Supplementary Figure 1 from Blockade of the Ubiquitin Protease UBP43 Destabilizes Transcription Factor PML/RARα and Inhibits the Growth of Acute Promyelocytic Leukemia

Telomeres are the capping ends of chromosomes that protect the loss of genetic material and prevent chromosomal instability. In human tissue-specific stem/progenitor cells, telomere length (TL) is maintained by the telomerase complex, which consists of a reverse transcriptase catalytic subunit (TERT) and an RNA template (TERC). Very short telomeres and loss-of-function mutations in the TERT and TERC genes have been reported in acute myeloid leukemia, but the role of telomeres in acute promyelocytic leukemia (APL) has not been well established. We report the results for a large cohort of 187 PML/RARα-positive APL patients. No germline mutations in the TERT or TERC genes were identified. Codon 279 and 1062 TERT polymorphisms were present at a frequency similar to that in the general population. TL measured in blood or marrow mononuclear cells at diagnosis was significantly shorter in the APL patients than in healthy volunteers, and shorter telomeres at diagnosis were significantly associated with high-risk disease. For patients who achieved complete remission, the median increase in TL from diagnosis to remission (delta TL) was 2.0 kilobase (kb), and we found delta TL to be the most powerful predictor of overall survival when compared with well-established risk factors for poor outcomes in APL.

Activating internal tandem duplication (ITD) mutations in the fms-like tyrosine kinase 3 (FLT3) gene (FLT3-ITD) are associated with poor outcome in acute myeloid leukemia, but their prognostic impact in acute promyelocytic leukemia (APL) remains controversial. Here, we screened for FLT3-ITD mutations in 171 APL patients, treated with all-trans retinoic acid (ATRA) and anthracycline-based chemotherapy. We identified FLT3-ITD mutations in 35 patients (20 %). FLT3-ITD mutations were associated with higher white blood cell counts (P < 0.0001), relapse-risk score (P = 0.0007), higher hemoglobin levels (P = 0.0004), higher frequency of the microgranular morphology (M3v) subtype (P = 0.03), and the short PML/RARA (BCR3) isoform (P < 0.0001). After a median follow-up of 38 months, FLT3-ITD(positive) patients had a lower 3-year overall survival rate (62 %) compared with FLT3-ITD(negative) patients (82 %) (P = 0.006). The prognostic impact of FLT3-ITD on survival was retained in multivariable analysis (hazard ratio: 2.39, 95 % confidence interval [CI] 1.17-4.89; P = 0.017). Nevertheless, complete remission (P = 0.07), disease-free survival (P = 0.24), and the cumulative incidence of relapse (P = 0.94) rates were not significantly different between groups. We can conclude that FLT3-ITD mutations are associated with several hematologic features in APL, in particular with high white blood cell counts. In addition, FLT3-ITD may independently predict a shorter survival in patients with APL treated with ATRA and anthracycline-based chemotherapy.

The addition of arsenic trioxide (ATO) to frontline therapy of acute promyelocytic leukemia (APL) has been shown to result in significant improvements in disease-free survival (DFS). FLT3 mutations are frequently observed in APL, but its prognostic significance remains unclear. We analyzed 245 newly diagnosed adult patients with APL treated on intergroup trial C9710 and evaluated previously defined biological and prognostic factors and their relationship to FLT3 mutations and to additional karyotypic abnormalities. FLT3 mutations were found in 48% of patients, including 31% with an internal tandem duplication (FLT3-ITD), 14% with a point mutation (FLT3-D835) and 2% with both mutations. The FLT3-ITD mutant level was uniformly low, < 0.5. Neither FLT3 mutation had an impact on remission rate, induction death rate, DFS or overall survival (OS). The addition of ATO consolidation improved outcomes regardless of FLT3 mutation type or level, initial white blood cell count, PML-RARA isoform type or transcript level. The presence of a complex karyotype was strongly associated with an inferior OS independently of post-remission treatment. In conclusion, the addition of ATO to frontline therapy overcomes the impact of previously described adverse prognostic factors including FLT3 mutations. However, complex karyotype is strongly associated with an inferior OS despite ATO therapy.

Many tumors are believed to be maintained by a small number of cancer stem-like cells, where cure is thought to require eradication of this cell population. In this study, we investigated the dynamics of acute promyelocytic leukemia (APL) before and during therapy with regard to disease initiation, progression, and therapeutic response. This investigation used a mathematical model of hematopoiesis and a dataset derived from the North American Intergroup Study INT0129. The known phenotypic constraints of APL could be explained by a combination of differentiation blockade of PML-RARα-positive cells and suppression of normal hematopoiesis. All-trans retinoic acid (ATRA) neutralizes the differentiation block and decreases the proliferation rate of leukemic stem cells in vivo. Prolonged ATRA treatment after chemotherapy can cure patients with APL by eliminating the stem-like cell population over the course of approximately one year. To our knowledge, this study offers the first estimate of the average duration of therapy that is required to eliminate stem-like cancer cells from a human tumor, with the potential for the refinement of treatment strategies to better manage human malignancy.

Telomeres are the ends of linear chromosomes composed of tandem hexameric nucleotide repeats, maintained by a telomerase enzyme complex, made of reverse transcriptase catalytic subunit (TERT) and an RNA template (TERC). Present in very specific cells, such as embryonic and adult stem cells, the telomerase complex functions to elongate and slow the attrition of telomeric repeats associated with cellular division during replicative cellular age, as well as conferring stability to the ends of the chromosomes. In humans, both telomere length (TL) and loss-of-function mutations in the telomerase complex genes have been linked to malignant transformation and alteration of normal hematopoiesis. Though very short telomeres and germline mutations in TERT and TERC are associated with acute myeloid leukemia (Calado et al. PNAS 2009), the association between TL or telomerase complex integrity and outcome in patients (pts) with acute promyelocytic leukemia (APL) has not been thoroughly analyzed.

Summary The KMT 2E ( MLL 5 ) gene encodes a histone methyltransferase implicated in the positive control of genes related to haematopoiesis. Its close relationship with retinoic acid–induced granulopoiesis suggests that the deregulated expression of KMT 2E might lead acute promyelocytic leukaemia ( APL ) blasts to become less susceptible to the conventional treatment protocols. Here, we assessed the impact of KMT 2E expression on the prognosis of 121 APL patients treated with ATRA and anthracycline‐based chemotherapy. Univariate analysis showed that complete remission ( P = 0·006), 2‐year overall survival ( OS ) ( P = 0·005) and 2‐year disease‐free survival ( DFS ) rates ( P = 0·037) were significantly lower in patients with low KMT 2E expression; additionally, the 2‐year cumulative incidence of relapse was higher in patients with low KMT 2E expression ( P = 0·04). Multivariate analysis revealed that low KMT 2E expression was independently associated with lower remission rate (odds ratio [ OR ]: 7·18, 95% confidence interval [ CI ]: 1·71–30·1; P = 0·007) and shorter OS (hazard ratio [ HR ]: 0·27, 95% CI : 0·08–0·87; P = 0·029). Evaluated as a continuous variable, KMT 2E expression retained association with poor remission rate ( OR : 10·3, 95% CI : 2·49–43·2; P = 0·001) and shorter survival ( HR : 0·17, 95% IC : 0·05–0·53; P = 0·002), while the association with DFS was of marginal significance ( HR : 1·01; 95% CI : 0·99–1·02; P = 0·06). In summary, low KMT 2E expression may predict poor outcome in APL patients.

We report a long-term follow-up (median 11.8 years) of the First North American Intergroup Study. 379 patients were randomized to induction with ATRA or to chemotherapy. All complete responders (CR) received consolidation chemotherapy, then randomized to 1 year ATRA or observation. 245 patients received ATRA sometime during the study: 195 (80%) achieved a CR. Nine (4.6%) relapsed late (>3 years from CR), the last occurred after 4.6 years; 7 of them were still alive after 5.5-15 years. In APL patients, late relapses are uncommon, and those who sustain CR >5 years can be considered cured.

Thanks to modern treatment with all-trans retinoic acid and chemotherapy, acute promyelocytic leukemia (APL) is now the most curable type of leukemia. However, this progress has not yielded equivalent benefit in developing countries. The International Consortium on Acute Promyelocytic Leukemia (IC-APL) was established to create a network of institutions in developing countries that would exchange experience and data and receive support from well-established US and European cooperative groups. The IC-APL formulated expeditious diagnostic, treatment, and supportive guidelines that were adapted to local circumstances. APL was chosen as a model disease because of the potential impact on improved diagnosis and treatment. The project included 4 national coordinators and reference laboratories, common clinical record forms, 5 subcommittees, and laboratory and data management training programs. In addition, participating institutions held regular virtual and face-to-face meetings. Complete hematological remission was achieved in 153/180 (85%) patients and 27 (15%) died during induction. After a median follow-up of 28 months, the 2-year cumulative incidence of relapse, overall survival (OS), and disease-free survival (DFS) were 4.5%, 80%, and 91%, respectively. The establishment of the IC-APL network resulted in a decrease of almost 50% in early mortality and an improvement in OS of almost 30% compared with historical controls, resulting in OS and DFS similar to those reported in developed countries.

Abstract Background The MLL5 gene encodes a histone methyltransferase implicated in positive control of several genes related to hematopoiesis. Its close relationship with retinoic acid–induced granulopoiesis suggests that the deregulated expression of MLL5 might lead acute promyelocytic leukemia (APL) blasts to become less susceptible to differentiation-inducing ATRA effects. Here, we retrospectively determined the MLL5 transcript levels in samples from APL patients enrolled in the International Consortium on Acute Promyelocytic Leukemia (IC-APL) trial and analyzed its relationship with clinical and laboratory features, hematologic recovery, relapse, and survival. The results of the IC-APL have been previously reported (Blood 2013, 121(11) pp: 1935). In brief, complete hematological remission (CR) was achieved by 153/180 patients enrolled in Brazil, Mexico, Chile and Uruguay and after a median follow up of 28 months, the 2-year cumulative incidence of relapse (CIR), overall survival (OS) and disease-free survival (DFS) were 4.5%, 80% and 91%, respectively. Design and Methods One hundred and twenty-one APL patients (age, 15-73y) from seven different Brazilian institutions and treated according to the IC-APL protocol were included. The treatment schedule was identical to that adopted in the PETHEMA/HOVON LPA2005 trial, except for the replacement of idarubicin by daunorubicin. ATRA treatment was initiated immediately in all cases in which the diagnosis of APL was suspected based on morphology. As normal controls, total bone marrow (BM, n=8) and peripheral blood (PB, n=101) cells from healthy donors (age, 18-60y) were collected and mononuclear cells were isolated by Ficoll-Hypaque density gradient centrifugation. Additionally, 28 CBF-leukemia samples were included. Gene expression profile was analyzed by real-time quantitative PCR using the ABL FusionQuant Standard Kit as an endogenous control. Based on the continuous distribution of MLL5/ABL expression on APL samples (Figure 1), we adopted the median value of MLL5/ABL expression as cut-off to dichotomize APL patients in “low” and “high” MLL5 transcript levels. Results MLL5 expression was not different between APL, CBF-leukemia and healthy donors samples (Figure 1; P=0.19). There was no relevant difference between APL patients with low (n=62) and high (n=59) MLL5 transcript levels with respect to clinical and laboratory features, although high MLL5 transcript levels were more likely to be present in female gender (P=0.029). Overall, 102 (84%) achieved CR. Patients with low MLL5 transcript levels had significantly lower CR rate than patients with high MLL5 transcript levels (74% vs 95%; P=0.002). Twelve patients (10%) experienced early mortality (i.e., death during induction therapy) due to hemorrhage (n=6; 50%), disease progression (n=1; 8.3%), thrombosis (n=1; 8.3%), therapy-related infection (n=3; 25%) and differentiation syndrome (n=1; 8.3%); MLL5 transcript levels had no impact on early mortality (P=0.155). With a follow-up of 33 months among survivors (range, 1-72 months), patients with low MLL5 transcript levels had significantly lower 2-year OS (P=0.005) and 2-year DFS rate (P=0.037) than patients with high MLL5 transcript levels. Up to January 2013, a total of six relapses (5%) had been recorded. The 2-year CIR among patients with low and high MLL5 transcript levels was 14% (95%CI: 5% to 27%) and 2% (95%CI: 0.1% to 11%), respectively (P=0.04). We have further evaluated the prognostic impact of MLL5 transcript levels in those patients who remain alive after induction therapy (107 patients). Low MLL5 transcript levels were predictive of lower CR rate (P=0.042) and 2-year OS rate (P=0.009), but had no impact on DFS (P=0.106). Conclusion Our results show that MLL5 transcript levels may predict lower remission rate, short survival and higher risk of relapse in APL patients treated with ATRA and anthracycline-based chemotherapy. This is the first report describing the MLL5 expression as a prognostic factor in APL; nevertheless, our results should be confirmed in an independent cohort. Disclosures: No relevant conflicts of interest to declare.

Objectives: Several clinical trials conducted in Europe and US reported favorable outcomes of patients with APL treated with the combination of all trans retinoic acid (ATRA) and anthracyclines. Nevertheless, the results observed in developing countries with the same regimen was poorer, mainly due to high early mortality mainly due bleeding. The International Consortium on Acute Promyelocytic Leukemia (IC-APL) is an initiative of the International Members Committee of the ASH and the project aims to reduce this gap through the establishment of international network, which was launched in Brazil, Mexico and Uruguay.Methods: The IC-APL treatment protocol is similar to the PETHEMA 2005, but changing idarubicin to daunorubicin. All patients with a suspected diagnosis of APL were immediately started on ATRA, while bone marrow samples were shipped to a national central lab where genetic verification of the diagnosis was performed. The immunofluorescence using an anti-PML antibody allowed a rapid confirmation of the diagnosis and, the importance of supportive measures was reinforced.Results: The interim analysis of 97 patients enrolled in the IC-APL protocol showed that complete remission (CR) rate was 83% and the 2-year overall survival and disease-free survival were 80% and 90%, respectively. Of note, the early mortality rate was reduced to 7.5%.Discussion: The results of IC-APL demonstrate the impact of educational programs and networking on the improvement of the leukemia treatment outcome in developing countries.

e15191 Background: Inactivation of KLF6 by mutations and alternative splicing has been implicated in PC. However, there are significant discrepancies among human PC studies concerning the prevalence and significance of KLF6 mutations, and the prevalence of splice variants (SVs) has not been established. The objective of this study was to assess the prevalence and role of KLF6 splicing in human PC progression. Methods: Human specimens were obtained from the tissue banks of three institutions. Representative epithelium of 74 frozen PC specimens [25 primary hormone-naïve (HN) PCs, 21 primary hormone-deprived (HD) PCs, 6 castrate-resistant metastases] and 22 normal prostates (NPs) were microdissected. KLF6 cDNA was amplified by PCR, cloned and sequenced in both forward and reverse directions and compared to the KLF6 GenBank sequence. The transcriptional activity of KLF6 SVs was evaluated in co-transfection assays with a p21-reporter, and their effect on LNCaP cell growth was determined by MTT assays. Results: Sixteen different SVs were identified: 13 were novel; 14 affected the DNA binding domain (DBD) and were transcriptionally inactive. The incidence of SVs was increased in PC versus NP (75% vs 45%; p=0.01) and in HD-PC versus HN-PC (90% vs 64%; p=0.04) HD-PC were significantly enriched in SVs lacking the DBD. Forced expression of dysfunctional KLF6 SVs increased growth of LNCaP cells in androgen-depleted media. Conclusions: KLF6 SVs lacking p21-reporter transactivation activity occur in NP epithelium but are markedly increased in incidence, variety and level of expression in PC tissues that survive androgen-deprivation. This enrichment may enable resistance to hormone therapy and a proliferative advantage under castrate conditions.

Abstract Abstract 1407 Background: Aberrant expression of MLL5, BAALC, ID1, and WT1 genes is frequently associated with inferior outcome in cytogenetically normal acute myeloid leukemia patients (Damm et al. Blood 2011; 117(17):4561–8). The expression levels of these genes vary in patients with acute promyelocytic leukemia (APL), but the clinical significance of these findings remains unclear. Objective: (1) to determine if the gene expression levels of MLL5, BAALC, ID1, and WT1 are associated with clinical outcome of APL patients treated with ATRA and anthracycline-based chemotherapy, (2) to generate an integrative score (IS) based on these potential prognostic factors and clinical parameters and (3) to use this score for outcome prediction in APL. Design and Methods: One hundred and fifty APL patients (age, 15–73y) from seven different Brazilian institutions and treated according to the IC-APL protocol were included. The treatment schedule was identical to the PETHEMA-LPA 2005, except for the replacement of idarubicin by daunorubicin; ATRA treatment was initiated immediately in all cases in which the diagnosis of APL was suspected based on morphology. Gene expression profile was analyzed by Real-time PCR. Integer weights for the IS were derived from Cox proportional hazard model, using overall survival (OS) as outcome parameter. Hazard ratios (HR) for OS were calculated for each variable separately (Table 1). Variables with P<0.05 in univariate analyses were included in the model. Variables considered for the model inclusion consisted in 2 clinical (WBC counts, albumin levels) and 5 molecular markers (FLT3-ITD status and gene expression levels of MLL5, BAALC, ID1, and WT1). Other candidates, such as age, platelet count, gender, ECOG performance status, PML breakpoint and FAB subtype were not significant and not included in the score. The HR were converted to integer weights according to the following: variables with HR < 1 were excluded from analyses; variables with HR 3 1 and < 1.5 were assigned a weight of 1; variables with HR 3 1.5 and < 2.5 were assigned a weight of 2; variables with HR 3 2.5 were assigned a weight of 3. The final score was the sum of these integer weights. Based on maximally selected rank statistics, the scores were grouped into 3 risk-groups: 0–5 (low-IS), 6–9 (intermediate-IS), and > 9 (high-IS). Results: The integrative weights of variables analyzed are summarized in Table 1. The IS was modeled in 137 patients (median score: 6; range, 1–17). According to PETHEMA-GIMEMA relapse risk criteria, 22%, 23% and 70% of patients assigned in the low-IS (n=46), intermediate-IS (n=57) and high-IS (n=34) groups were deemed high-risk of relapse (P<0.001). Overall, 118 (86%) patients achieved CR; the remaining 19 patients (14%) experienced early death due to hemorrhage (n=12), therapy-related infection (n=6) and differentiation syndrome (n=1). Induction mortality was significantly higher in the high-IS group (low: 2%; intermediate: 15%; high: 26%) (P=0.001). CR was achieved in the low-, intermediate-, and high-IS group in 98%, 84%, and 73% of the patients, respectively (P=0.007). With a follow-up of 24 months among survivors, patients assigned in the high-IS group had a lower 2-y OS rate (63%) compared with those in the intermediate- (80%) and low-IS groups (97%; P<0.001). Eight relapses were recorded. The IS was not predictive of relapses (P=0.351). Conclusions: Our results suggest that MLL5, BAALC, ID1, and WT1 expression levels are associated with clinical outcome and that the IS may become a useful tool for outcome prediction in APL. Disclosures: Lo-Coco: Cephalon: Speakers Bureau; Boehringer Ingelheim: Membership on an entity's Board of Directors or advisory committees. Löwenberg:Skyline Diagnostics: Membership on an entity's Board of Directors or advisory committees.

Inactivation of the transcription factor/tumor suppressor Krüppel-like factor 6 (KLF6) has been described in prostate cancer (PC). This study investigated the prevalence and significance of KLF6 exon 2 mutations and splice variants (SVs) in different stages of human PC progression. By using laser-capture microdissection and recombinant clone isolation of DNA sequences to enhance sensitivity, base changes were found in 20 (24.7%) of 81 PC tissues versus 1 (4%) of 25 normal prostate tissues (P = 0.02). Of 26 base changes, 54% produced nonsynonymous mutations. Only three mutations had driver characteristics (PCs, 4%; NPs, 0%). By using microdissection of fresh-frozen tissues and recombinant isolation of RNA sequences, SVs were found in 39 (75%) of 52 PCs and in 10 (45%) of 22 NPs (P = 0.01). Sixteen different SVs, including 13 unique SVs, were identified that used cryptic splicing sites and encoded nonfunctional KLF6 proteins. PCs that had survived hormone (androgen)-deprivation therapy (n = 21) had a significantly higher (P < 0.05) incidence, number, and expression level of nonfunctional SVs than either NPs (n = 22) or hormone-naïve PCs (n = 25). Forced expression of nonfunctional SVs conferred a survival advantage of androgen-dependent LNCaP cells under castration-simulated culture conditions. Together, these data suggest that decreased availability of functional KLF6 contributes to clinical PC progression. This decrease arises infrequently by somatic mutation and more commonly by the acquisition of SVs that provide a survival advantage under castrate conditions, enabling resistance to hormone therapy.

Mutations in the all-trans retinoic acid (ATRA)-targeted ligand binding domain of PML-RARα (PRα/LBD+) have been implicated in the passive selection of ATRA-resistant acute promyelocytic leukemia clones leading to disease relapse. Among 45 relapse patients from the ATRA/chemotherapy arm of intergroup protocol C9710, 18 patients harbored PRα/LBD+ (40%), 7 of whom (39%) relapsed Off-ATRA selection pressure, suggesting a possible active role of PRα/LBD+. Of 41 relapse patients coanalyzed, 15 (37%) had FMS-related tyrosine kinase 3 internal tandem duplication mutations (FLT3-ITD+), which were differentially associated with PRα/LBD+ depending on ATRA treatment status at relapse: positively, On-ATRA; negatively, Off-ATRA. Thirteen of 21 patients (62%) had additional chromosome abnormalities (ACAs); all coanalyzed PRα/LBD mutant patients who relapsed off-ATRA (n = 5) had associated ACA. After relapse Off-ATRA, ACA and FLT3-ITD+ were negatively associated and were oppositely associated with presenting white blood count and PML-RARα type: ACA, low, L-isoform; FLT3-ITD+, high, S-isoform. These exploratory results suggest that differing PRα/LBD+ activities may interact with FLT3-ITD+ or ACA, that FLT3-ITD+ and ACA are associated with different intrinsic disease progression pathways manifest at relapse Off-ATRA, and that these different pathways may be short-circuited by ATRA-selectable defects at relapse On-ATRA. ACA and certain PRα/LBD+ were also associated with reduced postrelapse survival.

In 2 patients with As 2 O 3 -refractory acute promyelocytic leukemia (APL), Goto et al identified missense mutations in the PML-region B2-box domain that neighbor the dicysteine motif recently found to be critical for PML-RARα degradation, a requirement for curative anti-APL effect. 1

Abstract 258This icon denotes a clinically relevant abstract

Abstract Abstract 3552 The International Consortium on Acute Promyelocytic Leukemia (IC-APL) is an initiative of the International Members Committee of the ASH that aims to improve the treatment outcome of acute promyelocytic leukemia (APL) patients in developing countries, which was launched in Mexico, Brazil, Chile and Uruguay. The protocol is identical to the PETHEMA-LPA2005, except for the replacement of idarubicin by daunorubicin. In our interim analysis, estimated 2-year overall and disease-free survivals are 80% and 90%, respectively. The 2-year cumulative incidence of relapse was 5.6%. The median follow-up among survivors was 23 months (range: 1 – 56 months). A secondary aim of IC-APL was to establish molecular monitoring for minimal residual disease (MRD) as a standard practice for APL patients in these countries and to use the results obtained to guide therapy. According to the IC-APL protocol, testing for the PML-RARA fusion transcript was to be performed at diagnosis, end of induction (optional), after the third cycle of consolidation, and every 3 months during maintenance. Considering that real-time quantitative polymerase chain reaction (RQ-PCR) provides a number of advantages compared to conventional non-quantitative reverse transcriptase PCR (RT-PCR), we retrospectively compared the results obtained by both techniques. We analyzed 400 bone marrow (BM) samples from 97 patients with de novo APL enrolled in the IC-APL protocol in Brazil. Of the 97 patients, 78 were considered eligible. The mean age was of 35.8 years with 46 males. Among eligible patients, 49 corresponded to bcr1 ; one to bcr2 and 28 to bcr3 subtype of PML breakpoint. To quantify the fusion transcript PML-RARA we used standardized assays developed in the Europe Against Cancer (EAC) program, normalized to the expression of the ABL gene. The results were compared to plasmid standards (Ipsogen, Marseille) and expressed as Normalized Copy Numbers (NCN). Follow-up samples were considered PCR positive when PML-RARA transcripts amplified with Cycle Threshold (Ct) values of ≤40 in at least 2 out of 3 replicates, according to EAC criteria. A total of 71 samples at diagnosis, 50 at the end of induction, 47 after the third consolidation, 202 during maintenance phase and 30 samples after completion of treatment were analyzed. The median NCN of PML-RARA transcripts at diagnosis was 0.5151 and 0.5092 for the bcr1 and bcr3 subtypes, respectively. At the end of induction there was a reduction of about 3 logs (0.0004 for bcr1 and 0.0005 for bcr3). In this phase, six discrepant cases were observed, all presenting positivity by RQ-PCR. None of these cases relapsed and presented consecutive negative results. Considering samples obtained at the end of consolidation, we detected one case of molecular persistence detected by both methods, and two discrepant results, one positive by RT and another by RQ-PCR. Both cases did not relapse. Among samples collected during maintenance and after the end of treatment, two patients (2.5%) relapsed. Both of them were molecular relapses, defined as the detection (in the context of morphological remission) of the PML-RARA transcript by RT-PCR in two consecutive samples collected 15 days apart. RQ-PCR analysis provided much earlier warning of recurring disease, testing positive 5 and 6 months, respectively, before documentation of molecular relapse by conventional RT-PCR assay. Figure 1 show the kinetics of NCN in these two cases. Our results reinforce that the PML-RARA transcript may be detected after induction but this finding was not of prognostic value. However, our study underlines the importance of sequential monitoring to distinguish patients likely to be cured following front-line therapy from those destined to relapse. The RQ-PCR technique was shown to be more sensitive than RT-PCR, providing earlier warning of impending relapse, thereby allowing greater opportunity for successful delivery of pre-emptive therapy. Finally, our results demonstrate that the implementation of the IC-APL allowed the improvement of laboratory standards in parallel to advances in clinical management. Disclosures: Pasquini: Novartis: Membership on an entity's Board of Directors or advisory committees, Speakers Bureau; Bristol Myers Squibb: Speakers Bureau. Pagnano:Novartis: Speakers Bureau; Bristol Myers Squibb: Speakers Bureau.

Abstract Abstract 3536 ΔNp73 is an alternative TP73 gene transcript lacking the transactivation (TA) domain that is generated via alternative splicing and/or P2 promoter. The encoded protein acts as a potent transdominant inhibitor of wild type TP53 and full-length TAp73. In several human malignancies the unbalanced expression of transcriptionally active (TAp73) and inactive (ΔNp73) variants correlates with treatment outcome. We have previously reported that higher ΔN/TA isoform expression ratio was associated with poorer prognosis and resistance to cytarabine induced apoptosis in patients with acute myeloid leukemia (AML) (Lucena-Araujo et al., 2008). In acute promyelocytic leukemia (APL), both isoforms are expressed, but the clinical significance remains unknown. The aim of this study was to determine whether the ΔN/TA expression ratio was associated with treatment outcome of APL patients and to investigate the mechanisms by which ΔNp73 may contribute to PML-RARa+ cell survival. Using isoform-specific probes for ΔNp73 and TAp73, their expression was analyzed in 166 APL patients by Real-time quantitative polymerase chain reaction (RQ-PCR). Patients were divided into tertiles for ΔN/TA expression ratio (median value=23.62; 33rd/66th percentiles=12.8/42.3) and their clinical and laboratory characteristics were compared. Patients in the highest tertile presented higher white blood cells (WBC) counts than those in intermediate/lower tertiles (p <0.001), but no significant differences were observed for age, gender, PML breakpoint, or platelets count. Higher ΔN/TA expression ratio values were significantly associated with the presence of FLT3-ITD (p =0.001). Treatment outcome was obtained for 131 APL patients enrolled in the APL99 (n=41) and IC-APL (n=90) trials. The mean follow-up was 29.1 months, ranging from 1 to 85.5 months. The mean overall survival (OS) of all patients was of 66.8 months [95%CI; 60.8 to 72.8], whereas it was of 67.1 months [56.5 to 77.7] for patients in the lower and 41.7 months [32 to 51.4] for those in the higher tertile for ΔN/TA expression ratio (p=0.014, Figure 1A). Univariate analysis identified WBC counts above 10,000/μl (p =0.003), FLT3-ITD mutation (p =0.011) and ΔN/TA expression ratio (p =0.014) as predictive factors for OS. However, in multivariate Cox analysis, these three prognostic factors were not independent. Until April 2011, a total of eight relapses (6.1%) were recorded. The disease free survival (DFS) rate at five-years for all patients was of 88.3% ± 4.2% and the mean DFS was of 76.1 months [71.2 to 80.9]. DFS was significantly shorter in patients at the higher tertile ΔN/TA expression ratio compared with patients at the lower tertile (72.1 ± 11.2% vs 97.1 ± 2.8%, respectively; p <0.001; Figure 1B) and was the only variable found to be significant in the univariate analysis. To test the functional significance of the association of PML-RARa with high ΔNp73 gene expression, primary murine bone marrow cells from hCG-PML-RARa transgenic mice were transfected with MSCV-based retroviral vector carrying the ΔNp73 cDNA upstream of IRES-GFP cassette (PML-pMIG-ΔN). Expression of ΔNp73 in PML-RARa+ cells increased cell proliferation rate by 2.5-fold compared to PML-RARa+ transfected with the empty vector (p =0.03). This increase resulted from accumulation of cells at the G2/M phase (5.79 ± 0.08% for PML-pMIG vs 9.8 ± 0.35% for PML-pMIG-ΔN, p <0.001), as well as at S phase of the cell cycle (27.74 ± 0.89% for PML-pMIG vs. 36.78 ± 0.81% for PML-pMIG-ΔN, p =0.001). In addition, transfection of ΔNp73 resulted in resistance to cytarabine-induced apoptosis. After 24h of culture with 50μg/ml of cytarabine (ED-50%), the fractional effect for the drug (% Anexin V-positive in (treated – untreated) cells /100 - % Anexin V-positive in non-treated cells) was 32.1% for PML-pMIG-ΔN and 54.8% for PML-pMIG (p <0.001). In conclusion, ΔN/TA expression ratio was associated with shorter OS and DFS in APL, which may reflect increased cell proliferation and apoptosis resistance due to ΔNp73 activity.Figure 1.Overall (A) and disease-free survival (B) in acute promyelocytic leukemia patients according to ΔN/TA expression ratio.Figure 1. Overall (A) and disease-free survival (B) in acute promyelocytic leukemia patients according to ΔN/TA expression ratio. Disclosures: No relevant conflicts of interest to declare.