Rosemary E. Gale

Insertional mutagenesis combined with acquired somatic mutations causes leukemogenesis following gene therapy of SCID-X1 patients

X-linked SCID (SCID-X1) is amenable to correction by gene therapy using conventional gammaretroviral vectors. Here, we describe the occurrence of clonal T cell acute lymphoblastic leukemia (T-ALL) promoted by insertional mutagenesis in a completed gene therapy trial of 10 SCID-X1 patients. Integration of the vector in an antisense orientation 35 kb upstream of the protooncogene LIM domain only 2 (LMO2) caused overexpression of LMO2 in the leukemic clone. However, leukemogenesis was likely precipitated by the acquisition of other genetic abnormalities unrelated to vector insertion, including a gain-of-function mutation in NOTCH1, deletion of the tumor suppressor gene locus cyclin-dependent kinase 2A (CDKN2A), and translocation of the TCR-beta region to the STIL-TAL1 locus. These findings highlight a general toxicity of endogenous gammaretroviral enhancer elements and also identify a combinatorial process during leukemic evolution that will be important for risk stratification and for future protocol design.

PI3-kinase/Akt is constitutively active in primary acute myeloid leukaemia cells and regulates survival and chemoresistance via NF-kB, MAPkinase and p53 pathways

The phosphoinositide 3-kinase (PI3-kinase) signalling pathway plays a key role in the regulation of cell survival and proliferation. We show that the PI3-kinase/Akt pathway is constitutively active in primary acute myeloid leukaemia (AML) cells and that blockade by the selective inhibitor LY294002 reduces survival of the total blast population (mean 52%). The ERK/MAPK module is also constitutively active and treatment with the MAPKK inhibitor U0126 reduces cell survival by 22%. In 10 of 18 samples, PI3-kinase contributes to MAPK activation as incubation with LY294002 leads to a marked reduction in its phosphorylation. PI3-kinase inhibition reduces survival of the CD34+38− AML progenitor subset by 44%, whereas MAPKK inhibition has little effect. Reporter assays in primary AML cells show that blocking PI3-kinase leads to a marked reduction of constitutive NF-kB activity and promotes p53-mediated transcription. This is associated with a synergistic interaction between LY294002 and Ara-C. An inducible activated form of Akt protects normal myeloid cells from Ara-C and etoposide-mediated apoptosis. These results show that blocking PI3-kinase has direct antileukaemic effects and potentiates the response to conventional cytotoxics via a number of targets including NF-kB, p53 and MAPK. Inhibitors of PI3-kinase and Akt may be useful in the treatment of AML.

Acquired skewing of X-chromosome inactivation patterns in myeloid cells of the elderly suggests stochastic clonal loss with age

More frequent skewing of X‐chromosome inactivation patterns (XCIPs) occurs in the white blood cells of elderly females; this study was performed to determine whether this occurs in myeloid or lymphoid lineages. XCIPs were analysed in purified neutrophils and T cells from 80 females  > 75 years and the results were compared with 23 cord blood and 94 younger adult blood samples. The degree of XCIP skewing in cord blood and younger adult blood cells was similar, with 3–4% having  > 90% expression of one allele. Skewing was markedly increased in the neutrophils of elderly females, with 33% having  > 90% expression of one allele (P < 0.0001). Extreme skewing was present in only 9% of the elderly T‐cell samples and no evidence of T‐cell clonality was found by PCR analysis of the TCRγ gene. The high level of acquired skewing of the XCIPs in myeloid cells of the elderly suggests that with time there is a change in stem cell usage with stochastic loss of some of the original stem cells. This has major implications for the use of XCIP analysis in the diagnosis of myeloid malignancies in the elderly and for gene therapy into haemopoietic stem cells.

Ollier disease and Maffucci syndrome are caused by somatic mosaic mutations of IDH1 and IDH2.

Ollier disease and Maffucci syndrome are characterized by multiple central cartilaginous tumors that are accompanied by soft tissue hemangiomas in Maffucci syndrome. We show that in 37 of 40 individuals with these syndromes, at least one tumor has a mutation in isocitrate dehydrogenase 1 (IDH1) or in IDH2, 65% of which result in a R132C substitution in the protein. In 18 of 19 individuals with more than one tumor analyzed, all tumors from a given individual shared the same IDH1 mutation affecting Arg132. In 2 of 12 subjects, a low level of mutated DNA was identified in non-neoplastic tissue. The levels of the metabolite 2HG were measured in a series of central cartilaginous and vascular tumors, including samples from syndromic and nonsyndromic subjects, and these levels correlated strongly with the presence of IDH1 mutations. The findings are compatible with a model in which IDH1 or IDH2 mutations represent early post-zygotic occurrences in individuals with these syndromes.

Assessment of Minimal Residual Disease in Standard-Risk AML.

BACKGROUND Despite the molecular heterogeneity of standard-risk acute myeloid leukemia (AML), treatment decisions are based on a limited number of molecular genetic markers and morphology-based assessment of remission. Sensitive detection of a leukemia-specific marker (e.g., a mutation in the gene encoding nucleophosmin [NPM1]) could improve prognostication by identifying submicroscopic disease during remission. METHODS We used a reverse-transcriptase quantitative polymerase-chain-reaction assay to detect minimal residual disease in 2569 samples obtained from 346 patients with NPM1-mutated AML who had undergone intensive treatment in the National Cancer Research Institute AML17 trial. We used a custom 51-gene panel to perform targeted sequencing of 223 samples obtained at the time of diagnosis and 49 samples obtained at the time of relapse. Mutations associated with preleukemic clones were tracked by means of digital polymerase chain reaction. RESULTS Molecular profiling highlighted the complexity of NPM1-mutated AML, with segregation of patients into more than 150 subgroups, thus precluding reliable outcome prediction. The determination of minimal-residual-disease status was more informative. Persistence of NPM1-mutated transcripts in blood was present in 15% of the patients after the second chemotherapy cycle and was associated with a greater risk of relapse after 3 years of follow-up than was an absence of such transcripts (82% vs. 30%; hazard ratio, 4.80; 95% confidence interval [CI], 2.95 to 7.80; P<0.001) and a lower rate of survival (24% vs. 75%; hazard ratio for death, 4.38; 95% CI, 2.57 to 7.47; P<0.001). The presence of minimal residual disease was the only independent prognostic factor for death in multivariate analysis (hazard ratio, 4.84; 95% CI, 2.57 to 9.15; P<0.001). These results were validated in an independent cohort. On sequential monitoring of minimal residual disease, relapse was reliably predicted by a rising level of NPM1-mutated transcripts. Although mutations associated with preleukemic clones remained detectable during ongoing remission after chemotherapy, NPM1 mutations were detected in 69 of 70 patients at the time of relapse and provided a better marker of disease status. CONCLUSIONS The presence of minimal residual disease, as determined by quantitation of NPM1-mutated transcripts, provided powerful prognostic information independent of other risk factors. (Funded by Bloodwise and the National Institute for Health Research; Current Controlled Trials number, ISRCTN55675535.).

Prognostic significance of CEBPA mutations in a large cohort of younger adult patients with acute myeloid leukemia: impact of double CEBPA mutations and the interaction with FLT3 and NPM1 mutations.

PURPOSE To determine the clinical relevance of mutations in the CCAAT/enhancer binding protein alpha (CEBPA) gene in acute myeloid leukemia (AML) and to examine factors that might modify prognostic impact. PATIENTS AND METHODS The entire CEBPA coding sequence was screened in 1,427 young adult patients with AML, excluding acute promyelocytic leukemia, using denaturing high-performance liquid chromatography and direct sequencing. RESULTS Of 107 patients (7%) with CEBPA mutations, 48 patients (45%) had one mutation (CEBPA-single), and 59 patients (55%) had two mutations (CEBPA-double). The incidence of CEBPA-double patients was similar in intermediate cytogenetic risk patients with and without a normal karyotype (6% and 5%, respectively). CEBPA-double patients had evidence of a lower coincidence with FLT3/ITDs (P = .04) and were highly unlikely to have an NPM1 mutation (P < .0001). CEBPA-double but not CEBPA-single patients had a significantly better overall survival (OS) at 8 years (34%, 31%, and 54% for CEBPA-wild-type [WT], CEBPA-single, and CEBPA-double, respectively, P = .004). This benefit was lost in the presence of a FLT3/ITD (OS for CEBPA-WT, CEBPA-single, and CEBPA-double FLT3/ITD-negative patients: 36%, 35%, 59%, respectively, P = .002; OS for CEBPA-WT, CEBPA-single, and CEBPA-double FLT3/ITD-positive patients: 26%, 21%, 14%, respectively, P = .05). There was no evidence of any additional favorable benefit for a CEBPA-single mutation in the presence of an NPM1 mutation (OS, 45%, 44%, and 56%, P = .2, for NPM1-positive/CEBPA-WT, NPM1-positive/CEBPA-single, and NPM1-negative/CEBPA-double patients, respectively). CONCLUSION Screening for CEBPA mutations can be restricted to patients with intermediate-risk cytogenetics lacking an FLT3/ITD or NPM1 mutation. Only the presence of a CEBPA-double mutation should be used for therapy risk stratification.

The prognostic significance of IDH2 mutations in AML depends on the location of the mutation

We have investigated the prognostic significance of isocitrate dehydrogenase 2 (IDH2) mutations in 1473 younger adult acute myeloid leukemia patients treated in 2 United Kingdom Medical Research Council trials. An IDH2 mutation was present in 148 cases (10%), 80% at R140 and 20% at R172. Patient characteristics and outcome differed markedly between the 2 mutations. IDH2(R140) significantly correlated with nucleophosmin mutations (NPM1(MUT)), whereas IDH2(R172) cases generally lacked other molecular mutations. An IDH2(R140) mutation was an independent favorable prognostic factor for relapse (P = .004) and overall survival (P = .008), and there was no significant heterogeneity with regard to NPM1 or FLT3 internal tandem duplication (FLT3/ITD) genotype. Relapse in FLT3/ITD(WT)NPM1(MUT)IDH2(R140) patients was lower than in favorable-risk cytogenetics patients in the same cohort (20% and 38% at 5 years, respectively). The presence of an IDH2(R172) mutation was associated with a significantly worse outcome than IDH2(R140), and relapse in FLT3/ITD(WT)NPM1(WT)IDH2(R172) patients was comparable with adverse-risk cytogenetics patients (76% and 72%, respectively).

The prognostic significance of IDH1 mutations in younger adult patients with acute myeloid leukemia is dependent on FLT3/ITD status

Mutations in the isocitrate dehydrogenase gene (IDH1) were recently described in patients with acute myeloid leukemia (AML). To investigate their prognostic significance we determined IDH1 status in 1333 young adult patients, excluding acute promyelocytic leukemia, treated in the United Kingdom MRC AML10 and 12 trials. A mutation was detected in 107 patients (8%). Most IDH1(+) patients (91%) had intermediate-risk cytogenetics. Mutations correlated significantly with an NPM1 mutation (P < .0001) but not a FLT3/ITD (P = .9). No difference in outcome between IDH1(+) and IDH1(-) patients was found in univariate or multivariate analysis, or if the results were stratified by NPM1 mutation status. However, when stratified by FLT3/ITD status, an IDH1 mutation was an independent adverse factor for relapse in FLT3/ITD(-) patients (P = .008) and a favorable factor in FLT3/ITD(+) patients (P = .02). These results suggest that metabolic changes induced by an IDH1 mutation may influence chemoresistance in a manner that is context-dependent.

Flt3 mutations and leukaemia

It is now 20 years since a molecular classification of acute myeloid leukaemia (AML) was initiated through the recognition of a number of leukaemia-specific cytogenetic abnormalities and their role as independent prognostic factors (Bloomfield et al, 1984). The intensive study of some of these markers, in particular the fusion transcripts from chromosomal translocations, has provided considerable insight into the underlying disease pathogenesis through characterization of the resulting aberrant gene products and their biological and clinical consequences. This has now been incorporated into the recent World Health Organization (WHO) classification of haemopoietic and lymphoid neoplasms in which AML patients with four well-defined recurring cytogenetic abnormalities have been put together as a subgroup (Harris et al, 1999). In general, however, patients are divided into three different risk groups based on cytogenetics: those with favourable, intermediate or standard, and poor risk disease. In some centres, the response to initial therapy is also taken into account (Wheatley et al, 1999). This classification has paved the way for a move from the indiscriminate use of high-dose chemotherapy for all patients to a more risk-adapted treatment approach. Patients with favourable cytogenetics, i.e. t(15;17), t(8;21) and inv(16), have particularly benefited from an improved understanding of the molecular pathology of their disease through identification of potential therapeutic targets. For example, the addition of all-trans retinoic acid (ATRA) during induction chemotherapy for acute promyelocytic leukaemia (APL) patients with the PML ⁄RARa (promyelocytic leukaemia ⁄ retinoic acid receptor alpha) fusion gene has increased the 5-year survival a further 20–30% compared with chemotherapy alone (Tallman et al, 1997; Burnett et al, 1999). Patients with poor risk disease have adverse survival, which hardly exceeds 20% at 5 years (Grimwade et al, 1998). They often have complete or partial loss of genetic material, e.g. )7, )5, del(5q), del(3q) or complex karyotypes, abnormalities that are currently less suitable candidates for targeted therapy. Approximately two-thirds of newly diagnosed AML patients, however, have intermediate ⁄ standard risk disease; their remission rate is similar to that of patients with favourable disease, but their outcome is hampered by an increased relapse rate (Grimwade et al, 1998). Although some of these patients have identifiable cytogenetic abnormalities that may allow the development of novel therapies, the majority of patients, 50% of all patients, have a normal karyotype. There has therefore been much interest in identifying further molecular markers of leukaemia that could help to improve the prognostic stratification of patients. In addition, with clear evidence for a multistep pathogenesis from both mouse models of leukaemia and the variable outcome for patients within defined cytogenetic groups, knowledge of co-operating mutations may further assist in improving therapy. Mutations in growth factor receptors and their downstream signalling molecules have long been obvious candidates for causing dysregulation of the delicate balance between proliferation and differentiation in haemopoietic cells. Many years of searching have now started to bear fruit with the demonstration that acquired mutations in the tyrosine kinase receptor gene, FLT3, are common in AML and have a major impact on prognosis. This review will outline the current knowledge on these mutations and their biological and clinical significance in leukaemia.

The replication rate of human hematopoietic stem cells in vivo

Hematopoietic stem cells (HSCs) replicate (self-renew) to create 2 daughter cells with capabilities equivalent to their parent, as well as differentiate, and thus can both maintain and restore blood cell production. Cell labeling with division-sensitive markers and competitive transplantation studies have been used to estimate the replication rate of murine HSCs in vivo. However, these methods are not feasible in humans and surrogate assays are required. In this report, we analyze the changing ratio with age of maternal/paternal X-chromosome phenotypes in blood cells from females and infer that human HSCs replicate on average once every 40 weeks (range, 25-50 weeks). We then confirm this estimate with 2 independent approaches, use the estimate to simulate human hematopoiesis, and show that the simulations accurately reproduce marrow transplantation data. Our simulations also provide evidence that the number of human HSCs increases from birth until adolescence and then plateaus, and that the ratio of contributing to quiescent HSCs in humans significantly differs from mouse. In addition, they suggest that human marrow failure, such as the marrow failure that occurs after umbilical cord blood transplantation and with aplastic anemia, results from insufficient numbers of early progenitor cells, and not the absence of HSCs.