Malignant primary tumors of scalp with cranial extension: multidisciplinary surgical strategies and outcomes

OBJECTIVE

Malignant cancers arising in the scalp may exhibit calvarial invasion, dural extension, and rarely cerebral involvement. Typically, such lesions require a multidisciplinary approach involving both neurosurgery and plastic surgery for optimal resection and reconstruction. The authors present a retrospective analysis of patients with scalp malignancies who underwent resection and reconstruction.

METHODS

Patients presenting with scalp malignancies (1993-2021, n = 84) who required neurosurgical assistance for tumor resection were prospectively entered into a database. These data were retrospectively reviewed for this case series. The extent of neurosurgical resection was classified into four levels of involvement: scalp (level I), calvarial (level II), dural (level III), or intraparenchymal (level IV). Complications and evidence of local, locoregional, or regional recurrence were documented.

RESULTS

Patients underwent level I (n = 2), level II (n = 61), level III (n = 13), and level IV (n = 8) resections. Pathologies consisted of primarily squamous cell carcinoma (n = 50, 59.5%), basal cell carcinoma (n = 11, 13.1%), and melanoma (n = 9, 10.7%), with infrequent lesions including sarcoma, atypical fibroxanthoma, and malignant fibrous histiocytoma. For cases requiring a cranioplasty, 92.2% were done using titanium mesh and 7.8% with methylmethacrylate. At a mean follow-up of 35.5 ± 45.9 months, the overall survival was 48.8% (n = 41) and recurrence-free survival was 31.0% (n = 43). Scalp-based reconstruction involving plastic surgery was performed in 75 (89.3%) patients. The most commonly used free flap was a latissimus dorsi muscle flap (n = 46, 61.3%). One or more postoperative complications occurred in 21.4% of all patients, the most common being wound dehiscence or delayed wound healing in 13% (n = 11).

CONCLUSIONS

A multidisciplinary approach with aggressive neurosurgical resection is associated with good outcomes in patients with primary malignant scalp tumors, despite invasive disease on presentation. This analysis suggests that aggressive resection (level II and higher) is effective at reducing locoregional recurrence and is not associated with a higher risk of complications relative to resection without craniectomy. As most patients require scalp reconstruction to close the postresection defect, usually with vascularized free tissue transfer, involving a plastic surgeon in the surgical planning and execution is essential.

Introduction of the L-PAP Flap: Bipedicled, Conjoined, and Stacked Thigh-Based Flaps for Autologous Breast Reconstruction

SUMMARY

Thigh-based flaps are increasingly popular options for autologous breast reconstruction in the setting of abdominal donor-site inadequacy, previous surgery, or patient preference, but the volume and skin associated with these flaps are often lacking relative to the abdomen. An individualized, shared decision-making approach to donor-site selection was adopted based on body shape, surgical history, lifestyle, reconstructive needs, and patient expectations. Different thigh-based flaps combined in stacked, bipedicled, or conjoined configurations were selected to maximize efficient use of available soft-tissue skin and volume while optimizing donor-site aesthetics. A total of 23 thigh-based stacked, bipedicled, or conjoined profunda artery perforator (PAP), lateral thigh perforator (LTP), or gracilis musculocutaneous flap components were used in six patients. Configurations included bilateral stacked PAP and LTP flaps, bipedicled posterolateral thigh flaps based on the LTP and PAP (L-PAP), and bipedicled thigh flaps based on the gracilis and PAP pedicles. Most anastomoses were performed to the antegrade and retrograde internal mammary vessels; intraflap anastomoses were performed in one case. There were no partial or total flap losses. There was one donor-site seroma. Design of stacked, bipedicled, and conjoined thigh-based flaps using multiple conventional flap components allows for tailored approaches to donor-site utilization based on individual body shape in selected patients. Bipedicled design with the L-PAP flap represents one strategy in appropriate cases to overcome skin and volume deficiencies while facilitating coning and projection.

CLINICAL QUESTION/LEVEL OF EVIDENCE

Therapeutic, IV.

Outcomes of Targeted Muscle Reinnervation and Regenerative Peripheral Nerve Interfaces for Chronic Pain Control in the Oncologic Amputee Population

BACKGROUND

Outcomes of targeted muscle reinnervation (TMR) and regenerative peripheral nerve interface (RPNI) in the oncologic population are limited. We sought to examine the safety and effectiveness of TMR and RPNI in controlling postamputation pain in the oncologic population.

STUDY DESIGN

A retrospective cohort study of consecutive patients who underwent oncologic amputation followed by immediate TMR or RPNI was conducted from November 2018 to May 2022. The primary study outcome was postamputation pain, assessed using the Numeric Pain Scale and Patient-Reported Outcomes Measurement Information System (PROMIS) for residual limb pain (RLP) and phantom limb pain (PLP). Secondary outcomes included postoperative complications, tumor recurrence, and opioid use.

RESULTS

Sixty-three patients were evaluated for a mean follow-up period of 11.3 months. The majority of patients (65.1%) had a history of previous limb salvage. At final follow-up, patients had an average Numeric Pain Scale score for RLP of 1.3 ± 2.2 and for PLP, 1.9 ± 2.6. The final average raw PROMIS measures were pain intensity 6.2 ± 2.9 (T-score 43.5), pain interference 14.6 ± 8.3 (T-score 55.0), and pain behavior 39.0 ± 22.1 (T-score 53.4). Patient opioid use decreased from 85.7% preoperatively to 37.7% postoperatively and morphine milligram equivalents decreased from a mean of 52.4 ± 53.0 preoperatively to 20.2 ± 38.4 postoperatively.

CONCLUSIONS

In the oncologic population TMR and RPNI are safe surgical techniques associated with significant reductions in RLP, PLP, and improvements in patient-reported outcomes. This study provides evidence for the routine incorporation of TMR and RPNI in the multidisciplinary care of oncologic amputees.

Outcomes of Abdominal Wall Reconstruction with a Bovine versus a Porcine Acellular Dermal Matrix: A Propensity Score-Matched Analysis

BACKGROUND

Abdominal wall reconstruction (AWR) is one of the most commonly performed procedures, yet large comparative studies comparing outcomes of AWR using bovine acellular dermal matrix (BADM) and porcine acellular dermal matrix (PADM) are lacking.

METHODS

In this retrospective cohort study of patients who underwent AWR from March of 2005 to June of 2019, the primary comparative outcome measure was hernia recurrence with BADM versus PADM. The secondary outcome was the incidence of surgical-site occurrence (SSO) and surgical-site infection. A propensity score matching approach was applied to compare the clinical outcomes between the two study groups.

RESULTS

The authors identified 725 patients who underwent AWR using BADM (50.5%) or PADM (49.5%). Their mean ± SD age was 59.8 ± 11.5 years, mean body mass index was 31.4 ± 6.7 kg/m 2 , and mean follow-up time was 42 ± 29 months. With propensity score matching, 219 matched pairs were identified. Hernia recurrence rates in BADM (11.4%) and PADM (13.7%) groups did not differ significantly ( P = 0.793). SSO (26.5% versus 29.2%; P = 0.518) and SSI (13.2% versus 11%; P = 0.456) rates did not differ significantly in the PADM and BADM groups, respectively. Conditional logistic regression model and marginal Cox proportional hazards regression model determined that type of acellular dermal matrix was not significantly associated with SSOs (adjusted OR, 1.11; 95% CI, 0.74 to 1.70; P = 0.589) or hernia recurrence (adjusted hazard ratio, 0.85; 95% CI, 0.50 to 1.42; P = 0.52).

CONCLUSIONS

Both BADMs and PADMs provide durable, long-term outcomes. The hernia recurrence and postoperative surgical complication rates were not significantly different between BADM and PADM.

CLINICAL QUESTION/LEVEL OF EVIDENCE

Therapeutic, III.

Allograft vs Xenograft Bioprosthetic Mesh in Tissue Expander Breast Reconstruction: A Blinded Prospective Randomized Controlled Trial

BACKGROUND

With the increased use of acellular dermal matrix (ADM) in breast reconstruction, the number of available materials has increased. There is a relative paucity of high-quality outcomes data for use of different ADMs.

OBJECTIVES

The goal of this study was to compare the outcomes between a human (HADM) and a bovine ADM (BADM) in implant-based breast reconstruction.

METHODS

A prospective, single-blinded, randomized controlled trial was conducted to evaluate differences in outcomes between HADM and BADM for patients undergoing immediate tissue expander breast reconstruction. Patients with prior radiation to the index breast were excluded. Patient and surgical characteristics were collected and analyzed.

RESULTS

From April 2011 to June 2016, a total of 90 patients were randomized to a mesh group, with 68 patients (HADM, 36 patients/55 breasts; BADM, 32 patients/48 breasts) included in the final analysis. Baseline characteristics were similar between the 2 groups. No significant differences in overall complication rates were identified between HADM (n = 14, 25%) and BADM (n = 13, 27%) (P = 0.85). Similar trends were identified for major complications and complications requiring reoperation. Tissue expander loss was identified in 7% of HADM patients (n = 4) and 17% of BADM patients (n = 8) (P = 0.14).

CONCLUSIONS

Similar complication and implant loss rates were found among patients undergoing immediate tissue expander breast reconstruction with HADM or BADM. Regardless of what material is used, careful patient selection and counseling, and cost consideration, help to improve outcomes and sustainability in immediate breast reconstruction.

LEVEL OF EVIDENCE: 2

Surgical and Ophthalmologic Outcomes of Reconstruction in Patients Treated With Eye-Sparing Surgery and Radiation Therapy for Tumors of the Lacrimal Drainage Apparatus

INTRODUCTION

Tumors involving the lacrimal drainage apparatus can be effectively treated with oncologic eye-sparing resection, immediate reconstruction, and adjuvant radiation. The extirpative technique is well described, whereas the reconstructive approach and outcomes are limited and largely anecdotal. The present study describes the largest series in the literature evaluating outcomes after reconstruction after globe-preserving oncologic resection.

METHODS

A retrospective review was performed for all patients undergoing reconstruction after resection of lacrimal gland tumors from 2008 to 2019. Reconstruction and ophthalmologic outcomes were assessed.

RESULTS

Of the 17 patients included, 2 underwent complex repair, 6 were reconstructed with a locoregional flap, and 9 underwent free flap reconstruction. All patients were treated with adjuvant radiation therapy. The median follow-up was 19 months (range, 5-126 months). Defects reconstructed with free flaps had lower rates of wound dehiscence and fistula formation compared with those reconstructed with other techniques (11% vs 25%, P = 0.45). Patients undergoing reconstruction with free tissue transfer also tended to have lower rates of ectropion, keratopathy and decreased visual acuity compared with those undergoing nonmicrosurgical reconstruction (33% vs 50%, P = 0.48; 11% vs 38%, P = 0.20; 56% vs 75%, P = 0.40, respectively). These differences were not statistically significant.

CONCLUSIONS

This is the first study to compare reconstructive and functional outcomes of nonmicrosurgical reconstruction and microsurgical free tissue transfer in the setting of eye-sparing surgery for tumors of the lacrimal drainage apparatus. Although various reconstructive options are feasible, microsurgical free tissue transfer is often used at our center and is associated with reliable outcomes.

Risk-adjustment models in patients undergoing head and neck surgery with reconstruction

BACKGROUND

With the current focus on value-based outcomes and reimbursement models, perioperative risk adjustment is essential. Specialty surgical outcomes are not well predicted by the American College of Surgeons National Surgical Quality Improvement Program (ACS NSQIP); the Head and Neck-Reconstructive Surgery NSQIP was created as a specialty-specific platform for patients undergoing head and neck surgery with flap reconstruction. This study aims to investigate risk prediction models in these patients.

METHODS

The Head and Neck-Reconstructive Surgery NSQIP collected data on patients undergoing head and neck surgery with flap reconstruction from August 1, 2012 to October 20, 2016. Multivariable logistic regression models were created for 9 outcomes (postoperative ventilator dependence, pneumonia, superficial recipient surgical site infection, presence of tracheostomy/nasoenteric (NE)/gastrostomy/gastrojejunostomy(G/GJ) tube 30 days postoperatively, conversion from NE to G/GJ tube, unplanned return to the operating room, length of stay > 7 days). External validation was completed with a more contemporary cohort.

RESULTS

A total of 1095 patients were included in the modelling cohort and 407 in the validation cohort. Models performed well predicting tracheostomy, NE, G/GJ tube presence at 30 days postoperatively and conversion from NE to G/GJ tube (c-indices = 0.75-0.91). Models for postoperative pneumonia, superficial recipient surgical site infection, ventilator dependence > 48 h, and length of stay > 7 days were fair (concordance [c]-indices = 0.63-0.69). The predictive model for unplanned return to the operating room was poor (c-index = 0.58).

CONCLUSIONS AND RELEVANCE

Reliable and discriminant risk prediction models were able to be created for postoperative outcomes using the specialty-specific Head and Neck-Reconstructive Surgery Specific NSQIP.

Cancer Surgery Scheduling During and After the COVID-19 First Wave: The MD Anderson Cancer Center Experience

OBJECTIVE

To summarize the multi-specialty strategy and initial guidelines of a Case Review Committee in triaging oncologic surgery procedures in a large Comprehensive Cancer Center and to outline current steps moving forward after the initial wave.

SUMMARY OF BACKGROUND DATA

The impetus for strategic rescheduling of operations is multifactorial and includes our societal responsibility to minimize COVID-19 exposure risk and propagation among patients, the healthcare workforce, and our community at large. Strategic rescheduling is also driven by the need to preserve limited resources. As many states have already or are considering to re-open and relax stay-at-home orders, there remains a continued need for careful surgical scheduling because we must face the reality that we will need to co-exist with COVID-19 for months, if not years.

METHODS

The quality officers, chairs, and leadership of the 9 surgical departments in our Division of Surgery provide specialty-specific approaches to appropriately triage patients.

RESULTS

We present the strategic approach for surgical rescheduling during and immediately after the COVID-19 first wave for the 9 departments in the Division of Surgery at The University of Texas MD Anderson Cancer Center in Houston, Texas.

CONCLUSIONS

Cancer surgeons should continue to use their oncologic knowledge to determine the window of opportunity for each surgical procedure, based on tumor biology, preoperative treatment sequencing, and response to systemic therapy, to safely guide patients through this cautious recovery phase.

A reconstructive algorithm of oncologic defects of the upper trunk and shoulder girdle: Factors predicting complexity and outcomes

BACKGROUND

Locally advanced malignancies of the upper torso and shoulder girdle (UT-SG) necessitate extensive resection and complex reconstruction. Due to the infrequent nature of these operations, a global reconstructive algorithm has not been defined.

METHODS

A retrospective review of all patients who received reconstructive surgery following malignant tumor extirpation in the UT-SG from 2008 to 2018 at the University of Texas MD Anderson Cancer Center. Factors predicting the need for flap reconstruction and risk for postoperative complications were evaluated.

RESULTS

In total, 252 procedures met inclusion criteria. The most common pathology was sarcoma (76%) and 52% were primary tumors. The median defect area was 112 cm² (range 4-1350 cm² ). Reconstructive techniques included pedicled flaps (46%), local tissue rearrangement (38%), and free flaps (16%). On univariate analysis, the probability of needing a free flap increased 39% when the defect size increased by 100 cm² . The strongest independent predictors of requiring a free flap were major vessel exposure (adjusted odds ratio [OR] = 4.92, 95% confidence interval [CI], 1.36-17.84, P = .015) and major peripheral nerve exposure (adjusted OR = 3.2, 95% CI, 1.1-9.2, P = .031).

CONCLUSION

Despite the aggressive nature of their malignancies, patients requiring an UT-SG resection demonstrate high survival rates and therefore demand a durable reconstruction. Exposed critical structures and defect size were predictive of free tissue transfer.

Soft-Tissue Reconstruction after Composite Vertebrectomy and Chest Wall Resection for Spinal Tumors

BACKGROUND

Oncologic resections involving both the spine and chest wall commonly require immediate soft-tissue reconstruction. The authors hypothesized that reconstructions of composite resections involving both the thoracic spine and chest wall would have a higher complication rate than reconstructions for resections limited to the thoracic spine alone.

METHODS

The authors performed a retrospective analysis of all consecutive patients who underwent a thoracic vertebrectomy and soft-tissue reconstruction from 2002 to 2017. Patients were divided into two groups: those whose defect was limited to the thoracic spine and those who required a composite resection involving the chest wall.

RESULTS

One hundred patients were included. Composite resection patients had larger defects, as indicated by a greater incidence of multilevel vertebrectomies (70.2 percent versus 17 percent; p = 0.001). Thoracic spine patients were older (58.2 ± 10.4 years versus 48.6 ± 13.9 years; p < 0.001) and had a greater incidence of metastatic disease (88.7 percent versus 38.3 percent; p = 0.001). Univariate and multivariate logistic regression analyses demonstrated that composite resections were not significantly associated with a higher rate of surgical, medical, or overall complications. Multivariate logistic regression analysis of composite resection subgroup demonstrated that flap separation of the spinal cord from the intrapleural space was protective against complications (OR, 0.22; 95 percent CI, 0.05 to 0.81; p = 0.03).

CONCLUSIONS

Despite the large defect size in composite resection patients, there was no increase in complications compared to thoracic spine patients. In composite resection patients, separating the exposed spinal cord from the intrapleural space with well-vascularized soft tissue was protective against complications.

CLINICAL QUESTION/LEVEL OF EVIDENCE

Risk, II.

Clinically available reinforcing materials for soft tissue reconstruction

Navigating the rapidly evolving field of materials for soft tissue reinforcement is challenging given the volume of clinically available options. Additionally, the current generally accepted classifications of these mesh materials confound the understanding of their utility by grouping disparate materials that have attributes overlapping category boundaries and that do not fully consider their clinically functionality. This review article highlights, from a materials science perspective, the most important attributes of these materials to improve the clinical decision-making process in the selection of the most appropriate features and design for the patient, surgery and clinical need. These characteristics include the physical attributes that directly impact the surgical procedure and immediate postoperative mechanical requirements as well as the post-implantation properties such as an adequate reinforcement time, strength of the resulting tissue and infection risk profile.

HIF-dependent hematopoietic factors regulate the development of the embryonic vasculature

Hypoxia inducible factors (HIFs) regulate adaptive responses to changes in oxygen (O(2)) tension during embryogenesis, tissue ischemia, and tumorigenesis. Because HIF-deficient embryos exhibit a number of developmental defects, the precise role of HIF in early vascular morphogenesis has been uncertain. Using para-aortic splanchnopleural (P-Sp) explant cultures, we show that deletion of the HIF-beta subunit (ARNT) results in defective hematopoiesis and the inhibition of both vasculogenesis and angiogenesis. These defects are rescued upon the addition of wild-type Sca-1(+) hematopoietic cells or recombinant VEGF. Arnt(-/-) embryos exhibit reduced levels of VEGF protein and increased numbers of apoptotic hematopoietic cells. These results suggest that HIF coordinates early endothelial cell emergence and vessel development by promoting hematopoietic cell survival and paracrine growth factor production.

Hypoxia-inducible factors 1alpha and 2alpha regulate trophoblast differentiation

Placental development initially occurs in a low-oxygen (O2) or hypoxic environment. In this report we show that two hypoxia-inducible factors (HIFs), HIF1alpha and HIF2alpha, are essential for determining murine placental cell fates. HIF is a heterodimer composed of HIFalpha and HIFbeta (ARNT) subunits. Placentas from Arnt-/- and Hif1alpha-/- Hif2alpha-/- embryos exhibit defective placental vascularization and aberrant cell fate adoption. HIF regulation of Mash2 promotes spongiotrophoblast differentiation, a prerequisite for trophoblast giant cell differentiation. In the absence of Arnt or Hifalpha, trophoblast stem cells fail to generate these cell types and become labyrinthine trophoblasts instead. Therefore, HIF mediates placental morphogenesis, angiogenesis, and cell fate decisions, demonstrating that O2 tension is a critical regulator of trophoblast lineage determination. This novel genetic approach provides new insights into the role of O2 tension in the development of life-threatening pregnancy-related diseases such as preeclampsia.

Targeted mutation of the murine arylhydrocarbon receptor nuclear translocator 2 (Arnt2) gene reveals partial redundancy with Arnt

The ubiquitously expressed basic helix-loop-helix (bHLH)-PAS protein ARNT (arylhydrocarbon receptor nuclear transporter) forms transcriptionally active heterodimers with a variety of other bHLH-PAS proteins, including HIF-1alpha (hypoxia-inducible factor-1alpha) and AHR (arylhydrocarbon receptor). These complexes regulate gene expression in response to hypoxia and xenobiotics, respectively, and mutation of the murine Arnt locus results in embryonic death by day 10.5 associated with placental, vascular, and hematopoietic defects. The closely related protein ARNT2 is highly expressed in the central nervous system and kidney and also forms complexes with HIF-1alpha and AHR. To assess unique roles for ARNT2 in development, and reveal potential functional overlap with ARNT, we generated a targeted null mutation of the murine Arnt2 locus. Arnt2(-/-) embryos die perinatally and exhibit impaired hypothalamic development, phenotypes previously observed for a targeted mutation in the murine bHLH-PAS gene Sim1 (Single-minded 1), and consistent with the recent proposal that ARNT2 and SIM1 form an essential heterodimer in vivo [Michaud, J. L., DeRossi, C., May, N. R., Holdener, B. C. & Fan, C. (2000) Mech. Dev. 90, 253-261]. In addition, cultured Arnt2(-/-) neurons display decreased hypoxic induction of HIF-1 target genes, demonstrating formally that ARNT2/HIF-1alpha complexes regulate oxygen-responsive genes. Finally, a strong genetic interaction between Arnt and Arnt2 mutations was observed, indicating that either gene can fulfill essential functions in a dose-dependent manner before embryonic day 8.5. These results demonstrate that Arnt and Arnt2 have both unique and overlapping essential functions in embryonic development.

Placental cell fates are regulated in vivo by HIF-mediated hypoxia responses

Placental development is profoundly influenced by oxygen (O2) tension. Human cytotrophoblasts proliferate in vitro under low O2 conditions but differentiate at higher O2 levels, mimicking the developmental transition they undergo as they invade the placental bed to establish the maternal–fetal circulation in vivo. Hypoxia-inducible factor-1 (HIF-1), consisting of HIF-1α and ARNT subunits, activates many genes involved in the cellular and organismal response to O2deprivation. Analysis of Arnt−/− placentas reveals an aberrant cellular architecture due to altered cell fate determination of Arnt−/− trophoblasts. Specifically, Arnt−/− placentas show greatly reduced labyrinthine and spongiotrophoblast layers, and increased numbers of giant cells. We further show that hypoxia promotes the in vitro differentiation of trophoblast stem cells into spongiotrophoblasts as opposed to giant cells. Our results clearly establish that O2 levels regulate cell fate determination in vivo and that HIF is essential for mammalian placentation. The unique placental phenotype of Arnt−/− animals also provides an important tool for studying the disease of preeclampsia. Interestingly, aggregation of Arnt−/− embryonic stem (ES) cells with tetraploid wild-type embryos rescues their placental defects; however, these embryos still die from yolk sac vascular and cardiac defects.

Placental cell fates are regulated in vivo by HIF-mediated hypoxia responses

Placental development is profoundly influenced by oxygen (O(2)) tension. Human cytotrophoblasts proliferate in vitro under low O(2) conditions but differentiate at higher O(2) levels, mimicking the developmental transition they undergo as they invade the placental bed to establish the maternal-fetal circulation in vivo. Hypoxia-inducible factor-1 (HIF-1), consisting of HIF-1alpha and ARNT subunits, activates many genes involved in the cellular and organismal response to O(2) deprivation. Analysis of Arnt(-/-) placentas reveals an aberrant cellular architecture due to altered cell fate determination of Arnt(-/-) trophoblasts. Specifically, Arnt(-/-) placentas show greatly reduced labyrinthine and spongiotrophoblast layers, and increased numbers of giant cells. We further show that hypoxia promotes the in vitro differentiation of trophoblast stem cells into spongiotrophoblasts as opposed to giant cells. Our results clearly establish that O(2) levels regulate cell fate determination in vivo and that HIF is essential for mammalian placentation. The unique placental phenotype of Arnt(-/-) animals also provides an important tool for studying the disease of preeclampsia. Interestingly, aggregation of Arnt(-/-) embryonic stem (ES) cells with tetraploid wild-type embryos rescues their placental defects; however, these embryos still die from yolk sac vascular and cardiac defects.

The combined functions of proapoptotic Bcl-2 family members bak and bax are essential for normal development of multiple tissues

Proapoptotic Bcl-2 family members have been proposed to play a central role in regulating apoptosis. However, mice lacking bax display limited phenotypic abnormalities. As presented here, bak(-/-) mice were found to be developmentally normal and reproductively fit and failed to develop any age-related disorders. However, when Bak-deficient mice were mated to Bax-deficient mice to create mice lacking both genes, the majority of bax(-/-)bak(-/-) animals died perinatally with fewer than 10% surviving into adulthood. bax(-/-)bak(-/-) mice displayed multiple developmental defects, including persistence of interdigital webs, an imperforate vaginal canal, and accumulation of excess cells within both the central nervous and hematopoietic systems. Thus, Bax and Bak have overlapping roles in the regulation of apoptosis during mammalian development and tissue homeostasis.

Multilineage embryonic hematopoiesis requires hypoxic ARNT activity

Although most cells undergo growth arrest during hypoxia, endothelial cells and placental cytotrophoblasts proliferate in response to low O(2). We demonstrate that proliferation of embryonic multilineage hematopoietic progenitors is also regulated by a hypoxia-mediated signaling pathway. This pathway requires HIF-1 (HIF-1alpha/ARNT heterodimers) because Arnt(-/-) embryoid bodies fail to exhibit hypoxia-mediated progenitor proliferation. Furthermore, Arnt(-/-) embryos exhibit decreased numbers of yolk sac hematopoietic progenitors. This defect is cell extrinsic, is accompanied by a decrease in ARNT-dependent VEGF expression, and is rescued by exogenous VEGF. Therefore, "physiologic hypoxia" encountered by embryos is essential for the proliferation or survival of hematopoietic precursors during development.

The importance of anchorage in determining a strained protein loop conformation

We examine the role of the conformational restriction imposed by constrained ends of a protein loop on the determination of a strained loop conformation. The Lys 116-Pro 117 peptide bond of staphylococcal nuclease A exists in equilibrium between the cis and trans isomers. The folded protein favors the strained cis isomer with an occupancy of 90%. This peptide bond is contained in a solvent-exposed, flexible loop of residues 112-117 whose ends are anchored by Val 111 and Asn 118. Asn 118 is constrained by 2 side-chain hydrogen bonds. We investigate the importance of this constraint by replacing Asn 118 with aspartate, alanine, and glycine. We found that removing 1 or more of the hydrogen bonds observed in Asn 118 stabilizes the trans configuration over the cis configuration. By protonating the Asp 118 side chain of N118D through decreased pH, the hydrogen bonding character of Asp 118 approached that of Asn 118 in nuclease A, and the cis configuration was stabilized relative to the trans configuration. These data suggest that the rigid anchoring of the loop end is important in establishing the strained cis conformation. The segment of residues 112-117 in nuclease A provides a promising model system for study of the basic principles that determine polypeptide conformations. Such studies could be useful in the rational design or redesign of protein molecules.