Orthopedic Resident and Patient Perception of Electronic Medical Record Use During the Clinic Visit

Background The transition from paper charts to electronic medical records (EMRs) has resulted in greater efficiency and reduced medical errors. This study aimed to examine the perception of patients and orthopedic residents regarding computer use during the clinic visit. Methodology This study utilized a cross-sectional cluster design. Orthopedic resident physicians were given a one-time general pre-visit survey. Additional surveys were given to patients and resident physicians post-visit. Surveys included questions that assessed satisfaction and the perceived impact of computer usage on doctor-patient interactions. Logistic generalized estimating equations were run to determine if there was an association between patient response and clinician assessment, adjusting for repeated measures within clinicians. Results A total of 80 patients and 15 residents completed the surveys. Results from the physician pre-visit survey showed that more residents perceived the computer as having a "negative" (47%) than "positive" (26%) effect on their relationship with patients. According to the post-visit analysis, patients perceived the residents' use of the EMR as having an overall positive effect on their ability to establish a personal connection and having a positive effect on their ability to give them attention. Conclusions Overall, there was little correlation between patient and resident perception of the computer's effect on their relationship. Patients generally perceived the computer as having a positive effect on their interaction with the residents even when residents had a negative perception of the computer's effect on their interaction.

Targeting Skeletal Muscle Dysfunction With L-Carnitine for the Treatment of Patients With Chronic Obstructive Pulmonary Disease

Chronic obstructive pulmonary disease (COPD) is a major medical problem and the world's third leading cause of death. COPD is a chronic disease with heterogeneous clinical symptoms, disease progression, and treatment responses. Besides pulmonary symptomatology, the common systemic clinical manifestations are cachexia, muscle weakness, and widespread comorbidities such as cardiovascular diseases, diabetes, osteoporosis, and infections. The adverse effects of pharmaceutical therapies contribute to the difficulty of health risk assessment and management of COPD patients. This review shows how skeletal muscle dysfunction and metabolic abnormalities contribute significantly to COPD patients' symptoms, functional activities, quality of life, and overall disease outcomes. Based on the clinical evidence of L-carnitine and derivatives as metabolic and muscle bioenergetic enhancers, we propose broader research and implementation of this nutraceutical agent as an effective, inexpensive, and safe adjuvant therapeutic for the long-term management of COPD patients. Moreover, we believe the management of COPD as a chronic disease should be shifted from symptomatic reactive pharmaceutical intervention to more constructive and non-toxic approaches using a single or combination of natural and nutritional agents with potential muscle metabolic enhancing and immunomodulating activities to achieve a better overall outcome for the patients in terms of morbidity, mortality, and medical cost-reduction.

Successful Phytotherapy for Acute Refractory Radiation Cystitis in a Cervical Cancer Patient: A Case Report

BACKGROUND/AIM

Radiation cystitis is a frequent complication that can occur after therapeutic irradiation of pelvic cancers. The current treatment for this condition is complex and often ineffective.

CASE REPORT

We present a clinical case of a 54-year-old patient diagnosed with small cell cervical cancer FIGO stage IIIC who developed grade 2-3 radiation cystitis following post-operational chemoradiation therapy. The patient exhibited increased urinary urgency and frequency, dysuria, and low abdominal pain, which failed to respond to acupuncture and corticosteroid treatments. A course of Ich Nieu Khang phytotherapy tablets, resulted in significant improvement of symptoms within 24 hours of initiation. The symptoms resolved completely within 10 days, and ultrasonography documented a marked decrease in bladder wall thickening and improved bladder evacuation function. The phytotherapy was well-tolerated, and no side-effects were observed during the 60-day treatment period.

CONCLUSION

These findings suggest that phytotherapy may be viable for managing radiation cystitis. However, further controlled clinical trials are needed to confirm the efficacy of Ich Nieu Khang and promote its broader clinical applications.

Nowarta110 Topical Versus Placebo for the Treatment of Plantar Warts: A Phase I/II Randomized Controlled Clinical Trial

BACKGROUND/AIM

Plantar warts are cutaneous lesions on the plantar aspect of the foot caused by the infection of keratinocytes with the human papillomavirus (HPV). The severity and magnitude of warts can vary, but they cause pain and discomfort for all age groups. The treatment for plantar warts remains a continuing challenge. The purpose of this research was to compare the efficacy and safety of naturally derived Nowarta110 topical formula versus a matching placebo in treating plantar warts.

PATIENTS AND METHODS

The study is a randomized, double-blind, parallel assignment control interventional phase I/II clinical trial. This study included 54 patients with plantar warts. Patients were randomized to two groups: the placebo group, which included 26 patients treated with a matching placebo and the Nowarta110 group, which included 28 patients who received topical Nowarta110. The diagnosis of plantar warts was made by clinical examination. The treatment's efficacy and safety were assessed every week and after 6 weeks from the initiation of the intervention.

RESULTS

In the Nowata110 group, 18 patients (64.3%) were completely cleared of their warts, and 10 patients (35.7%) partially responded to the therapy with a 20% to 80% decrease in warts dimensions. In the placebo group, only 2 patients (7.7%) were completely cleared of their warts, and 3 patients (11.5%) partially responded to the intervention with a 10% to 35% decrease in warts dimensions. The difference was highly significant between the two groups. There was 1 event with minor pain as a side effect in the Nowarta110 group and 9 events of non-serious local side effects in the placebo group, which included 2 patients who dropped out.

CONCLUSION

Topical Nowarta110 is a safe, well-tolerated, and highly effective therapeutic modality in treating refractory and recurrent plantar warts. The breakthrough findings of the study encourage further extensive clinical trials to fully explore the prospect of Nowarta110 in managing all types of warts and HPV-related diseases.

Hyperlipidemia in tendon injury: chronicles of low-density lipoproteins

Hyperlipidemia impacts millions of people globally and has been the major risk factor for developing atherosclerosis and cardiovascular disease. Interestingly, hyperlipidemic subjects exhibit increased incidence of rotator cuff tendon injury (RCTI) and disorganization of tendon matrix. Low-density lipoproteins (LDL) and its oxidized form (ox-LDL) play a crucial role in hyperlipidemia-driven pro-inflammatory responses in multiple tissues including the tendon. The signaling of oxLDL upregulates the inflammatory cytokines, chemokines, adhesion molecules, and the activation of monocytes/macrophages/resident tendon cells and matrix metalloproteinases impairing the tendon homeostasis resulting in the alteration of extracellular matrix. In addition, the hyperlipidemia-driven immune response and subsequent oxidative stress promote degenerative responses in the tendon tissue. However, the pathological mechanisms underlying the occurrence of RCTI in hyperlipidemia and the effect of ox-LDL in tendon matrix are currently unknown. The present review focuses on the implications and perspectives of LDL/oxLDL on the increased incidence of RCTI.

Preoperative Determination of the Size of the Semitendinosus and Gracilis Tendon by Multidetector Row CT Scanner for Anterior Cruciate Ligament Reconstruction

Accurately measuring the length and diameter of the hamstring tendon autograft preoperatively is important for planning anterior cruciate ligament (ACL) reconstructive surgery. The purpose of this study was to assess the reliability of three-dimensional computed tomography (3D CT) scanning technique to produce the actual measurement of the gracilis and semitendinosus (GT and ST, respectively) tendon grafts' length and diameter for surgery. Ninety patients were scheduled for ACL reconstruction with hamstring autograft. Before the surgery, patients were examined under the multidetector row CT scanner and the ST and GT tendons were qualitatively measured by a volume-rendering technique. The length of ST and GT was measured with 3D CT compared with the length of the harvested ST and GT. The cross-sectional area (CSA) of ST and GT measured with 3D CT compared with the ST and GT graft diameter. Tendon size measured preoperatively and during surgery were statistically compared and correlated. The GT tendons length and cross-sectional area measured during surgery was both shorter and smaller compared with the ST tendon. GT and ST tendon length were correlated to patients' body index such as the height and weight (p < 0.05). However, the correlation levels were low to medium (r = 0.23-0.49). There was strong correlation between the lengths of GT (r = 0.76; p < 0.001) and ST (r = 0.87; p < 0.001) measured with the 3D CT and tendon length at surgery. There was a moderate correlation between graft diameter measured at surgery and 3D CT cross-sectional area (r = 0.31; p < 0.05). A multidetector row CT scanner can determine the ST and GT tendons' length and diameter. These measurements can be used for preoperative planning to help determine the surgical method and counsel patients on appropriate graft choices prior to surgery.

Investigating Particulate Production in The Operating Suite Following the Use of Waterless Alcohol Based Dry Scrub Versus Traditional Hand Washing and Drying with Commonly Used Surgical Towels: An Experimental Study

BACKGROUND

An often-overlooked item that could cause contamination in the operating suite are the towels used for hand drying following surgical scrub. The purpose of this current study was to determine if there was a difference in the particulate count from different hand drying methods following surgical hand preparation.

METHODS

Three simulated hand drying groups were established: disposable sterilized surgical towels, reusable sterilized surgical towels, and a waterless alcohol-based dry rub. Particle size measurements of 0.3 µm, 5.0 µm, and 10.0 µm were collected at time zero and repeated every minute for 5 minutes for a total of 10 trials each.

RESULTS

Both the reusable and disposable towels produced significantly more particle matter in all size groups compared to the alcohol scrub control group. A comparison analysis and ANOVA testing demonstrated that alcohol dry scrub produced significantly fewer particles compared to both the disposable blue towels (P<0.01) and the reusable green towels (P<0.01). Disposable towels produced significantly more particles in the 0.3 µm count compared to reusable towels (P<0.05).

CONCLUSIONS

An alcohol-based dry rub without using a towel yielded the lowest amount of particulate formation in this experimental model, while reusable surgical towels produced the highest number of particles.

LEVEL OF EVIDENCE

Level II Experimental Study.

Food and Drug Administration's Position on Commonly Injected Biologic Materials in Orthopaedic Surgery

The use of biologically based therapies is becoming a popular less-invasive therapy for relieving pain and promoting tissue regeneration. The most commonly used biologics are autologous adipose-derived products, bone marrow aspirations, and platelet-rich plasma (PRP). Birth tissue is a common allogenic source of biologics, including umbilical cord, placental membranes, and amniotic fluid. Injected biologics, depending on the indication and how they are processed, formulated, delivered, and promoted, can be subject to different regulatory pathways. The aim of this review is to provide an overview of these products and procedures and educate the musculoskeletal community about the relevant current Food and Drug Administration (FDA) regulations.

"Smart Exosomes": A Smart Approach for Tendon Regeneration

Shoulder tendon injuries are the common musculoskeletal disorder resulting in significant pain and disability. These injuries are characterized by chronic inflammation and tissue degeneration. Tendon pathology exhibits poor innate healing ability, enhanced inflammation, disorganized collagen fibers, calcification, and scar tissue formation affecting the normal healing process. Extracellular vesicle, especially exosomes, treatment has been emerging as a potential regenerative strategy improving the outcomes and biomechanical properties, accelerating tenocyte proliferation and migration, reducing inflammation, and facilitating the healing at tendon-bone interface. In this article, we critically reviewed the potential role of exosomes in tendon regeneration and their applications to accelerate the healing response following injury. In addition, the article provides novel insights on the concept of "Smart Exosomes" by programming/manipulating the secretome contents and functions of exosomes in the management of shoulder tendon injury.

Governmental Regulations and Increasing Food and Drug Administration Oversight of Regenerative Medicine Products: What's New in 2020?

The United States Food and Drug Administration (FDA) is responsible for protecting and promoting public health through rules and regulations. Over the past few years, the field of regenerative medicine and cell therapy have garnered significant interest, and this evolving new biology is changing fast and challenging regulatory bodies. The FDA has published a series of guidance documents outlining steps to protect consumers against potentially dangerous and unproven treatments. The agency has offered a grace period for "stem cell clinics" until November 2020 to come into compliance by obtaining Investigational New Drug applications and working to secure premarket approval of their products. With the documentation of hundreds of "stem cell clinics," the FDA needs to enforce the adherence to their outlined standards to protect patients. The aim of this review was to provide an overview of these FDA regulations and some current issues within the industry. The purpose is to educate and inform the musculoskeletal community about the current government regulations of this new expanding biology. LEVEL OF EVIDENCE: Level V, expert opinion.

Zinc sulfide nanoparticles improve skin regeneration

Wound healing has been intensely studied to expedite recovery times and reduce scarring. However, current technologies fail to achieve regenerative capabilities, leaving wounds with scarring and lack of skin accessories. The recent emergence of nanotechnology has provided a new clinical modality of zinc nanoparticles in wound care. This present study investigated Zinc Sulfide nanoparticles (ZnS-NP) on wound healing in vitro with 2D and 3D models and in vivo with rat full-thickness wound model. ZnS-NP inhibited fetal bovine serum-stimulated rat skin fibroblast cell proliferation, altered cytoskeletal organization, and reduced collagen synthesis as well as contractile activity. ZnS-NP regulated redox homeostatsis and promoted fibroblast viability in 3D hypoxia conditions. In the rat full-thickness wound model, ZnS-NP reduced wound contraction, enhanced re-epithelization, and promoted skin appendage formation. The biological activities of ZnS-NPs determined in our current study may suggest promising practical applications for topical or systemic treatment for wound repair.

Effect of temperature on pharmacokinetics of enrofloxacin in mud crab, Scylla serrata (Forsskål), following oral administration

The study was conducted to evaluate the pharmacokinetics of enrofloxacin following a single oral gavage (10 mg kg(-1)) in mud crab, Scylla serrata, at water temperatures of 19 and 26 degrees C. Enrofloxacin concentration in haemolymph was determined using high-performance liquid chromatography (HPLC). A multiple and repeated haemolymph sampling from the articular cavity of crab periopods was developed. The haemolymph of an individual crab was successfully sampled up to 11 times from the articular cavity. The profile of haemolymph enrofloxacin concentration of an individual crab versus time was thus achieved. The mean haemolymph enrofloxacin concentration versus time was described by a two-compartment model with first-order absorption at two water temperatures. The peak concentrations of haemolymph enrofloxacin at 19 and 26 degrees C were 7.26 and 11.03 mug mL(-1), at 6 and 2 h, respectively. The absorption and distribution half-life time ( and t(1/2alpha)) at 19 degrees C were 3.7 and 4.5 h, respectively, which were markedly larger than the corresponding values (1.1 and 1.5 h) at 26 degrees C; the elimination half-life time (t(1/2beta)) was 79.1 and 56.5 h at 19 and 26 degrees C, respectively. The area under curve (AUC), total body clearance (Cl) and mean residence time (MRT(0-infinity)) at 19 degrees C were 636.0 mg L(-1) h, 0.016 L h(-1) kg(-1) and 102.5 h, respectively; the corresponding values at 26 degrees C were 583.4 mg L(-1) h, 0.018 L h(-1) kg(-1)and 63.7 h. These results indicate that enrofloxacin is absorbed and eliminated more rapidly in mud crab at 26 degrees C than at 19 degrees C.

Density functional theory investigation of the reactions of isodihalomethanes (CH(2)X-X where X = Cl, Br, or I) with ethylene: substituent effects on the carbenoid behavior of the CH(2)X-X species

We investigated the chemical reactions of isodihalomethane (CH(2)X-X) and CH(2)X radical species (where X = Cl, Br, or I) with ethylene and the isomerization reactions of CH(2)X-X using density functional theory calculations. The CH(2)X-X species readily reacts with ethylene to give the cyclopropane product and an X(2) product via a one-step reaction with barrier heights of approximately 2.9 kcal/mol for CH(2)I-I, 6.8 kcal/mol for CH(2)Br-Br, and 8.9 kcal/mol for CH(2)Cl-Cl. The CH(2)X reactions with ethylene proceed via a two-step reaction mechanism to give a cyclopropane product and X atom product with much larger barriers to reaction. This suggests that photocyclopropanation reactions using ultraviolet excitation of dihalomethanes most likely occurs via the isodihalomethane species and not the CH(2)X species. The isomerization reactions of CH(2)X-X had barrier heights of approximately 14.4 kcal/mol for CH(2)I-I, 11.8 kcal/mol for CH(2)Br-Br, and 9.1 kcal/mol for CH(2)Cl-Cl. We compare our results for the CH(2)X-X carbenoids to results from previous calculations of the Simmons-Smith-type carbenoids (XCH(2)ZnX) and Li-type carbenoids (LiCH(2)X) and discuss their differences and similarities as methylene transfer agents.

Effect of recombinant bactericidal/permeability-increasing protein on endotoxin translocation and lipopolysaccharide-binding protein/CD14 expression in rats after thermal injury

OBJECTIVE

To investigate the potential mechanisms underlying the in vivo effect of recombinant bactericidal/permeability-increasing protein (rBPI21) on endogenous bacteria or endotoxin translocation and lipopolysaccharide-binding protein/CD14 expression secondary to thermal injury.

DESIGN

Prospective, randomized, controlled animal study.

SETTING

College hospital animal research laboratory.

SUBJECTS

Thirty-six male Wistar rats weighing 250-300 g.

INTERVENTIONS

The rats were anesthetized, and a 35% total body surface area full-thickness burn was created. Animals were randomized to receive treatment with either rBPI21 or the control protein (albumin). rBPI21 (2 mg/kg body wt, BPI group) or a protein control preparation (burn group) in the same dose was administered in an intravenous bolus at 30 mins and 4 hrs after thermal injury. All animals were killed at 12 and 24 hrs postburn (six to ten rats for each interval). In addition, eight rats were taken as normal controls.

MEASUREMENT AND MAIN RESULTS

Our data showed that treatment with rBPI21 was effective in preventing endotoxin translocation secondary to severe burns. Meanwhile, tissue lipopolysaccharide-binding protein, CD14, and tumor necrosis factor-alpha mRNA expression in various organs were inhibited markedly by rBPI21 secondary to acute insults (p <.05-.01). Furthermore, significant reduction in serum aminoleucine transferase concentrations and elevation in intestinal diamine oxidase activities in the rBPI21-treated group were found compared with controls (p <.05-.01).

CONCLUSIONS

These findings indicate that endotoxin accumulated in local sites after thermal injury can markedly up-regulate lipopolysaccharide-binding protein/CD14 and tumor necrosis factor-alpha mRNA expression in various organs. Meanwhile, up-regulation of lipopolysaccharide-binding protein/CD14 expression would be the major molecular mechanism of increasing sensitivity to endogenous endotoxin response after burns. Early treatment with rBPI21may be effective in attenuating multiple organ damage resulting from gut-origin endotoxin translocation. This might be associated with the down-regulation effects of tissue lipopolysaccharide-binding protein and CD14 gene expression by the use of rBPI21.

Isodiiodomethane is the methylene transfer agent in cyclopropanation reactions with olefins using ultraviolet photolysis of diiodomethane in solutions: a density functional theory investigation of the reactions of isodiiodomethane, iodomethyl radical, and iodomethyl cation with ethylene

We examine the chemical reactions of the isodiiodomethane (CH2I-I), .CH2I and CH2I(+) species with ethylene using density functional theory computations. The CH2I-I species readily reacts with ethylene to give the cyclopropane product and an I2 product via a one-step reaction with a barrier height of approximately 2.9 kcal/mol. However, the.CH2I and CH2I(+) species have much more difficult pathways (with larger potential barriers) to react with ethylene via a two-step reaction mechanism. Comparison of experimental results to our present calculation results indicates that the CH2I-I photoproduct species is most likely the methylene transfer agent for the cyclopropanation reaction of olefins via ultraviolet photoexcitation of diiodomethane.

Repair of large insertion/deletion heterologies in human nuclear extracts is directed by a 5' single-strand break and is independent of the mismatch repair system

The repair of 12-, 27-, 62-, and 216-nucleotide unpaired insertion/deletion heterologies has been demonstrated in nuclear extracts of human cells. When present in covalently closed circular heteroduplexes or heteroduplexes containing a single-strand break 3' to the heterology, such structures are subject to a low level repair reaction that occurs with little strand bias. However, the presence of a single-strand break 5' to the insertion/deletion heterology greatly increases the efficiency of rectification and directs repair to the incised DNA strand. Because nick direction of repair is independent of the strand in which a particular heterology is placed, the observed strand bias is not due to asymmetry imposed on the heteroduplex by the extrahelical DNA segment. Strand-specific repair by this system requires ATP and the four dNTPs and is inhibited by aphidicolin. Repair is independent of the mismatch repair proteins MSH2, MSH6, MLH1, and PMS2 and occurs by a mechanism that is distinct from that of the conventional mismatch repair system. Large heterology repair in nuclear extracts of human cells is also independent of the XPF gene product, and extracts of Chinese hamster ovary cells deficient in the ERCC1 and ERCC4 gene products also support the reaction.

A requirement for protein kinase C inhibition for calcium-triggered apoptosis in acute lymphoblastic leukemia cells

We have evaluated the cytotoxicities of the combinations of calcium mobilizers and PKC inhibitors against human acute lymphoblastic leukemia (ALL) cells. Here we report that calcium mobilizers alone or PKC inhibitors alone do not induce apoptosis in human ALL cells. However, the combinations of calcium mobilizers with potent inhibitors of PKC cause significant apoptosis in ALL cells. Our results provide experimental evidence that PKC blocks Ca2+-triggered apoptosis in human ALL cells. Thus, PKC inhibitors can be used to enhance the antileukemic activity of chemical or biological agents that trigger an apoptotic calcium signal in ALL cells. The exquisite sensitivity of ALL cells to calcium-dependent apoptosis in the presence of PKC inhibitors could provide the basis for new treatment programs against ALL.

Calphostin C triggers calcium-dependent apoptosis in human acute lymphoblastic leukemia cells

Recent studies have demonstrated that the naturally occurring perylenequinone antibiotic calphostin C is a potent inhibitor of protein kinase C and can induce apoptosis in some tumor cell lines by an as yet unknown mechanism. Here we demonstrate that calphostin C induces dose-dependent apoptosis in DT40 chicken lymphoma B-cells, and targeted disruption of lyn, syk, btk, PLCgamma2, or IP3R genes does not prevent or attenuate its cytotoxicity. In our study, calphostin C also induced rapid apoptosis in human acute lymphoblastic leukemia (ALL) cell lines ALL-1 (BCR-ABL+ pre-pre-B ALL), RS4;11 (MLL-AF4+ pro-B ALL), NALM-6 (pre-B ALL), DAUDI (Burkitt's/B-cell ALL), MOLT-3 (T-ALL), and JURKAT (T-ALL), whereas other potent PKC inhibitors did not. In biochemical studies, calphostin C was discovered to induce rapid calcium mobilization from intracellular stores of ALL cell lines, and its cytotoxicity against ALL cell lines was well correlated with the magnitude of this calcium signal. Calphostin C-induced apoptosis was markedly suppressed by BAPTA/AM, a cell-permeable Ca2+ chelator as well as NiCl2, an inhibitor of Ca2+/Mg2+-dependent endonucleases. Inhibition of the Ca2+/calmodulin-dependent phosphatase calcineurin with perfluoreperazine dimadeate (a calmodulin antagonist) or cyclosporin A (a specific inhibitor of calcineurin) also reduced the magnitude of calphostin C-induced apoptosis in ALL cell lines. Calphostin C was capable of inducing calcium mobilization and apoptosis in freshly obtained primary leukemic cells from children with ALL. Taken together, our results provide unprecedented evidence that calphostin C triggers a Ca2+-dependent apoptotic signal in human ALL cells.

Altered intermediate filament expression in human neuroblastoma cells transformed by a growth-promoting agent derived from schizophrenic CSF

1. Transformed (TR) cell lines showed faster doubling times and higher cell densities at confluence, as well as altered morphology, changing from flat epitheloid to smaller round or bipolar shapes. Since such morphological changes are suggestive of alterations in intermediate filaments, we have analyzed the expression of both vimentin and neurofilament. 2. Immunohistochemical analysis of vimentin showed a redistribution from a cytoplasmic network to a perinuclear accumulation in TR cell lines. 3. Western blot analysis demonstrated that the vimentin content was decreased 60-90%. The content of the 70-kD neurofilament protein was also decreased in TR cells, but its intracellular distribution was indistinguishable from that in the control cell lines.

Hypermutability and mismatch repair deficiency in RER+ tumor cells

A subset of sporadic colorectal tumors and most tumors developing in hereditary nonpolyposis colorectal cancer patients display frequent alterations in microsatellite sequences. Such tumors have been thought to manifest replication errors (RER+), but the basis for the alterations has remained conjectural. We demonstrate that the mutation rate of (CA)n repeats in RER+ tumor cells is at least 100-fold that in RER- tumor cells and show by in vitro assay that increased mutability of RER+ cells is associated with a profound defect in strand-specific mismatch repair. This deficiency was observed with microsatellite heteroduplexes as well as with heteroduplexes containing single base-base mismatches and affected an early step in the repair pathway. Thus, a true mutator phenotype exists in a subset of tumor cells, the responsible defect is likely to cause transitions and transversions in addition to microsatellite alterations, and a biochemical basis for this phenotype has been identified.

An alkylation-tolerant, mutator human cell line is deficient in strand-specific mismatch repair

The human lymphoblastoid MT1 B-cell line was previously isolated as one of a series of mutant cells able to survive the cytotoxic effects of N-methyl-N'-nitro-N-nitrosoguanidine (MNNG). MT1 cells nevertheless remain sensitive to mutagenesis by MNNG and display a mutator phenotype. These phenotypes have been attributed to a single genetic alteration postulated to confer a defect in strand-specific mismatch repair, a proposal that attributes the cytotoxic effect of DNA alkylation in wild-type cells to futile attempts to correct mispairs that arise during replication of alkylated template strands. Our results support this view. MNNG-induced mutations in the HPRT gene of MT1 cells are almost exclusively G.C-->A.T transitions, while spontaneous mutations observed in this mutator cell line are single-nucleotide insertions, transversions, and A.T-->G.C transitions. In vitro assay has demonstrated that the MT1 line is in fact deficient in strand-specific correction of all eight base-base mispairs. This defect, which is manifest at or prior to the excision stage of the reaction, is due to simple deficiency of a required activity because MT1 nuclear extracts can be complemented by a partially purified HeLa fraction to restore in vitro repair. These findings substantiate the idea that strand-specific mismatch repair contributes to alkylation-induced cytotoxicity and imply that this process serves as a barrier to spontaneous transition, transversion, and insertion/deletion mutations in mammalian cells.

Human strand-specific mismatch repair occurs by a bidirectional mechanism similar to that of the bacterial reaction

Nuclear extracts prepared from a HeLa cell line have been previously shown to support strand-specific repair of heteroduplex DNAs containing a site-specific, strand-specific incision (Holmes, J.J., Clark, S., and Modrich, P. (1990) Proc. Natl. Acad. Sci. U.S.A. 87, 5837-5841; Thomas, D.C., Roberts, J.D., and Kunkel, T.A. (1991) J. Biol. Chem. 266, 3744-3751). Further analysis of the substrate specificity of the reaction has shown that in addition to G-T, A-C, G-G, and C-C, nuclear extracts also recognize and correct in a strand-specific manner A-A, A-G, T-T, and C-T mismatches, with repair in each case being inhibited by aphidicolin. The rate of repair of a circular G-T heteroduplex was found to decrease monotonically with increasing separation between the mismatch and the strand break that targets repair, as viewed along the shorter path joining the two sites in the circular substrate. This decrease is independent of the polarity of the incised strand, suggesting that the human pathway of mismatch correction may possess a bidirectional excision capability similar to that of the Escherichia coli methyl-directed system. This possibility was confirmed by analysis of excision tracts associated with the reaction. Inhibition of DNA synthesis by aphidicolin or by omission of exogenous dNTPs leads to the mismatch-provoked formation of a single-strand gap that spans the shorter path between the strand break and the mismatch, irrespective of the polarity of the incised strand. Formation of these gaps, which extend from the site of the strand break to terminate at a number of discrete sites in the region 90 to 170 nucleotides beyond the mismatch, is therefore independent of the relative orientation of the two sites. Based on similar mismatch specificities and common features of mechanism, we have concluded that the human strand-specific mismatch repair system is functionally homologous to the bacterial methyl-directed pathway.