Perspectives on joint EANM/SNMMI/ANZSNM practice guidelines/procedure standards for [18F]FDG PET/CT imaging during immunomodulatory treatments in patients with solid tumors

Response assessment in the context of immunomodulatory treatments represents a major challenge for the medical imaging community and requires a multidisciplinary approach with involvement of oncologists, radiologists, and nuclear medicine specialists. There is evolving evidence that [¹⁸F]FDG PET/CT is a useful diagnostic modality for this purpose. The clinical indications for, and the principal aspects of its standardization in this context have been detailed in the recently published "Joint EANM/SNMMI/ANZSNM practice guidelines/procedure standards on recommended use of [¹⁸F]FDG PET/CT imaging during immunomodulatory treatments in patients with solid tumors version 1.0". These recommendations arose from a fruitful collaboration between international nuclear medicine societies and experts in cancer treatment. In this perspective, the key elements of the initiative are reported, summarizing the core aspects of the guidelines for radiologists and nuclear medicine physicians. Beyond the previous guidelines, this perspective adds further commentary on how this technology can advance development of novel therapeutic approaches and guide management of individual patients.

Joint EANM/SNMMI/ANZSNM practice guidelines/procedure standards on recommended use of [18F]FDG PET/CT imaging during immunomodulatory treatments in patients with solid tumors version 1.0

PURPOSE

The goal of this guideline/procedure standard is to assist nuclear medicine physicians, other nuclear medicine professionals, oncologists or other medical specialists for recommended use of [18F]FDG PET/CT in oncological patients undergoing immunotherapy, with special focus on response assessment in solid tumors.

METHODS

In a cooperative effort between the EANM, the SNMMI and the ANZSNM, clinical indications, recommended imaging procedures and reporting standards have been agreed upon and summarized in this joint guideline/procedure standard.

CONCLUSIONS

The field of immuno-oncology is rapidly evolving, and this guideline/procedure standard should not be seen as definitive, but rather as a guidance document standardizing the use and interpretation of [18F]FDG PET/CT during immunotherapy. Local variations to this guideline should be taken into consideration.

PREAMBLE

The European Association of Nuclear Medicine (EANM) is a professional non-profit medical association founded in 1985 to facilitate worldwide communication among individuals pursuing clinical and academic excellence in nuclear medicine. The Society of Nuclear Medicine and Molecular Imaging (SNMMI) is an international scientific and professional organization founded in 1954 to promote science, technology and practical application of nuclear medicine. The Australian and New Zealand Society of Nuclear Medicine (ANZSNM), founded in 1969, represents the major professional society fostering the technical and professional development of nuclear medicine practice across Australia and New Zealand. It promotes excellence in the nuclear medicine profession through education, research and a commitment to the highest professional standards. EANM, SNMMI and ANZSNM members are physicians, technologists, physicists and scientists specialized in the research and clinical practice of nuclear medicine. All three societies will periodically put forth new standards/guidelines for nuclear medicine practice to help advance the science of nuclear medicine and improve service to patients. Existing standards/guidelines will be reviewed for revision or renewal, as appropriate, on their fifth anniversary or sooner, if indicated. Each standard/guideline, representing a policy statement by the EANM/SNMMI/ANZSNM, has undergone a thorough consensus process, entailing extensive review. These societies recognize that the safe and effective use of diagnostic nuclear medicine imaging requires particular training and skills, as described in each document. These standards/guidelines are educational tools designed to assist practitioners in providing appropriate and effective nuclear medicine care for patients. These guidelines are consensus documents based on current knowledge. They are not intended to be inflexible rules or requirements of practice, nor should they be used to establish a legal standard of care. For these reasons and those set forth below, the EANM, SNMMI and ANZSNM caution against the use of these standards/guidelines in litigation in which the clinical decisions of a practitioner are called into question. The ultimate judgment regarding the propriety of any specific procedure or course of action must be made by medical professionals considering the unique circumstances of each case. Thus, there is no implication that an action differing from what is laid out in the guidelines/procedure standards, standing alone, is below standard of care. To the contrary, a conscientious practitioner may responsibly adopt a course of action different from that set forth in the standards/guidelines when, in the reasonable judgment of the practitioner, such course of action is indicated by the condition of the patient, limitations of available resources or advances in knowledge or technology subsequent to publication of the guidelines/procedure standards. The practice of medicine involves not only the science, but also the art of dealing with the prevention, diagnosis, alleviation and treatment of disease. The variety and complexity of human conditions make it impossible for general guidelines to consistently allow for an accurate diagnosis to be reached or a particular treatment response to be predicted. Therefore, it should be recognized that adherence to these standards/ guidelines will not ensure a successful outcome. All that should be expected is that practitioners follow a reasonable course of action, based on their level of training, current knowledge, clinical practice guidelines, available resources and the needs/context of the patient being treated. The sole purpose of these guidelines is to assist practitioners in achieving this objective. The present guideline/procedure standard was developed collaboratively by the EANM, the SNMMI and the ANZSNM, with the support of international experts in the field. They summarize also the views of the Oncology and Theranostics and the Inflammation and Infection Committees of the EANM, as well as the procedure standards committee of the SNMMI, and reflect recommendations for which the EANM and SNMMI cannot be held responsible. The recommendations should be taken into the context of good practice of nuclear medicine and do not substitute for national and international legal or regulatory provisions.

[Comparing prediction performances of 18F-FDG PET and CGFL/Curie nomogram to predict pathologic complete response after neoadjuvant chemotherapy for HER2-positive breast cancers]

OBJECTIVES

The aim of this study was to compare the value of ¹⁸F-fluorodesoxyglucose positron emission tomography (¹⁸F-FDG PET/CT) with CGFL/Curie nomogram to predict a pathologic complete response (pCR) after neoadjuvant chemotherapy (NAC) in women with human epidermal growth factor 2 (HER2)-positive breast cancer treated by trastuzumab.

METHODS

Fifty-one women with HER2-positive breast cancer treated with trastuzumab plus taxane-based NAC were retrospectively included from January 2005 to December 2015. For ¹⁸F-FDG PET/CT, the analyzed predictor was the maximum standardized uptake value of the primary tumor and axillary nodes after the first course of NAC (PET₂.SUV_max). pCR was defined by no residual infiltrative tumor but in situ tumor was accepted. Accuracy of CGFL/Curie nomogram and PET₂.SUV_max was evaluated measuring sensitivity (Se), specificity (Sp), positive predictive value (PPV) and negative predictive value (NPV). Combined prediction was evaluated testing predictor's associations.

RESULTS

For CGFL/Curie nomogram's performances, Se, Sp, PPV and NPV were respectively: 76% (95%CI: 58-90%), 57% (95%CI: 43-66%), 55% (95%CI: 42-65), 77% (95%CI: 59-90%). For PET₂.SUV_max's performances, Se, Sp, PPV and NPV were respectively: 67% (95%CI: 48-81%), 77% (95%CI: 64-97%), 67% (95%CI: 48-82%), 77% (95%CI: 64-87%). ROC curves for these predictors were similar; the areas under the curve were 0.6 (95%CI: 0.56-0.64) for PET₂.SUV_max and 0.55 (95%CI: 0.50-0.59) for CGFL/Curie nomogram. Combined prediction was efficient with Se at 80%, VPN at 76%, Sp at 78% and VPP at 81%.

CONCLUSIONS

CGFL/Curie nomogram and PET₂.SUV_max were two efficient predictors of pCR in patients with HER2-positive breast cancer. Combined prediction has an improved accuracy.

[Comparison of the prognostic value of different clinical, angiographic and scintigraphic scores in stable coronary patients after acute coronary syndrome]

INTRODUCTION

To date, there is no consensus regarding the follow-up of asymptomatic coronary patients with an intermediate risk of events. Indeed, most of cardiovascular events (CVE) occur in asymptomatic patients, hence the clinician's interest in establishing risk stratification scores. In asymptomatic patient, the risk assessment after acute coronary syndrome (ACS) can currently be based on 3 types of score: clinical with, for example, the REACH score; angiographic with the residual SYNTAX score; imaging with different scintigraphic scores. These scores differ widely in terms of evaluation criteria and period of analysis. The aim of our study was therefore, in stable and asymptomatic coronary patients after ACS, to compare these different predictive scores; to establish that the combination of these scores makes it possible to optimize the risk assessment during the follow-up.

METHODS

Our retrospective study included 236 revascularized patients after ACS. Three different risk scores were collected: 1) the residual SYNTAX score, calculated at the time of revascularization; 2) the scintigraphic risk score described by Sharir et al., performed 3 to 12 months after the event and taking into account the extent of ischemia (SDS) and the poststress left ventricular ejection fraction (LVEF). Patients with LVEF <50% and/or moderate to severe ischemic disease (SDS≥2) were considered with an intermediate or high scintigraphic risk; 3) the REACH clinical score calculated on the day of the scintigraphic examination. After the myocardial scintigraphic exam, patients had a 1-year follow-up and CVE were recorded. Continuous data were analyzed either by Student's t-test or non-parametric Mann-Whitney test. The dichotomous data were compared either by the χ² test or by Fisher's exact test.

RESULTS

Forty-eight patients (20.1%) had a CVE during the 1-year follow-up. Thirty patients (13.8%) had a high residual SYNTAX score (≥8) without any correlation observed between the residual SYNTAX score and CVE (P=0.359). 148 patients (57.7%) had a high REACH clinical score (≥11) with no significant correlation observed with CVE (P=0.079). Lastly, 34 patients (14.4%) had an intermediate or high scintigraphic score, this imaging score being strongly correlated with a greater number of CVE (P<0.001). Multivariate analysis revealed 3 independent factors associated with CVE: a scintigraphic score> 2 (OR [(95% CI): 5.530 [2.426-12.605] P<0.001); Peripheral Arterial Obstructive Disease (PAOD) (OR [95% CI]: 8.531 [2.540-28.660] P<0.001); diabetes (OR [95% CI]: 2.86 [1.262-6.517] P=0,012).

CONCLUSION

The combination of the scintigraphic score with two clinical factors, such as PAOD and diabetes, provides optimal prognostic value in the evaluation of asymptomatic and stable patients after ACS. Our study therefore highlights the importance of optimizing evaluation strategies in the follow-up of these patients who remain at risk of post-revascularization CVE.

18FDG PET/CT in the early assessment of non-small cell lung cancer response to immunotherapy: frequency and clinical significance of atypical evolutive patterns

PURPOSE

This prospective study aimed (1) to assess the non-small cell lung cancer (NSCLC) evolutive patterns to immunotherapy using FDG-PET and (2) to describe their association with clinical outcome.

DESIGN

Fifty patients with metastatic NSCLC were included before pembrolizumab or nivolumab initiation. FDG-PET scan was performed at baseline and after 7 weeks of treatment (PET_interim1) and different criteria/parameters of tumor response were assessed, including PET response criteria in solid tumors (PERCIST). If a first PERCIST progressive disease (PD) without clinical worsening was observed, treatment was continued and a subsequent FDG-PET (PET_interim2) was performed at 3 months of treatment. Pseudo-progression (PsPD) was defined as a PERCIST response/stability on PET_interim2 after an initial PD. If a second PERCIST PD was assessed on PET_interim2, a homogeneous progression of lesions (termed immune homogeneous progressive-disease: iPD_homogeneous) was distinguished from a heterogeneous evolution (termed immune dissociated-response: iDR). A durable clinical benefit (DCB) of immunotherapy was defined as treatment continuation over a 6-month period. The association between PET evolutive profiles and DCB was assessed.

RESULTS

Using PERCIST on PET_interim1, 42% (21/50) of patients showed a response or stable disease, most of them (18/21) reached a DCB. In contrast, 58% (29/50) showed a PD, but more than one-third (11/29) were misclassified as they finally reached a DCB. No standard PET_interim1 criteria could accurately distinguished responding from non-responding patients. Treatment was continued in 19/29 of patients with a first PERCIST PD; the subsequent PET_interim2 demonstrated iPD_homogeneous, iDR and PsPD in 42% (8/19), 26% (5/19), and 32% (6/19), respectively. Whereas no patients with iPD_homogeneous experienced a DCB, all patients with iDR and PsPD reached a clinical benefit to immunotherapy.

CONCLUSION

In patients with a first PD on PERCIST and treatment continuation, a subsequent PET identifies more than half of them with iDR and PsPD, both patterns being strongly associated with a clinical benefit of immunotherapy.

Severe cardiomyopathy revealing antineutrophil cytoplasmic antibodies-negative eosinophilic granulomatosis with polyangiitis

Eosinophilic granulomatosis with polyangiitis (EGPA) is a rare form of systemic vasculitis in which cardiac involvement is frequent and severe, and accounts for half of EGPA-related deaths. ANCA-positive EGPA differs from ANCA-negative EGPA in that the former is significantly associated with renal involvement, peripheral neuropathy and biopsy proven vasculitis, whereas the latter is associated with cardiac involvement. Herein, we report a case of EGPA with myocarditis in a woman, who was successfully treated with steroids and cyclophosphamide. This report highlights the importance of diagnosing cardiac involvement in EGPA early, especially in ANCA-negative patients.

Predictors of post-stress LVEF drop 6 months after reperfused myocardial infarction: a gated myocardial perfusion SPECT study

OBJECTIVE

To identify the predictive factors of myocardial stunning as assessed by the drop in post-stress Left Ventricular Ejection Fraction (LVEF) in patients with a recent history of myocardial infarction (MI).

METHODS

We prospectively included 215 consecutive patients admitted for acute MI who underwent percutaneous coronary intervention with a greater than or equal to grade-3 TIMI flow in the culprit vessel. Six months after discharge, a post-stress/rest 99mTc-sestamibi gated SPECT was performed. The perfusion score was evaluated visually using a 17-segment model. The LVEF drop was considered significant if the post-stress LVEF was ≥ 5% below the rest LVEF (QGS® software).

RESULTS

A post-stress LVEF drop was observed in 51 (24%) patients. Patients with an LVEF drop were more likely than patients with a stable post-stress LVEF to have diabetes (22% vs. 10%, p = 0.048), significant ischemia (SDS > 2) (51% vs. 28% p = 0.003) and higher rest LVEF [62% (56-69) vs. 56% (49-63) p < 0.001]. In contrast, summed rest score, related to infarct size, did not differ between the groups. Multivariate logistic regression analysis identified SDS > 2 (OR 3.78, 95% CI 1.8-7.92, p < 0.001), diabetes (OR 3.35, 95% CI 1.33-8.49; p = 0.011) and rest LVEF (OR 1.08, 95% CI 1.04-1.12, p < 0.001) as independent explanatory variables of an LVEF drop.

CONCLUSION

In patients with recent MI and post-procedural grade-3 TIMI flow, ischemia and diabetes were independent predictive factors of myocardial stunning. The higher incidence of reversible perfusion abnormalities validates the model of myocardial stunning in the post-MI period, and excludes the potential involvement of myocardial necrosis.

Changes in 18F-FDG tumor metabolism after a first course of neoadjuvant chemotherapy in breast cancer: influence of tumor subtypes

BACKGROUND

The aim of this study is to evaluate the impact of the different breast cancer subtypes on the tumor (18)F-FDG uptake at baseline and on its changes after the first course of neoadjuvant chemotherapy (NAC).

PATIENTS AND METHODS

One hundred and fifteen women with newly diagnosed, large or locally advanced breast cancer undergoing NAC were included. Estrogen receptor (ER), progesterone receptor (PR) and HER2 status were used to define three major tumor subtypes: triple negative (TN) (ER-/PR-/HER2-), luminal (ER+ and/or PR+; HER2-) and HER2 positive (HER2+). Using Fluorine-18 fluorodeoxyglucose positron emission tomography, the tumoral standard uptake value (SUV) maximal index was measured at baseline and just before the second course of NAC.

RESULTS

TN tumors presented the highest baseline SUV (11.3 ± 8.5; P < 0.0001). The decrease of SUV after the first course of NAC (ΔSUV) was significantly higher in TN and HER2-positive subtypes (-45% ± 25% and -57% ± 30%, respectively) than in luminal one (-19% ± 35%; P < 0.0001). ΔSUV was a predictive factor of the pathological complete response only in HER2-positive tumors (cut-off = -75%; P < 0.03) with an accuracy of 76%.

CONCLUSION

The baseline (18)F-FDG tumoral uptake but also its early response to NAC is different according to the immunohistological subtypes of breast cancer.

Abstract P5-01-06: 18[F]-FDG Tumoral Uptake and Metabolic Response after 1 Cycle of Primary Chemotherapy According to Tumoral Phenotype of Breast Cancer

To compare the 18[F]-FDG-PET (PET) metabolic characteristics in triple-negative, luminal and positive HER2 breast cancer. Material and Methods: One hundred and forty-five patients with newly diagnosed locally advanced breast cancer were evaluated. Twenty one metastatic patients (discovered in baseline PET) were excluded and 9 patients were lost. One hundred and fifteen had undergone FDG PET before and after the first course of neoadjuvant chemotherapy. Breast cancer lesions were imaged at 80-90 minutes after administration of FDG. Maximum standardized uptake values (SUV) were measured at both time point (SUV1 and SUV 2). Metabolic response was measured by the relative decrease of SUV (ΔSUV). Using Immunohistochemistry as a surrogate for expression profiling, the tumours were classified as follows: triple negative defined by the lack of oestrogen receptors, progesterone receptors and human epidermal growth factor receptors 2 (HER2) expression, luminal (hormonal receptor positive, HER2 negative) and HER2 positive (overexpression of HER 2). Relationships between baseline [18F]-FDG uptake and clinical, histopathological and biological parameters were assessed by Mann-Whitney test. Relationships between SUV1, SUV2, ΔSUV and the tumoral phenotype were assessed by Kruskal-Wallis test.

Results: In the overall population the mean ±SD SUV1, SUV2 and ΔSUV values were 6.7± 5.9, 3.7± 3 and −36.7% ± 35.7%, respectively. Significant relationships were found between baseline FDG uptake and initial clinical and histopathological parameters : a high mitotic activity (p≥0.0001), a high nuclear pleomorphism (≥0.0002), a high tumour grading (p≥0.0001) and a negative oestrogen hormonal receptor status (p≥0.0001) were associated with a high baseline SUV. Patient age, lymph node involvement, architectural differentiation, progesterone hormonal receptor and HER2 status were not found to be related with SUV values.

Metabolic characteristics (mean ± SD) according to tumoral phenotype:

Conclusion: Our results show a significant positive relationship between the baseline FDG uptake and the proliferation markers, the tumour grading, the number of mitoses and the nuclear pleomorphism. A significant negative relationship was found between the baseline FDG uptake and the oestrogen hormonal receptor status. The triple negative phenotype was associated with higher baseline FDG uptake and higher residual FDG uptake after one course of chemotherapy commensurate with their aggressive biology. Luminal tumours showed a lower ΔSUV reflecting their lower chemosensibility.

Abstract P5-01-09: Evaluation of Tumor Blood Flow with Dynamic 18F-Fluorodeoxyglucose Positron Emission Tomography: Correlation with Microvessel Density

Background: Breast cancer is typically associated with an increase in tumor blood flow (due to angiogenesis) and tumor metabolism. Thus, these 2 key parameters are essential to characterize aggressiveness of breast tumors. Moreover, angiogenesis is a potential target for specific therapies. Positron Emission Tomography (PET) with 18F-Fluorodeoxyglucose (FDG) has become the gold standard for in vivo evaluation of tumor metabolism. A method for simultaneous measurement of blood flow and metabolism from a single injection of FDG may be an important addition for functional imaging of breast tumors. Unfortunately, there is no consensus on the validity of dynamic FDG acquisition for the evaluation of tumor blood flow.

Objective: to compare a new technique for simultaneous evaluation of tumor blood flow and metabolism, using a dynamic acquisition of FDG PET, with proliferation and endothelial cell markers. Material and methods: Twenty-eight patients with new diagnosed locally advanced breast cancer were included. In addition to diagnosis and tumor grading, biopsy samples of each tumor were used to assess the Ki-67 index of proliferation and the immuno-staining for CD31 (a panendothelial cell marker) and CD105 (a proliferation-related endothelial cell marker) (both expressed as number of labeled vessels counted on 10 consecutive high power fields, 400X magnification). All patients underwent FDG PET at least one week after sample biopsy and before any treatment. Dynamic 2 minutes acquisition was performed immediately after intravenous injection of 5 MBq/Kg of FDG; tumor Blood Flow (BF, in ml/min/g) was then calculated using a single compartiment kinetic model. Static acquisition was performed 60 minutes after injection for quantification of delayed FDG tumor uptake (“Standardized Uptake Value”, SUV), reflecting tumor metabolism.

Results: Pathologic and PET data were available for all patients. The SUV measured on delayed PET images was strongly positively correlated with the expression of Ki-67 (r=+0.693; P&lt;0.0001). In contrast, there was no significant correlation between SUV and endothelial markers (CD31 and CD105). Tumor BF was positively correlated with the expression of CD31 (r=+0.392; p=0.039) and CD105 (r=+0.470; p=0.016). In contrast, there was no significant correlation between BF and Ki-67. Patients were categorized according to low (inframedian) or high (supramedian) tumor BF. Patients with high tumor BF showed a higher mean CD31 expression (236±169 vs 124±47, p=0.025) and a higher mean CD105 expression (110±73 vs 51±14; p=0.006) when compared with low tumor BF. In contrast there was no significant difference concerning SUV and Ki-67.

Conclusion: Tumor BF quantified by dynamic FDG PET is related to tumor microvessel density evaluated by immunohistochemistry. Thus, dynamic FDG PET is a valuable tool to evaluate both tumor blood flow and metabolism. Further investigations are needed to confirm the respective role of these 2 key parameters for elaboration of therapeutic strategies and response assessment in locally advanced breast cancer.

Citation Information: Cancer Res 2010;70(24 Suppl):Abstract nr P5-01-09.

Abstract P5-01-05: Prognostic Value at 4 Years of FDG PET, after the First Course of Neoadjuvant Chemotherapy in Breast Cancer

FDG-PET may be a powerful examination to evaluate the effectiveness of neoadjuvant chemotherapy: it appears to be possible to predict the preoperative histological response after the first cycle of chemotherapy. The aim of this prospective study was to investigate the prognostic value of the early decrease of tumoral glucidic metabolism, evaluated with FDG-PET, after the first cycle of neoadjuvant chemotherapy in breast cancer. Material and methods: FDG-PET was performed before and after the first cycle of neoadjuvant chemotherapy in 92 patients with large or locally advanced, non inflammatory, breast cancer. The change of 18F-FDG tumoral uptake was calculated from the maximum Standard Uptake Value corrected for body surface and glycemia (ΔSUVmax). Disease free survival (DFS) was assessed, and the prognostic value of ΔSUVmax was first studied among all patients. Using immunohistochemistry as a surrogate for expression profiling, patients were then classified according to tumoral phenotypes as follow: triple negative (defined by the lack of estrogen receptor (ER), progesterone receptor (PR), and human epidermal growth factor receptor 2 (HER2) expression), luminal (ER positive or PR positive, HER2 negative) and HER2 positive.

Results: The median follow-up was 46.3 months (range: 37–52 months). Twenty one patients had recurrent disease, 9 of whom died. Using univariate Cox regression analysis with all patients, DFS showed significant correlation with initial tumor size measured with ultrasound scan (p=0.03), and ΔSUVmax after the first cycle of neoadjuvant chemotherapy (cut-off = -45%, p=0.01). Using multivariate cox regression analysis, only ΔSUVmax remained an independent prognostic factor of DFS at four years (p=0.028). According to tumoral phenotypes, ΔSUVmax was a prognostic factor of DFS for the 50 patients with a luminal cancer : the risk of relapse was 8.7 times higher in women whose SUVmax decreased less than 24% after the first course of chemotherapy (p=0,04). No significant correlation between

ΔSUVmax and DFS was shown in the 22 triple negative phenotypes. For the 31 patients overexpressing HER2, a tumoral SUVmax lower than 1,8 after one cycle of chemotherapy tended to be a favorable prognostic factor with a lower risk of relapse (p = 0,058) but ΔSUVmax had no significant prognostic value (p=0.08).

Conclusion: Using immunohistochemistry, the change of tumoral FDG uptake after the first cycle of neoadjuvant chemotherapy in operable breast cancer patients appears to be an early surrogate marker of disease free survival for luminal tumors in this study. For HER2 positive tumors, FDG-PET seems to provide prognostic information but, as for the triple negative tumors, it was not statistically significant. FDG-PET might be useful to guide the early therapeutic choice for breast cancer treated with neoadjuvant chemotherapy.

Citation Information: Cancer Res 2010;70(24 Suppl):Abstract nr P5-01-05.

Efficient repair of A/C mismatches in mouse cells deficient in long-patch mismatch repair

A previously unrecognized mismatch repair activity is described. Extracts of immortalized MSH2-deficient mouse fibroblasts did not correct most single base mispairs. The same extracts carried out efficient repair of A/C mismatches. A/G mispairs were less efficiently corrected and there was no significant repair of A/A. MLH1-defective mouse extracts also repaired an A/C mispair. A/C correction by Msh2(-/-) mouse cell extracts was not affected by antibodies against the PMS2 protein, which inhibited long-patch mismatch repair. A/C repair activity is thus independent of MutSalpha, MutSbeta and MutLalpha. A/C mismatches were corrected 5-fold more efficiently by extracts of Msh2 knockout mouse cells than by comparable extracts prepared from hMSH2- or hMLH1-deficient human cells. MSH2-independent A/C correction by mouse cell extracts did not require a nick in the circular duplex DNA substrate. Repair involved replacement of the A and was associated with the resynthesis of a limited stretch of </=25 bases of DNA.

Control of recombination rate during transformation of Streptococcus pneumoniae: an overview

Despite the fact that natural transformation was described long ago in Streptococcus pneumoniae, only a limited number of recombination genes have been identified. Two of them have recently been characterized at the molecular level, recA which encodes a protein essential for homologous recombination and mmsA which encodes the homologue of the Escherichia coli RecG protein. After a survey of the available information regarding the function of RecA, RecG, and other proteins such as the mismatch repair proteins HexA and HexB that can affect the outcome of recombinants, the different levels at which horizontal genetic exchange can be controlled are discussed. It is shown that the specific induction of the recA gene which occurs in competent cells is required for full recombination proficiency. Results regarding the ability of the Hex generalized mismatch repair system to prevent recombination between partially divergent sequences during transformation are also summarized. A structural analysis of homeologous recombinants which suggests that formation of mosaic recombinants can occur independently of mismatch repair in a single-step transformation is also reported. Finally, arguments in favor of an evolutionary origin of transformation as a means of genome evolution are discussed and the different types of recombination events observed which could potentially contribute to S. pneumoniae genome evolution are listed.

The DNA-dependent protein kinase catalytic activity regulates DNA end processing by means of Ku entry into DNA

The DNA-dependent protein kinase (DNA-PK) is required for double-strand break repair in mammalian cells. DNA-PK contains the heterodimer Ku and a 460-kDa serine/threonine kinase catalytic subunit (p460). Ku binds in vitro to DNA termini or other discontinuities in the DNA helix and is able to enter the DNA molecule by an ATP-independent process. It is clear from in vitro experiments that Ku stimulates the recruitment to DNA of p460 and activates the kinase activity toward DNA-binding protein substrates in the vicinity. Here, we have examined in human nuclear cell extracts the influence of the kinase catalytic activity on Ku binding to DNA. We demonstrate that, although Ku can enter DNA from free ends in the absence of p460 subunit, the kinase activity is required for Ku translocation along the DNA helix when the whole Ku/p460 assembles on DNA termini. When the kinase activity is impaired, DNA-PK including Ku and p460 is blocked at DNA ends and prevents their processing by either DNA polymerization, degradation, or ligation. The control of Ku entry into DNA by DNA-PK catalytic activity potentially represents an important regulation of DNA transactions at DNA termini.

Mismatch repair and differential sensitivity of mouse and human cells to methylating agents

The long-patch mismatch repair pathway contributes to the cytotoxic effect of methylating agents and loss of this pathway confers tolerance to DNA methylation damage. Two methylation-tolerant mouse cell lines were identified and were shown to be defective in the MSH2 protein by in vitro mismatch repair assay. A normal copy of the human MSH2 gene, introduced by transfer of human chromosome 2, reversed the methylation tolerance. These mismatch repair defective mouse cells together with a fibroblast cell line derived from an MSH2-/- mouse, were all as resistant to N-methyl-N-nitrosourea as repair-defective human cells. Although long-patch mismatch repair-defective human cells were 50- to 100-fold more resistant to methylating agents than repair-proficient cells, loss of the same pathway from mouse cells conferred only a 3-fold increase. This discrepancy was accounted for by the intrinsic N-methyl-N-nitrosourea resistance of normal or transformed mouse cells compared with human cells. The >20-fold differential resistance between mouse and human cells could not be explained by the levels of either DNA methylation damage or the repair enzyme O6-methylguanine-DNA methyltransferase. The resistance of mouse cells to N-methyl-N-nitrosourea was selective and no cross-resistance to unrelated DNA damaging agents was observed. Pathways of apoptosis were apparently intact and functional after exposure to either N-methyl-N-nitrosourea or ultraviolet light. Extracts of mouse cells were found to perform 2-fold less long-patch mismatch repair. The reduced level of mismatch repair may contribute to their lack of sensitivity to DNA methylation damage.

Frameshift mismatch recognition by the human MutS alpha complex

DNA mismatch binding by an extensively purified hMutS alpha mismatch recognition complex was investigated using a bandshift assay. The complex bound to G. T mispairs and to looped structures containing an unpaired single or two adjacent bases. A CA loop was preferentially recognised if the unpaired bases formed part of a repeated sequence. In general, single base loops were also more favourably recognised by hMutS alpha when present in monotonic runs of two to five. In one series of substrates, based on a known hotspot for frameshift mutations in the hypoxanthine-guanine phosphoribosyltransferase gene, in which different length G, A, C or T tracts were flanked by closely similar sequences, an A loop was bound preferentially in the absence of adjacent As and a C loop in the absence of adjacent Cs. This preferential binding was influenced by the base immediately 5' to the loop. Thus, while repeated regions generally favour recognition of single base loops by hMutS alpha, other factors related to local sequence may influence this interaction.

Increased somatic recombination in methylation tolerant human cells with defective DNA mismatch repair

We have studied whether spontaneous intrachromosomal recombination is altered in methylation tolerant human cells with a defect in mismatch repair. Somatic recombination was analysed in HeLaMR cells containing the vector pTPSN, which carries two copies of the gene for hygromycin resistance. The hygromycin genes are both inactivated by an inserted HindIII linker but hygromycin-resistant clones can arise by recombination. The spontaneous rate of recombination in a clone of HeLaMR cells containing a single integrated copy of pTPSN (HeLaG1) was 3.1x10(-6)/cell per generation. Two methylation tolerant variants from HeLaG1 cells (clone 12 and clone 15) were isolated by exposure to MNNG. Clone 12 cells exhibited a 16-fold increase in spontaneous mutation rate at the HPRT gene and extensive microsatellite instability at both mono- and dinucleotide repeats. Microsatellite instability limited to mononucleotide repeats was found in clone 15, whereas the mutation rate at HPRT was not significantly affected. A mismatch binding defect in extracts of clone 15 could be complemented by exogenous GTBP but not by purified hMSH2 protein. These data suggest that clone 15 is defective in GTBP. Extracts of clone 12 were unable to correct a single C:T mispair and complementation by extracts of human colorectal carcinoma cells with known deficiencies in mismatch repair indicated a defect in hMutLalpha. Western blotting with antibodies against different human mismatch repair proteins showed that clone 12 cells did not express hPMS2 protein, but expression of hMLH1, hMSH2 and GTBP appeared normal. The spontaneous recombination rate of clone 12 was 19-fold higher than the parental HeLaG1 cells, whereas no increase was observed in clone 15. Analysis of individual recombinants showed that hygromycin resistance arose exclusively by gene conversion. Our data indicate that mismatch correction regulates somatic recombination in human cells.

Mismatch repair defects and O6-methylguanine-DNA methyltransferase expression in acquired resistance to methylating agents in human cells

Fifteen variants with >/=30-fold resistance to N-methyl-N-nitrosourea were isolated from the Burkitt's lymphoma Raji cell line. Eight had received a single treatment with a highly cytotoxic dose. The remainder, including the previously described RajiF12 cell line, arose following multiple exposures to initially moderate but escalating doses. Surprisingly, methylation resistance arose in three clones by reactivation of a previously silent O6-methylguanine-DNA methyltransferase gene. Five clones, including RajiF12, displayed the microsatellite instability and increased spontaneous mutation rates at the hypoxanthine-guanine phosphoribosyltransferase locus, consistent with deficiencies in mismatch repair. Defects in either the hMutSalpha or hMutLalpha mismatch repair complexes were identified in extracts of these resistant clones by in vitro complementation using extracts from colorectal carcinoma cell lines. Defects in hMutLalpha were confirmed by Western blot analysis. Remarkably, five methylation-resistant clones in which mismatch repair defects were demonstrated by biochemical assays did not exhibit significant microsatellite instability.

The mmsA locus of Streptococcus pneumoniae encodes a RecG-like protein involved in DNA repair and in three-strand recombination

We describe the characterization of a mutant strain of Streptococcus pneumoniae previously isolated on the basis of its sensitivity to Methyl Methane Sulphonate (MMS). The mutant strain also exhibited increased sensitivity to UV light and to X-rays, together with a reduced capacity for recombination and Hex-mediated generalized mismatch repair. We show that the original mutant contains two unlinked mutations in the mmsA and in the pms genes. The mmsA wild-type region was cloned and the nucleotide sequence of the mmsA gene was determined. mmsA encodes a polypeptide of 671 amino acids related to a large family of DNA-RNA helicases, with the highest similarity to Escherichia coli RecG, a protein involved in the branch migration of Holliday junctions. A plasmid carrying the intact mmsA coding region was shown to restore UV resistance to E. coli recG mutant strains. An mmsA-null mutant constructed by insertion of a chloramphenicol-resistance gene exhibited a 25-fold reduction in recombination during transformation. We suggest that MmsA recognizes and branch migrates three-strand transformation intermediates to extend donor-recipient heteroduplex regions. The mmsA-null mutant exhibited the other phenotypes of the original mutant, apart from mismatch-repair deficiency and, in addition, an alteration in colony-forming ability was noticed. In the pms mutant background, all phenotypes caused by the mmsA mutation were attenuated. Therefore, the pms mutation, although it affected mismatch repair and, to some extent, DNA repair and recombination, acted as a suppressor of the mmsA mutation.

Homeologous recombination and mismatch repair during transformation in Streptococcus pneumoniae: saturation of the Hex mismatch repair system

The ability of the Hex generalized mismatch repair system to prevent recombination between partially divergent (also called homeologous) sequences during transformation in Streptococcus pneumoniae was investigated. By using as donor in transformation cloned fragments 1.7-17.5% divergent in DNA sequence from the recipient, it was observed that the Hex system prevents chromosomal integration of the least and the most divergent fragments but frequently fails to do so for other fragments. In the latter case, the Hex system becomes saturated (inhibited) due to an excess of mismatches: it is unable to repair a single mismatch located elsewhere on the chromosome. Further investigation with chromosomal donor DNA, carrying only one genetically marked divergent region, revealed that a single divergent fragment can lead to saturation of the Hex system. Increase in cellular concentration of either HexA, the MutS homologue that binds mismatches, or HexB, the MutL homologue for which the essential role in repair as yet remains obscure, was shown to restore repair ability in previously saturating conditions. Investigation of heterospecific transformation by chromosomal DNA from two related streptococcal species, Streptococcus oralis and Streptococcus mitis, also revealed complete saturation of the Hex system. Therefore the Hex system is not a barrier to interspecies recombination in S. pneumoniae. These results are discussed in light of those described for the Mut system of Escherichia coli.

A highly conserved repeated DNA element located in the chromosome of Streptococcus pneumoniae

We report the discovery of a group of highly conserved DNA sequences located, in those cases studied, within intergenic regions of the chromosome of the Gram positive Streptococcus pneumoniae. The S. pneumoniae genome contains about 25 of these elements called BOX. From 5' to 3', BOX elements are composed of three subunits (boxA, boxB, and boxC) which are 59, 45 and 50 nucleotides long, respectively. BOX elements containing one, two and four copies of boxB have been observed; boxB alone was also detected in one instance. These elements are unrelated to the two most thoroughly documented families of repetitive DNA sequences present in the genomes of enterobacteria. BOX sequences have the potential to form stable stem-loop structures and one of these, at least, is transcribed. Most of these elements are located in the immediate vicinity of genes whose product has been implicated at some stage in the process of genetic transformation or in virulence of S. pneumoniae. This location raises the intriguing possibility that BOX sequences are regulatory elements shared by several coordinately controlled genes, including competence-specific and virulence-related genes.