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Simon N Powell

Publications and source records attributed to Simon N Powell.

33 records · Page 2Linked to original sources
PubMed2004

ATR affecting cell radiosensitivity is dependent on homologous recombination repair but independent of nonhomologous end joining.

ATR is one of the most important checkpoint proteins in mammalian cells responding to DNA damage. Cells defective in normal ATR activity are sensitive to ionizing radiation (IR). The mechanism by which ATR protects the cells from IR-induced killing remains unclear. DNA double-strand breaks (DSBs) induced by IR are critical lesions for cell survival. Two major DNA DSB repair pathways exist in mammalian cells: homologous recombination repair (HRR) and nonhomologous end joining (NHEJ). We show that the doxycycline (dox)-induced ATR kinase dead (ATRkd) cells have the similar inductions and rejoining rates of DNA DSBs compared with cells without dox induction, although the dox-induced ATRkd cells are more sensitive to IR and have the deficient S and G(2) checkpoints. We also show that the dox-induced ATRkd cells have a lower HRR efficiency compared with the cells without dox induction. These results indicate that the effects of ATR on cell radiosensitivity are independent of NHEJ but are linked to HRR that may be affected by the deficient S and G(2) checkpoints.

Ataxia Telangiectasia Mutated Proteins↗
PubMed2004

BRCA1-BARD1 complexes are required for p53Ser-15 phosphorylation and a G1/S arrest following ionizing radiation-induced DNA damage.

BRCA1 is a major player in the DNA damage response. This is evident from its loss, which causes cells to become sensitive to a wide variety of DNA damaging agents. The major BRCA1 binding partner, BARD1, is also implicated in the DNA damage response, and recent reports indicate that BRCA1 and BARD1 co-operate in this pathway. In this report, we utilized small interfering RNA to deplete BRCA1 and BARD1 to demonstrate that the BRCA1-BARD1 complex is required for ATM/ATR (ataxia-telangiectasia-mutated/ATM and Rad3-related)-mediated phosphorylation of p53(Ser-15) following IR- and UV radiation-induced DNA damage. In contrast, phosphorylation of a number of other ATM/ATR targets including H2AX, Chk2, Chk1, and c-jun does not depend on the presence of BRCA1-BARD1 complexes. Moreover, prior ATM/ATR-dependent phosphorylation of BRCA1 at Ser-1423 or Ser-1524 regulates the ability of ATM/ATR to phosphorylate p53(Ser-15) efficiently. Phosphorylation of p53(Ser-15) is necessary for an IR-induced G(1)/S arrest via transcriptional induction of the cyclin-dependent kinase inhibitor p21. Consistent with these data, repressing p53(Ser-15) phosphorylation by BRCA1-BARD1 depletion compromises p21 induction and the G(1)/S checkpoint arrest in response to IR but not UV radia-tion. These findings suggest that BRCA1-BARD1 complexes act as an adaptor to mediate ATM/ATR-directed phosphorylation of p53, influencing G(1)/S cell cycle progression after DNA damage.

Ataxia Telangiectasia Mutated Proteins↗
PubMed2004

DNA damage induces p53-dependent BRCA1 nuclear export.

The tumor suppressor gene BRCA1 plays an important role in the response to DNA damage. BRCA1 function is regulated by a variety of mechanisms including transcriptional control, phosphorylation, and protein-protein interactions. Recent studies have shown that BRCA1 is a nuclear-cytoplasmic shuttle protein. Its subcellular localization is controlled by a nuclear localization signal-mediated nuclear import via the importin receptor pathway and a nuclear export signal-facilitated nuclear export through a CRM1-dependent pathway. Using the human breast cancer cell line, MCF7, the subcellular distribution of BRCA1 was assessed by immunohistochemical staining and Western blotting analyses of fractionated subcellullar extracts. Ionizing radiation stimulated BRCA1 nuclear export in a dose-dependent manner. This DNA damage-induced BRCA1 nuclear export utilized a CRM1-dependent mechanism and also required wild-type p53, whose function was abrogated by the E6 protein in MCF7 cells. In addition, the dependence on p53 was confirmed using a second cell type operating a tetracycline-inducible system. The effect of ionizing radiation on BRCA1 export was observed in every phase of the cell cycle, although BRCA1 localization did vary between the G(1), S, and G(2)/M phases. These results imply that, in addition to ATM-, ATR-, and Chk2-dependent phosphorylations, cytoplasmic relocalization of BRCA1 protein is a mechanism whereby BRCA1 function is regulated in response to DNA damage.

Active Transport, Cell Nucleus↗
PubMed2004

The effect of delaying radiation therapy for systemic chemotherapy on local-regional control in breast cancer.

BACKGROUND: It remains controversial as to whether the administration of chemotherapy prior to radiation following the surgical treatment for localized breast cancer compromises local control. METHODS: The outcome of 290 patients who received chemotherapy prior to breast or chest wall irradiation following curative breast cancer surgery was retrospectively analyzed to determine if delaying radiation from the time of surgery adversely affects local-regional control. The patients were divided into three groups according to the time interval from definitive surgery to the start of radiation: group 1, < 5 months; group 2, 5 to < 7 months, and group 3, 7+ months. Local-regional and distant failure events were analyzed according to the time delay from surgery to radiation. RESULTS: The median follow-up was 6.0 years (range, 7 months to 15 years). Loco-regional failure was observed in 22 patients, 18 of which occurred within the original radiation fields. There was no significant adverse effect on local control or distant failure from delaying radiation for systemic chemotherapy. Univariate and multivariate analyses revealed that positive margins after surgery were associated with increased local recurrence. CONCLUSION: Delaying radiation therapy for greater than 7 months, in order to administer chemotherapy following curative breast cancer surgery, does not compromise local control. However, positive margins were significantly associated with higher rates of local failure and even an increase in radiation boost dose was not able to fully counteract the increased risk of local failure.

Antineoplastic Combined Chemotherapy Protocols↗
PubMed2004

Chk2 phosphorylation of BRCA1 regulates DNA double-strand break repair.

The pathway determining malignant cellular transformation, which depends upon mutation of the BRCA1 tumor suppressor gene, is poorly defined. A growing body of evidence suggests that promotion of DNA double-strand break repair by homologous recombination (HR) may be the means by which BRCA1 maintains genomic stability, while a role of BRCA1 in error-prone nonhomologous recombination (NHR) processes has just begun to be elucidated. The BRCA1 protein becomes phosphorylated in response to DNA damage, but the effects of phosphorylation on recombinational repair are unknown. In this study, we tested the hypothesis that the BRCA1-mediated regulation of recombination requires the Chk2- and ATM-dependent phosphorylation sites. We studied Rad51-dependent HR and random chromosomal integration of linearized plasmid DNA, a subtype of NHR, which we demonstrate to be dependent on the Mre11-Rad50-Nbs1 complex. Prevention of Chk2-mediated phosphorylation via mutation of the serine 988 residue of BRCA1 disrupted both the BRCA1-dependent promotion of HR and the suppression of NHR. Similar results were obtained when endogenous Chk2 kinase activity was inhibited by expression of a dominant-negative Chk2 mutant. Surprisingly, the opposing regulation of HR and NHR did not require the ATM phosphorylation sites on serines 1423 and 1524. Together, these data suggest a functional link between recombination control and breast cancer predisposition in carriers of Chk2 and BRCA1 germ line mutations. We propose a dual regulatory role for BRCA1 in maintaining genome integrity, whereby BRCA1 phosphorylation status controls the selectivity of repair events dictated by HR and error-prone NHR.

Ataxia Telangiectasia Mutated Proteins↗
PubMed2003

Roles of BRCA1 and BRCA2 in homologous recombination, DNA replication fidelity and the cellular response to ionizing radiation.

Inheritance of one defective copy of either of the two breast cancer susceptibility genes, BRCA1 and BRCA2, predisposes individuals to breast and ovarian cancers. Current progress in determining the function of these genes suggests that they participate in a common pathway to facilitate orderly homologous recombination and thereby maintain genomic integrity. As a consequence of this defect in homologous recombination, tumors that arise in BRCA carriers are likely to be more sensitive to ionizing radiation. This review summarizes recent investigations about the nature of the defect in DNA repair, and highlights the unanswered questions about the tumor suppressor paradox of BRCA genes. The unsolved mystery is the other genetic changes that must occur to turn a BRCA-deficient cell from a nonviable cell into a tumor cell capable of endless growth.

Apoptosis↗
PubMed2003

Dose-volume analysis of radiotherapy for T1N0 invasive breast cancer treated by local excision and partial breast irradiation by low-dose-rate interstitial implant.

PURPOSE: To evaluate the toxicity of partial breast irradiation (RT) using escalating doses of low-dose-rate interstitial implant as the sole adjuvant local therapy for selected T1N0 breast cancer patients treated by wide local excision. The results of a European Organization for Research and Treatment of Cancer study have demonstrated a significant local control benefit using external beam RT to 65 Gy compared with 50 Gy. Thus, the tolerance of escalating doses of partial breast RT should be determined, because this approach may become a standard treatment for patients with early-stage breast cancer. METHODS AND MATERIALS: Between 1997 and 2001, 48 patients with T1N0M0 breast cancer were enrolled into an institutional review board-approved Phase I/II protocol using low-dose-rate brachytherapy implants after wide local excision and lymph node staging surgery. Brachytherapy was started 3-4 days after surgery at a dose rate of 50 cGy/h, using (192)Ir sources evenly spaced to cover 3 cm around the resection margins. Typically, 2-3 planes were used, with a median of 14 catheters (range 10-16). The total dose was escalated in three groups: 50 Gy (n = 19), 55 Gy (n = 16), and 60 Gy (n = 13). The implant volume was calculated and used to classify patients into quartiles: 76-127 cm(3) (n = 12), 128-164 cm(3) (n = 12), 165-204 cm(3) (n = 12), and >204 cm(3) (n = 12). Cosmesis, patient satisfaction, treatment-related complications, mammographic abnormalities, rebiopsies, and disease status were recorded at each scheduled patient visit. RESULTS: The median follow-up for all patients was 23.1 months (range 2-43). Very good to excellent cosmetic results were observed in 91.8% of patients. Ninety-two percent of patients were satisfied with their cosmetic outcome and said they would choose brachytherapy again over the standard course of external beam RT. Six perioperative complications occurred: two developed bleeding at the time of catheter removal, two had abscesses, one developed a hematoma, and one had a nonhealing sinus tract requiring surgical intervention. Significant fibrosis (moderate-to-severe scarring and thickening of the skin and breast) was noted in only 4 patients; 1 had received 55 Gy and 3 had received 60 Gy. Abnormal posttreatment mammograms were seen in 19 patients. Eight patients underwent rebiopsy for abnormalities found either by mammography or on physical examination; all proved to be fat necrosis or post-RT changes. The rebiopsy rates appeared to correlate with doses >/=55 Gy (6 [75%] of 8 compared with 29 [60%]of 48 overall) and implant volumes >/=128 cm(3) (7 [87.5%] of 8 compared with 36 [75%] of 48 overall). To date, no local, regional, or distant recurrences have been observed. CONCLUSION: Low-dose-rate implants up to 60 Gy were well-tolerated overall. With an implant dose of 60 Gy, the incidence of posttreatment fibrosis (25%) appeared to be increased. Only the long-term follow-up of this and other implant studies will allow an understanding of the total radiation dose necessary for tumor control and the volume of breast that requires treatment.

Adult↗
PubMed2003

Risk of lymphedema after regional nodal irradiation with breast conservation therapy.

PURPOSE: To evaluate the risk factors for lymphedema in patients receiving breast conservation therapy for early-stage breast cancer. METHODS AND MATERIALS: Between 1982 and 1995, 727 Stage I-II breast cancer patients were treated with breast conservation therapy at Massachusetts General Hospital. A retrospective analysis of the development of persistent arm edema was performed. Lymphedema was defined as a >2-cm difference in forearm circumference compared with the untreated side. The median follow-up was 72 months. Breast and regional nodal irradiation (BRNI) was administered in 32% of the cases and breast irradiation alone in 68%. RESULTS: Persistent arm lymphedema was documented in 21 patients. The 10-year actuarial incidence was 4.1%. The median time to edema was 39 months. The only significant risk factor for lymphedema was BRNI. The 10-year risk was 1.8% for breast irradiation alone vs. 8.9% for BRNI (p = 0.001). The extent of axillary dissection did not predict for lymphedema even within the subgroups of patients defined by the extent of irradiation. Most patients underwent Level I or II dissection. In this subgroup, the lymphedema risk at 10 years was 10.7% for BRNI vs. 1.0% for breast irradiation alone (p = 0.0003). CONCLUSION: Nodal irradiation was the only significant risk factor for arm lymphedema in patients receiving breast conservation therapy for early-stage breast cancer. Our data suggest that this risk is low with Level I/II dissection and breast irradiation. However, even after the addition of radiotherapy to the axilla and supraclavicular fossa, the development of lymphedema was only 1 in 10, lower than generally recognized.

Antineoplastic Combined Chemotherapy Protocols↗
PubMed2003

Education and training for radiation scientists: radiation research program and American Society of Therapeutic Radiology and Oncology Workshop, Bethesda, Maryland, May 12-14, 2003.

Current and potential shortfalls in the number of radiation scientists stand in sharp contrast to the emerging scientific opportunities and the need for new knowledge to address issues of cancer survivorship and radiological and nuclear terrorism. In response to these challenges, workshops organized by the Radiation Research Program (RRP), National Cancer Institute (NCI) (Radiat. Res. 157, 204-223, 2002; Radiat. Res. 159, 812-834, 2003), and National Institute of Allergy and Infectious Diseases (NIAID) (Nature, 421, 787, 2003) have engaged experts from a range of federal agencies, academia and industry. This workshop, Education and Training for Radiation Scientists, addressed the need to establish a sustainable pool of expertise and talent for a wide range of activities and careers related to radiation biology, oncology and epidemiology. Although fundamental radiation chemistry and physics are also critical to radiation sciences, this workshop did not address workforce needs in these areas. The recommendations include: (1) Establish a National Council of Radiation Sciences to develop a strategy for increasing the number of radiation scientists. The strategy includes NIH training grants, interagency cooperation, interinstitutional collaboration among universities, and active involvement of all stakeholders. (2) Create new and expanded training programs with sustained funding. These may take the form of regional Centers of Excellence for Radiation Sciences. (3) Continue and broaden educational efforts of the American Society for Therapeutic Radiology and Oncology (ASTRO), the American Association for Cancer Research (AACR), the Radiological Society of North America (RSNA), and the Radiation Research Society (RRS). (4) Foster education and training in the radiation sciences for the range of career opportunities including radiation oncology, radiation biology, radiation epidemiology, radiation safety, health/government policy, and industrial research. (5) Educate other scientists and the general public on the quantitative, basic, molecular, translational and applied aspects of radiation sciences.

Curriculum↗
PubMed2002

Radiotherapy and breast reconstruction: complications and cosmesis with TRAM versus tissue expander/implant.

PURPOSE: Radiotherapy (RT) has an important role in breast cancer treatment after modified radical mastectomy. Many of these patients also undergo breast reconstruction. We reviewed our institutions' experience to determine the outcome of patients treated with breast reconstruction and RT. METHODS AND MATERIALS: Between 1981 and 1999, 48 breast cancer patients underwent modified radical mastectomy, breast reconstruction, and ipsilateral breast RT during their treatment course. Reconstruction either preceded or followed RT. Autologous reconstruction with a transverse rectus abdominus myocutaneous (TRAM) flap was performed in 30 patients, and 18 underwent expander and implant (E/I) reconstruction. The primary endpoint was the quality of the reconstructed, irradiated breast, as measured by analyzing the actuarial incidence of complications. The cosmetic outcome was also assessed by multidisciplinary review of the follow-up visits. RESULTS: The median follow-up from reconstruction was 32 months. The actuarial 2-year complication rate was 53% for patients receiving E/I vs. 12% for those receiving TRAM reconstruction (p <0.01). No other patient or treatment-related factors had a significant impact on complications. The cosmetic outcome was also significantly better in the TRAM subgroup than in the E/I subgroup. CONCLUSION: The tolerance and cosmetic outcome of breast reconstruction for breast cancer patients in irradiated sites depends significantly on the type of reconstruction used.

Adult↗
PubMed2002

Dose-escalated total body irradiation and autologous stem cell transplantation for refractory hematologic malignancy.

PURPOSE: To evaluate the feasibility of dose escalation of total body irradiation (TBI) above the previously reported maximally tolerated dose, we have undertaken a Phase I-II trial of dose-escalated TBI with autologous peripheral blood stem cell transplantation (PBSCT) for chemotherapy-refractory lymphoma. METHODS AND MATERIALS: Nine lymphoma patients with primary refractory disease (PRD) or in resistant relapse (RR) received dose-escalated TBI and PBSCT. The three dose levels of fractionated TBI (200 cGy twice daily) were 1,600 cGy, 1,800 cGy, and 2,000 cGy. Lung blocks were used to reduce the TBI transmission dose by 50%, and the chest wall dose was supplemented to the prescribed dose using electrons. Shielding of the kidneys was performed to keep the maximal renal dose at 1,600 cGy. Three patients, two with non-Hodgkin's lymphoma (NHL) in RR and one with PRD Hodgkin's disease, received 1,600 cGy + PBSCT, three patients (two NHL in RR, one PRD) received 1,800 cGy + PBSCT, and three patients with NHL (two in RR, one PRD) received 2,000 cGy + PBSCT. RESULTS: Toxicities associated with this high-dose TBI regimen included reversible hepatic veno-occlusive disease in 1 patient, Grade 2 mucositis requiring narcotic analgesics in 8 patients, and neurologic toxicities consisting of a symmetrical sensory neuropathy (n = 4) and Lhermitte's syndrome (n = 1). Interstitial pneumonitis developed in 1 patient who received 1,800 cGy after receiving recombinant alpha-interferon (with exacerbation after rechallenge with interferon). Six (66%) patients achieved a response. Four (44%) patients achieved complete responses, three of which were of a duration greater than 1 year, and 2 (22%) patients achieved a partial response. One patient remains disease-free more than 5 years posttransplant. Corticosteroid-induced gastritis and postoperative infection resulted in the death of 1 patient in complete response, 429 days posttransplant. CONCLUSION: TBI in a dose range 1,600-2,000 cGy as preparative therapy for autologous PBSCT is feasible and has substantial activity in chemorefractory non-Hodgkin's and Hodgkin's lymphoma.

Adult↗
PubMed2002

Model-based predictions of BRCA1/2 mutation status in breast carcinoma patients treated at an academic medical center.

BACKGROUND: Women with an existing breast carcinoma diagnosis who are found to carry a BRCA1/2 mutation have a substantial risk of developing both a contralateral breast carcinoma and ovarian carcinoma. In a newly diagnosed breast carcinoma patient, this genetic information may influence the management of her disease. To assess the volume of patients who may need genetic services at the time of diagnosis, the authors determined the proportion of women with newly diagnosed breast carcinoma at the study institution who would be eligible for genetic testing. METHODS: Fifty consecutive women with new breast carcinoma who were attending a multidisciplinary clinic were interviewed. Detailed, three-generation pedigrees were collected for each patient by a genetic counselor. Three commonly used probability models were used to calculate each woman's predicted risk of harboring a germline BRCA1/2 mutation. RESULTS: Eleven of 50 patients (22% [95% confidence interval, 12-36%]) were calculated to have a > or = 10% probability of carrying a BRCA1/2 mutation by at least one mathematic model and should have been offered genetic counseling that included the discussion of genetic testing. There were considerable discrepancies between probability calculations among the three mathematic models. One of the 11 patients who was eligible for genetic testing pursued genetic counseling within 12 months of diagnosis. CONCLUSIONS: At a large academic medical center, a substantial proportion of unselected women attending a multidisciplinary clinic were found to have a > or = 10% risk of carrying a BRCA1/2 mutation. The actual number of patients eligible to receive BRCA1/2 genetic testing outweighs the number of patients seen for genetic counseling at the study institution. Finally, limited correlation was found between current predictive models.

Academic Medical Centers↗
PubMed2002

The molecular basis of radiosensitivity and chemosensitivity in the treatment of breast cancer.

The molecular basis of sensitivity to therapeutic radiation and chemotherapy is a complex product of cellular and tissue responses. Certain genetic factors can be highlighted as being of special importance in the response of breast cancers to treatment. The breast cancer susceptibility genes, BRCA1 and BRCA2, determine the phenotype of the tumor, with BRCA1- or BRCA2-deficient tumors showing marked sensitivity to ionizing radiation and drugs that produce double-strand breaks. However, the extent to which loss of BRCA1 or BRCA2 function occurs in sporadic cancer has not yet been determined. The ATM protein plays a significant role in determining the response to therapy, but how frequently the function of ATM is disrupted in breast cancer is debated. Although the p53 protein is a major determinant of the response to ionizing radiation and cytotoxic drugs, there is no consistency in how p53 affects the survival of cells, because an impairment of DNA repair is offset by reduced apoptosis. Growth factors that sustain the proliferation of breast cancer cells may impact the response to therapy by inhibiting apoptosis. Loss of cell-cycle checkpoint responses may result in increased sensitivity, particularly if the checkpoint controls the G2 transition. Overexpression of cyclin D, which shortens the duration of the G1 transition, is associated with mild radiation resistance, perhaps by inhibiting apoptosis. Overall, there is much more to be understood in the complex response of breast cancers to therapy, and many other proteins play important roles in the response to treatment. The focus of our investigation is on those genetic alterations in tumors that affect the response to therapy, which will ultimately allow strategies to achieve therapeutic gain.

Ataxia Telangiectasia↗