Beth M. Beadle

Structural bases of stability-function tradeoffs in enzymes

The structures of enzymes reflect two tendencies that appear opposed. On one hand, they fold into compact, stable structures; on the other hand, they bind a ligand and catalyze a reaction. To be stable, enzymes fold to maximize favorable interactions, forming a tightly packed hydrophobic core, exposing hydrophilic groups, and optimizing intramolecular hydrogen-bonding. To be functional, enzymes carve out an active site for ligand binding, exposing hydrophobic surface area, clustering like charges, and providing unfulfilled hydrogen bond donors and acceptors. Using AmpC beta-lactamase, an enzyme that is well-characterized structurally and mechanistically, the relationship between enzyme stability and function was investigated by substituting key active-site residues and measuring the changes in stability and activity. Substitutions of catalytic residues Ser64, Lys67, Tyr150, Asn152, and Lys315 decrease the activity of the enzyme by 10(3)-10(5)-fold compared to wild-type. Concomitantly, many of these substitutions increase the stability of the enzyme significantly, by up to 4.7kcal/mol. To determine the structural origins of stabilization, the crystal structures of four mutant enzymes were determined to between 1.90A and 1.50A resolution. These structures revealed several mechanisms by which stability was increased, including mimicry of the substrate by the substituted residue (S64D), relief of steric strain (S64G), relief of electrostatic strain (K67Q), and improved polar complementarity (N152H). These results suggest that the preorganization of functionality characteristic of active sites has come at a considerable cost to enzyme stability. In proteins of unknown function, the presence of such destabilized regions may indicate the presence of a binding site.

TP53 Disruptive Mutations Lead to Head and Neck Cancer Treatment Failure through Inhibition of Radiation-Induced Senescence

Purpose: Mortality of patients with head and neck squamous cell carcinoma (HNSCC) is primarily driven by tumor cell radioresistance leading to locoregional recurrence (LRR). In this study, we use a classification of TP53 mutation (disruptive vs. nondisruptive) and examine impact on clinical outcomes and radiation sensitivity. Experimental Design: Seventy-four patients with HNSCC treated with surgery and postoperative radiation and 38 HNSCC cell lines were assembled; for each, TP53 was sequenced and the in vitro radioresistance measured using clonogenic assays. p53 protein expression was inhibited using short hairpin RNA (shRNA) and overexpressed using a retrovirus. Radiation-induced apoptosis, mitotic cell death, senescence, and reactive oxygen species (ROS) assays were carried out. The effect of the drug metformin on overcoming mutant p53-associated radiation resistance was examined in vitro as well as in vivo, using an orthotopic xenograft model. Results: Mutant TP53 alone was not predictive of LRR; however, disruptive TP53 mutation strongly predicted LRR (P = 0.03). Cell lines with disruptive mutations were significantly more radioresistant (P < 0.05). Expression of disruptive TP53 mutations significantly decreased radiation-induced senescence, as measured by SA-β-gal staining, p21 expression, and release of ROS. The mitochondrial agent metformin potentiated the effects of radiation in the presence of a disruptive TP53 mutation partially via senescence. Examination of our patient cohort showed that LRR was decreased in patients taking metformin. Conclusions: Disruptive TP53 mutations in HNSCC tumors predicts for LRR, because of increased radioresistance via the inhibition of senescence. Metformin can serve as a radiosensitizer for HNSCC with disruptive TP53, presaging the possibility of personalizing HNSCC treatment. Clin Cancer Res; 18(1); 290–300. ©2011 AACR.

The impact of pregnancy on breast cancer outcomes in women ≤35 years†

Some evidence suggests that women with pregnancy‐associated breast cancers (PABC) have a worse outcome compared with historical controls. However, young age is a worse prognostic factor independently, and women with PABC tend to be young. The purpose of the current study was to compare locoregional recurrence (LRR), distant metastases (DM), and overall survival (OS) in young patients with PABC and non‐PABC.

Eat and Exercise During Radiotherapy or Chemoradiotherapy for Pharyngeal Cancers: Use It or Lose It

IMPORTANCE Data support proactive swallowing therapy during radiotherapy (RT) or chemoradiotherapy (CRT) for pharyngeal cancers. The benefits of adherence to a regimen of swallowing exercises and maintaining oral intake throughout treatment are reported, but independent effects are unclear. OBJECTIVE To evaluate the independent effects of maintaining oral intake throughout radiotherapy and adherence to preventive swallowing exercise. DESIGN Retrospective observational study. SETTING The University of Texas MD Anderson Cancer Center, Houston. PATIENTS The study included 497 patients treated with definitive RT or CRT for pharyngeal cancer (458 oropharynx, 39 hypopharynx) between 2002 and 2008. MAIN OUTCOMES AND MEASURES Swallowing-related end points were final diet after RT or CRT and duration of gastrostomy dependence. Primary independent variables included oral intake status at the end of RT or CRT (no oral intake, partial oral intake, or full oral intake) and adherence to a swallowing exercise regimen. Multiple linear regression and ordered logistic regression models were analyzed. RESULTS At the conclusion of RT or CRT, 131 patients (26%) had no oral intake and 74% maintained oral intake (167 partial [34%], 199 full [40%]). Fifty-eight percent (286 of 497) reported adherence to swallowing exercises. Maintenance of oral intake during RT or CRT and swallowing exercise adherence were independently associated with better long-term diet after RT or CRT (P = .045 and P < .001, respectively) and shorter duration of gastrostomy dependence (P < .001 and P = .007, respectively) in models adjusted for tumor and treatment burden. CONCLUSIONS AND RELEVANCE The data indicate independent, positive associations of maintenance of oral intake throughout RT or CRT and swallowing exercise adherence with long-term swallowing outcomes. Patients who either eat or exercise fare better than those who do neither. Patients who both eat and exercise have the highest rate of return to a regular diet and shortest duration of gastrostomy dependence.

CERVIX REGRESSION AND MOTION DURING THE COURSE OF EXTERNAL BEAM CHEMORADIATION FOR CERVICAL CANCER

PURPOSE To evaluate the magnitude of cervix regression and motion during external beam chemoradiation for cervical cancer. METHODS AND MATERIALS Sixteen patients with cervical cancer underwent computed tomography scanning before, weekly during, and after conventional chemoradiation. Cervix volumes were calculated to determine the extent of cervix regression. Changes in the center of mass and perimeter of the cervix between scans were used to determine the magnitude of cervix motion. Maximum cervix position changes were calculated for each patient, and mean maximum changes were calculated for the group. RESULTS Mean cervical volumes before and after 45 Gy of external beam irradiation were 97.0 and 31.9 cc, respectively; mean volume reduction was 62.3%. Mean maximum changes in the center of mass of the cervix were 2.1, 1.6, and 0.82 cm in the superior-inferior, anterior-posterior, and right-left lateral dimensions, respectively. Mean maximum changes in the perimeter of the cervix were 2.3 and 1.3 cm in the superior and inferior, 1.7 and 1.8 cm in the anterior and posterior, and 0.76 and 0.94 cm in the right and left lateral directions, respectively. CONCLUSIONS Cervix regression and internal organ motion contribute to marked interfraction variations in the intrapelvic position of the cervical target in patients receiving chemoradiation for cervical cancer. Failure to take these variations into account during the application of highly conformal external beam radiation techniques poses a theoretical risk of underdosing the target or overdosing adjacent critical structures.

Structural Milestones in the Reaction Pathway of an Amide Hydrolase: Substrate, Acyl, and Product Complexes of Cephalothin with AmpC β-Lactamase

Beta-lactamases hydrolyze beta-lactam antibiotics and are the leading cause of bacterial resistance to these drugs. Although beta-lactamases have been extensively studied, structures of the substrate-enzyme and product-enzyme complexes have proven elusive. Here, the structure of a mutant AmpC in complex with the beta-lactam cephalothin in its substrate and product forms was determined by X-ray crystallography to 1.53 A resolution. The acyl-enzyme intermediate between AmpC and cephalothin was determined to 2.06 A resolution. The ligand undergoes a dramatic conformational change as the reaction progresses, with the characteristic six-membered dihydrothiazine ring of cephalothin rotating by 109 degrees. These structures correspond to all three intermediates along the reaction path and provide insight into substrate recognition, catalysis, and product expulsion.

Improved survival using intensity-modulated radiation therapy in head and neck cancers: a SEER-Medicare analysis.

Intensity‐modulated radiation therapy (IMRT) is a technologically advanced, and more expensive, method of delivering radiation therapy with a goal of minimizing toxicity. It has been widely adopted for head and neck cancers; however, its comparative impact on cancer control and survival remains unknown. The goal of this analysis was to compare the cause‐specific survival (CSS) for patients with head and neck cancers treated with IMRT versus non‐IMRT from 1999 to 2007.

Patterns of Disease Recurrence Following Treatment of Oropharyngeal Cancer With Intensity Modulated Radiation Therapy

PURPOSE To report mature results of a large cohort of patients diagnosed with squamous cell carcinoma of the oropharynx who were treated with intensity modulated radiation therapy (IMRT). METHODS AND MATERIALS The database of patients irradiated at The University of Texas, M.D. Anderson Cancer Center was searched for patients diagnosed with oropharyngeal cancer and treated with IMRT between 2000 and 2007. A retrospective review of outcome data was performed. RESULTS The cohort consisted of 776 patients. One hundred fifty-nine patients (21%) were current smokers, 279 (36%) former smokers, and 337 (43%) never smokers. T and N categories and American Joint Committee on Cancer group stages were distributed as follows: T1/x, 288 (37%); T2, 288 (37%); T3, 113 (15%); T4, 87 (11%); N0, 88(12%); N1/x, 140 (18%); N2a, 101 (13%); N2b, 269 (35%); N2c, 122 (16%); and N3, 56 (7%); stage I, 18(2%); stage II, 40(5%); stage III, 150(19%); and stage IV, 568(74%). Seventy-one patients (10%) presented with nodes in level IV. Median follow-up was 54 months. The 5-year overall survival, locoregional control, and overall recurrence-free survival rates were 84%, 90%, and 82%, respectively. Primary site recurrence developed in 7% of patients, and neck recurrence with primary site control in 3%. We could only identify 12 patients (2%) who had locoregional recurrence outside the high-dose target volumes. Poorer survival rates were observed in current smokers, patients with larger primary (T) tumors and lower neck disease. CONCLUSIONS Patients with oropharyngeal cancer treated with IMRT have excellent disease control. Locoregional recurrence was uncommon, and most often occurred in the high dose volumes. Parotid sparing was accomplished in nearly all patients without compromising tumor coverage.

Ten-year recurrence rates in young women with breast cancer by locoregional treatment approach.

PURPOSE Young women with breast cancer have higher locoregional recurrence (LRR) rates than older patients. The goal of this study is to determine the impact of locoregional treatment strategy, breast-conserving therapy (BCT), mastectomy alone (M), or mastectomy with adjuvant radiation (MXRT), on LRR for patients 35 years or younger. METHODS AND MATERIALS Data for 668 breast cancers in 652 young patients with breast cancer were retrospectively reviewed; 197 patients were treated with BCT, 237 with M, and 234 with MXRT. RESULTS Median follow-up for all living patients was 114 months. In the entire cohort, 10-year actuarial LRR rates varied by locoregional treatment: 19.8% for BCT, 24.1% for M, and 15.1% for MXRT (p = 0.05). In patients with Stage II disease, 10-year actuarial LRR rates by locoregional treatment strategy were 17.7% for BCT, 22.8% for M, and 5.7% for MXRT (p = 0.02). On multivariate analysis, M (hazard ratio, 4.45) and Grade III disease (hazard ratio, 2.24) predicted for increased LRR. In patients with Stage I disease, there was no difference in LRR rates based on locoregional treatment (18.0% for BCT, 19.8% for M; p = 0.56), but chemotherapy use had a statistically significant LRR benefit (13.5% for chemotherapy, 27.9% for none; p = 0.04). CONCLUSIONS Young women have high rates of LRR after breast cancer treatment. For patients with Stage II disease, the best locoregional control rates were achieved with MXRT. For patients with Stage I disease, similar outcomes were achieved with BCT and mastectomy; however, chemotherapy provided a significant benefit to either approach.

Structural Basis for Imipenem Inhibition of Class C β-Lactamases

ABSTRACT To determine how imipenem inhibits the class C β-lactamase AmpC, the X-ray crystal structure of the acyl-enzyme complex was determined to a resolution of 1.80 Å. In the complex, the lactam carbonyl oxygen of imipenem has flipped by approximately 180° compared to its expected position; the electrophilic acyl center is thus displaced from the point of hydrolytic attack. This conformation resembles that of imipenem bound to the class A enzyme TEM-1 but is different from that of moxalactam bound to AmpC.