in disease-related pathology and recent technological achievements that decipher mutational landscapes and may facilitate pathological classification and personalized therapy.

A network of DNA damage surveillance systems is triggered by sensing of DNA lesions and the initiation of a signal transduction cascade that activates genome-protection pathways including nucleotide excision repair (NER). NER operates through coordinated assembly of repair factors into pre- and post-incision complexes. Recent work identifies RPA as a key regulator of the transition from dual incision to repair-synthesis in UV-irradiated non-cycling cells, thereby averting the generation of unprocessed repair intermediates. These intermediates could lead to recombinogenic events and trigger a persistent ATR-dependent checkpoint signaling. It is now evident that DNA damage signaling is not limited to NER proficient cells. ATR-dependent checkpoint activation also occurs in UV-exposed non-cycling repair deficient cells coinciding with the formation of endonuclease APE1-mediated DNA strand breaks. In addition, the encounter of elongating RNA polymerase II (RNAPIIo) with DNA damage lesions and its persistent stalling provides a strong DNA damage signaling leading to cell cycle arrest, apoptosis and increased mutagenesis. The mechanism underlying the strong and strand specific induction of UV-induced mutations in NER deficient cells has been recently resolved by the finding that gene transcription itself increases UV-induced mutagenesis in a strand specific manner via increased deamination of cytosines. The cell removes the RNAPIIo-blocking DNA lesions by transcription-coupled repair (TC-NER) without displacement of the DNA damage stalled RNAPIIo. Deficiency in TC-NER associates with mutations in the CSA and CSB genes giving rise to the rare human disorder Cockayne syndrome (CS). CSB functions as a repair coupling factor to attract NER proteins, chromatin remodelers and the CSA-E3-ubiquitin ligase complex to the stalled RNAPIIo; CSA is dispensable for attraction of NER proteins, yet in cooperation with CSB is required to recruit XAB2, the nucleosomal binding protein HMGN1 and TFIIS. The molecular mechanisms by which these proteins bring about efficient TC-NER and trigger signaling after transcription arrest remain elusive; particularly the role of chromatin remodeling in TC-NER needs to be clarified in the context of anticipated structural changes that allow repair and transcription restart.

Transcription-coupled nucleotide excision repair (TC-NER) allows RNA polymerase II (RNAPII)-blocking lesions to be rapidly removed from the transcribed strand of active genes. Defective TCR in humans is associated with Cockayne syndrome (CS), typically caused by defects in either CSA or CSB. Here, we show that CSB contains a ubiquitin-binding domain (UBD). Cells expressing UBD-less CSB (CSB(del)) have phenotypes similar to those of cells lacking CSB, but these can be suppressed by appending a heterologous UBD, so ubiquitin binding is essential for CSB function. Surprisingly, CSB(del) remains capable of assembling nucleotide excision repair factors and repair synthesis proteins around damage-stalled RNAPII, but such repair complexes fail to excise the lesion. Together, our results indicate an essential role for protein ubiquitylation and CSB's UBD in triggering damage incision during TC-NER and allow us to integrate the function of CSA and CSB in a model for the process.

Heterochromatin protein 1 (HP1) family members are chromatin-associated proteins involved in transcription, replication, and chromatin organization. We show that HP1 isoforms HP1-α, HP1-β, and HP1-γ are recruited to ultraviolet (UV)-induced DNA damage and double-strand breaks (DSBs) in human cells. This response to DNA damage requires the chromo shadow domain of HP1 and is independent of H3K9 trimethylation and proteins that detect UV damage and DSBs. Loss of HP1 results in high sensitivity to UV light and ionizing radiation in the nematode Caenorhabditis elegans, indicating that HP1 proteins are essential components of DNA damage response (DDR) systems. Analysis of single and double HP1 mutants in nematodes suggests that HP1 homologues have both unique and overlapping functions in the DDR. Our results show that HP1 proteins are important for DNA repair and may function to reorganize chromatin in response to damage.

Decondensation of chromatin is essential to facilitate access to DNA metabolizing processes such as transcription and DNA repair. Disruption of histone-DNA contacts by histone modification or by ATP dependent chromatin remodelling allows DNA-binding proteins to compete with histones for DNA. The efficiency of global genome nucleotide excision repair (GGR) that removes a variety of helix distorting DNA lesions is known to be affected by chromatin structure most notably demonstrated by the slow repair of heterochromatin. In addition, the efficiency of GGR to repair lesions in transcriptionally active genes requires functional CSA and B proteins. We found that repair of UV-photolesions in both strands of the active adenosine deaminase gene was delayed in CS cells when compared to normal human fibroblasts. We suggest that the lack of transcription recovery characteristic for CS cells exposed to DNA damaging agents, might lead to changes in the chromatin structure of active genes, causing less efficient repair of lesions in these genes when compared to normal cells.

Nucleotide excision repair (NER) removes damage from DNA in a tightly regulated multiprotein process. Defects in NER result in three different human disorders, xeroderma pigmentosum (XP), trichothiodystrophy (TTD) and Cockayne syndrome (CS). Two cases with the combined features of XP and CS have been assigned to the XP–D complementation group. Despite their extreme UV sensitivity, these cells appeared to incise their DNA as efficiently as normal cells in response to UV damage. These incisions were, however, uncoupled from the rest of the repair process. Using cell-free extracts, we were unable to detect any incision activity in the neighbourhood of the damage. When irradiated plasmids were introduced into unirradiated XP–D/CS cells, the ectopically introduced damage triggered the induction of breaks in the undamaged genomic DNA. XP–D/CS cells thus have a unique response to sensing UV damage, which results in the introduction of breaks into the DNA at sites distant from the damage. We propose that it is these spurious breaks that are responsible for the extreme UV sensitivity of these cells.

Transcriptional arrest caused by DNA damage is detrimental for cells and organisms as it impinges on gene expression and thereby on cell growth and survival. To alleviate transcriptional arrest, cells trigger a transcription-dependent genome surveillance pathway, termed transcription-coupled nucleotide excision repair (TC-NER) that ensures rapid removal of such transcription-impeding DNA lesions and prevents persistent stalling of transcription. Defective TC-NER is causatively linked to Cockayne syndrome, a rare severe genetic disorder with multisystem abnormalities that results in patients' death in early adulthood. Here we review recent data on how damage-arrested transcription is actively coupled to TC-NER in mammals and discuss new emerging models concerning the role of TC-NER-specific factors in this process.

Continued association of RPA with sites of incomplete nucleotide excision repair averts further incision events until repair is completed., Single-stranded DNA gaps that might arise by futile repair processes can lead to mutagenic events and challenge genome integrity. Nucleotide excision repair (NER) is an evolutionarily conserved repair mechanism, essential for removal of helix-distorting DNA lesions. In the currently prevailing model, NER operates through coordinated assembly of repair factors into pre- and post-incision complexes; however, its regulation in vivo is poorly understood. Notably, the transition from dual incision to repair synthesis should be rigidly synchronized as it might lead to accumulation of unprocessed repair intermediates. We monitored NER regulatory events in vivo using sequential UV irradiations. Under conditions that allow incision yet prevent completion of repair synthesis or ligation, preincision factors can reassociate with new damage sites. In contrast, replication protein A remains at the incomplete NER sites and regulates a feedback loop from completion of DNA repair synthesis to subsequent damage recognition, independently of ATR signaling. Our data reveal an important function for replication protein A in averting further generation of DNA strand breaks that could lead to mutagenic and recombinogenic events.

Nucleotide excision repair (NER) operates through coordinated assembly of repair factors into pre- and postincision complexes. The postincision step of NER includes gap-filling DNA synthesis and ligation. However, the exact composition of this NER-associated DNA synthesis complex in vivo and the dynamic interactions of the factors involved are not well understood. Using immunofluorescence, chromatin immunoprecipitation, and live-cell protein dynamic studies, we show that replication factor C (RFC) is implicated in postincision NER in mammalian cells. Small interfering RNA-mediated knockdown of RFC impairs upstream removal of UV lesions and abrogates the downstream recruitment of DNA polymerase delta. Unexpectedly, RFC appears dispensable for PCNA recruitment yet is required for the subsequent recruitment of DNA polymerases to PCNA, indicating that RFC is essential to stably load the polymerase clamp to start DNA repair synthesis at 3' termini. The kinetic studies are consistent with a model in which RFC exchanges dynamically at sites of repair. However, its persistent localization at stalled NER complexes suggests that RFC remains targeted to the repair complex even after loading of PCNA. We speculate that RFC associates with the downstream 5' phosphate after loading; such interaction would prevent possible signaling events initiated by the RFC-like Rad17 and may assist in unloading of PCNA.

Nucleotide excision repair (NER) is the most versatile DNA repair system that deals with the major UV photoproducts in DNA, as well as many other DNA adducts. The early steps of NER are well understood, whereas the later steps of repair synthesis and ligation are not. In particular, which polymerases are definitely involved in repair synthesis and how they are recruited to the damaged sites has not yet been established. We report that, in human fibroblasts, approximately half of the repair synthesis requires both pol kappa and pol delta, and both polymerases can be recovered in the same repair complexes. Pol kappa is recruited to repair sites by ubiquitinated PCNA and XRCC1 and pol delta by the classical replication factor complex RFC1-RFC, together with a polymerase accessory factor, p66, and unmodified PCNA. The remaining repair synthesis is dependent on pol epsilon, recruitment of which is dependent on the alternative clamp loader CTF18-RFC.

Defects in the DNA repair mechanism nucleotide excision repair (NER) may lead to tumors in xeroderma pigmentosum (XP) or to premature aging with loss of subcutaneous fat in Cockayne syndrome (CS). Mutations of mitochondrial (mt)DNA play a role in aging, but a link between the NER-associated CS proteins and base excision repair (BER)-associated proteins in mitochondrial aging remains enigmatic. We show functional increase of CSA and CSB inside mt and complex formation with mtDNA, mt human 8-oxoguanine glycosylase (mtOGG)-1, and mt single-stranded DNA binding protein (mtSSBP)-1 upon oxidative stress. MtDNA mutations are highly increased in cells from CS patients and in subcutaneous fat of aged Csbm/m and Csa−/− mice. Thus, the NER-proteins CSA and CSB localize to mt and directly interact with BER-associated human mitochondrial 8-oxoguanine glycosylase-1 to protect from aging- and stress-induced mtDNA mutations and apoptosis-mediated loss of subcutaneous fat, a hallmark of aging found in animal models, human progeroid syndromes like CS and in normal human aging.

The repair of melphalan-induced N-alkylpurine monoadducts and interstrand cross-links was examined in different repair backgrounds, focusing on four genes (beta-actin, p53, N-ras, and delta-globin) with dissimilar transcription activities. Adducts were found to be substrates for both global genome repair (GGR) and transcription-coupled repair (TCR), with TCR being less efficient than GGR. In nucleotide excision repair-deficient cells, adducts accumulated to similar levels in all four genes. The repair efficiency in different gene loci varied in a qualitatively and quantitatively similar way in both GGR-deficient and TCR-deficient backgrounds and correlated with transcriptional activity and local chromatin condensation. No strand-specific repair was found in GGR(+)/TCR(+) cells, implying that GGR dominated. Adducts were lost over two sharply demarcated phases: a rapid phase resulting in the removal within 1 hour of up to approximately 80% of the adducts, and a subsequent phase with t(1/2) approximately 36 to 48 hours. Following pretreatment of cells with alpha-amanitin, the rate of transcription, the state of chromatin condensation, and the repair efficiencies (both TCR and GGR) of the transcribed beta-actin, p53, and N-ras genes became similar to those of the nontranscribed delta-globin gene. In conclusion, a continuous, parallel variation of the state of transcription and local chromatin condensation, on one hand, and the rates of both GGR and TCR, on the other hand, have been shown. Grant support: ECNIS (Environmental Cancer, Nutrition and Individual Susceptibility) Network of Excellence of the European Union (contract no. 513943).

The encounter of elongating RNA polymerase II (RNAPIIo) with DNA lesions has severe consequences for the cell as this event provides a strong signal for P53-dependent apoptosis and cell cycle arrest. To counteract prolonged blockage of transcription, the cell removes the RNAPIIo-blocking DNA lesions by transcription-coupled repair (TC-NER), a specialized subpathway of nucleotide excision repair (NER). Exposure of mice to UVB light or chemicals has elucidated that TC-NER is a critical survival pathway protecting against acute toxic and long-term effects (cancer) of genotoxic exposure. Deficiency in TC-NER is associated with mutations in the CSA and CSB genes giving rise to the rare human disorder Cockayne syndrome (CS). Recent data suggest that CSA and CSB play differential roles in mammalian TC-NER: CSB as a repair coupling factor to attract NER proteins, chromatin remodellers and the CSA- E3-ubiquitin ligase complex to the stalled RNAPIIo. CSA is dispensable for attraction of NER proteins, yet in cooperation with CSB is required to recruit XAB2, the nucleosomal binding protein HMGN1 and TFIIS. The emerging picture of TC-NER is complex: repair of transcription-blocking lesions occurs without displacement of the DNA damage-stalled RNAPIIo, and requires at least two essential assembly factors (CSA and CSB), the core NER factors (except for XPC-RAD23B), and TC-NER specific factors. These and yet unidentified proteins will accomplish not only efficient repair of transcription-blocking lesions, but are also likely to contribute to DNA damage signalling events.

Decondensation of chromatin is essential to facilitate access to DNA metabolizing processes such as transcription and DNA repair. Disruption of histone-DNA contacts by histone modification or by ATP dependent chromatin remodelling allows DNA-binding proteins to compete with histones for DNA. The efficiency of global genome nucleotide excision repair (GGR) that removes a variety of helix distorting DNA lesions is known to be affected by chromatin structure most notably demonstrated by the slow repair of heterochromatin. In addition, the efficiency of GGR to repair lesions in transcriptionally active genes requires functional CSA and B proteins. We found that repair of UV-photolesions in both strands of the active adenosine deaminase gene was delayed in CS cells when compared to normal human fibroblasts. We suggest that the lack of transcription recovery characteristic for CS cells exposed to DNA damaging agents, might lead to changes in the chromatin structure of active genes, causing less efficient repair of lesions in these genes when compared to normal cells.

Defects in the XPD gene can result in several clinical phenotypes, including xeroderma pigmentosum (XP), trichothiodystrophy, and, less frequently, the combined phenotype of XP and Cockayne syndrome (XP-D/CS). We previously showed that in cells from two XP-D/CS patients, breaks were introduced into cellular DNA on exposure to UV damage, but these breaks were not at the sites of the damage. In the present work, we show that three further XP-D/CS patients show the same peculiar breakage phenomenon. We show that these breaks can be visualized inside the cells by immunofluorescence using antibodies to either gamma-H2AX or poly-ADP-ribose and that they can be generated by the introduction of plasmids harboring methylation or oxidative damage as well as by UV photoproducts. Inhibition of RNA polymerase II transcription by four different inhibitors dramatically reduced the number of UV-induced breaks. Furthermore, the breaks were dependent on the nucleotide excision repair (NER) machinery. These data are consistent with our hypothesis that the NER machinery introduces the breaks at sites of transcription initiation. During transcription in UV-irradiated XP-D/CS cells, phosphorylation of the carboxy-terminal domain of RNA polymerase II occurred normally, but the elongating form of the polymerase remained blocked at lesions and was eventually degraded.

In Schizosaccharomyces pombe , rad18 is an essential gene involved in the repair of DNA damage produced by ionizing radiation and in tolerance of UV‐induced DNA damage. The Rad18 protein is a member of the SMC (structural maintenance of chromosomes) superfamily, and we show that, like the other SMC proteins in condensin and cohesin, Rad18 is a component of a high‐molecular‐weight complex. This complex contains at least six other proteins, the largest of which is Spr18, a novel SMC family member closely related to Rad18, and likely to be its heterodimeric partner. SMC proteins have ATP‐binding domains at the N‐ and C‐termini, and two extended coiled‐coil domains separated by a hinge in the middle. We show that the N‐terminal ATP‐binding domain of Rad18 is essential for all functions, and overexpression of an N‐terminal mutant has a dominant‐negative effect. We have identified an important mutation (S1045A) near the C‐terminus of Rad18 that separates its repair and essential roles. Potential models for the role of the Rad18–Spr18 complex during DNA repair are discussed.

The accumulation of stochastic DNA damage throughout an organism's lifespan is thought to contribute to ageing. Conversely, ageing seems to be phenotypically reproducible and regulated through genetic pathways such as the insulin-like growth factor-1 (IGF-1) and growth hormone (GH) receptors, which are central mediators of the somatic growth axis. Here we report that persistent DNA damage in primary cells from mice elicits changes in global gene expression similar to those occurring in various organs of naturally aged animals. We show that, as in ageing animals, the expression of IGF-1 receptor and GH receptor is attenuated, resulting in cellular resistance to IGF-1. This cell-autonomous attenuation is specifically induced by persistent lesions leading to stalling of RNA polymerase II in proliferating, quiescent and terminally differentiated cells; it is exacerbated and prolonged in cells from progeroid mice and confers resistance to oxidative stress. Our findings suggest that the accumulation of DNA damage in transcribed genes in most if not all tissues contributes to the ageing-associated shift from growth to somatic maintenance that triggers stress resistance and is thought to promote longevity.

Abstract 122 The development of cancer is driven by the accumulation of scores of alterations affecting the structure and function of the genome. Equally important in this process are genetic alterations and epigenetic changes. Whereas the former disrupt normal patterns of gene expression, sometimes leading to the expression of abnormal, constitutively active proteins, the latter deregulate the mechanisms such as transcriptional control leading to the inappropriate silencing or activation of cancer-associated genes. In this report, the effect of differential epigenetic alterations on the clinical outcome in multiple myeloma (MM) patients is presented. Peripheral blood mononuclear cells (PBMC) from twelve human healthy volunteers (7M/5F; median age 41 years, range 28 to 52 years) and 32 MM patients (14M/18F; median age 59 years, range 23 to 71 years) who underwent high dose melphalan with autologous stem cell support (ASCT) as part of their first line therapy. Twenty-three patients achieved an objective response post-ASCT while 9 patients did not respond according to IMWG criteria as well as human primary fibroblasts with different repair backgrounds (VH25, GGR+/TCR+; CS1AN, GGR+/TCR-; XP21RO, GGR-/TCR+; XP25RO, GGR-/TCR-) were exposed to melphalan. Chromatin condensation (using micrococcal nuclease digestion), transcription activity (steady-state levels using RNA slot-blots and rates of transcription using run-off assay), as well as in vitro melphalan-induced damage formation/repair (monoadducts and interstrand cross-links using Southern blot) were measured in three transcriptionally active genes (the housekeeping beta-actin gene, the tumor suppressor p53 and the protooncogene N-ras) as well as in the non-transcribed delta-globin gene. In repair-deficient human primary fibroblasts, melphalan adducts accumulated to similar levels in all four genes studied, indicating that the state of transcription and the local chromatin condensation did not affect adduct formation. In all MM patients examined, a close association was observed between the locus-specific repair efficiency, the transcriptional activity and the chromatin condensation. Strikingly, the order of variation of DNA repair efficiency, transcriptional activity and local chromatin structure between different genes was beta-actin>p53>N-ras>delta-globin in all healthy volunteers and in 95% of responders to chemotherapy, while a perturbation of this order was found in 80% of non-responders. In agreement with our previous reports (Dimopoulos et al, J Clin Oncol 2005;23:4381-9; Dimopoulos et al, Haematologica 2007;92:1505-12) in the p53, N-ras and delta-globin gene, responders to HDM showed higher melphalan-induced damage and lower rates of repair compared to non-responders. These differences were statistically significant for all types of melphalan adducts. No difference in the repair activity between responders and non-responders was observed in the beta-actin gene. Also, in all genes examined, DNA damage formation and repair induced by in vitro melphalan treatment of PBMC taken from MM patients prior to therapy, correlates with the respective data obtained in vivo, i.e. after therapeutic administration of HDM. Furthermore, the kinetics of melphalan adduct formation/repair along the transcribed N-ras gene showed a 5' to 3'-end gradient of repair efficiency, with faster repair at the 5'-end. Interestingly, the difference in the repair activity between responders and non-responders becomes maximal at the 5'-end of the N-ras gene, decrescent towards the 3'-end of the gene. As for the delta-globin gene, an induction of the transcription activity, a higher “looseness” of the local chromatin structure and an increase in the repair efficiency was observed specifically in MM patients who did not respond to melphalan therapy. No difference between responders and non-responders was found in regions on both sides of the genes as well as at the overall genome repair, suggesting that in non-responders the repair system active in the removal of melphalan-induced adducts was not grossly affected. In conclusion, our study suggests that the DNA repair efficiency, the transcriptional activity and the local chromatin structure in different gene loci are tightly connected, and can be used as molecular markers to select those patients with MM who are more likely to benefit from HDM therapy. Disclosures: No relevant conflicts of interest to declare.