Adult stem cells reside in specialized microenvironments, or niches, that have an important role in regulating stem cell behavior. Therefore, tight control of niche number, size, and function is necessary to ensure the proper balance between stem cells and progenitor cells available for tissue homeostasis and wound repair. Identification of genetic networks that regulate stem cell niches are of particular interest for their potential roles in regenerative medicine. This dissertation work utilizes the Drosophila testis as a model for understanding how genetic factors regulate stem cell niche maintenance and regeneration. In Drosophila males, the germline stem cell niche is located at the tip of the testis where germline and somatic cyst stem cells surround the apical hub, a cluster of approximately 10-15 somatic cells that are required for stem cell self-renewal. Findings described in Chapter 2 led to the hypothesis that somatic cyst stem cells contribute to the apical hub and identified an important role for the Snail family transcription factor escargot within the stem cell niche. This work also identified adhesion to the hub is required for both germline and somatic cyst stem cell maintenance. Work presented in Chapter 3 demonstrates the cell type specific requirements of escargot within hub cells (maintenance) and somatic cyst stem cells (maintenance and regulation of proliferation). In addition, Chapter 3 provides direct evidence that the hub is absolutely required for germline and somatic cyst stem cell maintenance. Using both genetic and genomic approaches, we demonstrate that an interaction between escargot and the transcripton factor Stat92E coordinately act in regulating hub cell maintenance and somatic cyst stem cell activity. These data build the framework for findings described in Chapter 4 that provide preliminary evidence that the AP-4 transcription factor cropped, a putative transcriptional target of escargot, is required for hub maintenance. Futhermore, these data suggest that the hub is capable of regeneration during adulthood. This dissertation highlights the dynamic relationship between stem cells and niche support cells and provides insight into genetic programs that regulate niche size and function to support normal tissue homeostasis and organ regeneration during development and adulthood.

Objective

We sought to evaluate the effectiveness and safety of utilizing radiotherapy (RT) with standard fractionation, with or without intraoperative RT (IORT), to treat locally recurrent rectal cancer (LRRC).

Methods

Retrospective review of 25 patients with LRRC treated with standard fractionation RT from 2005 to 2011. 15 patients (60%) had prior pelvic RT and 10 (40%) had synchronous metastases. The median equivalent dose in 2-Gy fractions was 30 and 49.6 Gy in patients with and without prior RT, respectively. 23 patients (92%) received concurrent chemotherapy and 16 (64%) underwent surgical resection. Eight patients (33.3%, four with and four without prior RT) received IORT. A competing risks model was developed to estimate the cumulative incidence of local failure with death treated as a competing event.

Results

Median follow-up was 36.9 months after the date of local recurrence. 3-year rates of overall survival (OS), local control (LC) and death with LC were 51.6%, 73.3% and 69.2%, respectively. On multivariable analysis, surgical resection was significantly predictive of improved OS (p < 0.05). If surgical resection were removed from the multivariable model, given the collinearity between IORT delivery and surgical resection, then IORT also became a significant predictor of OS (p < 0.05). Systemic disease at the time of local recurrence was not associated with either LC or OS. No patient had grade ≥3 acute or late toxicity.

Conclusion

RT with standard fractionation is safe and effective in the treatment of patients with LRRC, even in patients with significant risk of systemic disease and/or history of prior RT. Advances in knowledge: The utility of RT with standard fractionation, generally with chemotherapy, in the treatment of LRRC is demonstrated. In this high-risk cohort of patients with a 40% incidence of synchronous metastatic disease, surgical resection of the recurrence was the major predictor of OS, though a benefit to IORT was also suggested. No patients had grade ≥3 acute or late toxicity, though 40% had undergone prior RT, underscoring the tolerability of standard fractionation RT in this setting.