MEDB-55. Single-cell transcriptomics reveals progenitor cells expressing a photoreceptor program as putative cells origin of MYC-driven Group 3 Medulloblastoma

Brain tumors are the leading cause of childhood cancer-related death. Medulloblastoma is the most common malignant pediatric brain tumor with about 70% survival. Medulloblastoma comprises four distinct subgroups respective of genomic and molecular drivers influencing tumorigenesis. It has been established that despite being considered a single disease entity, each subgroup arises from a distinct population of cells found within unique compartments of the developing brain. The cell of origin of Group 3 medulloblastoma, the most malignant medulloblastoma subgroups, is currently unknown and remains controversial. Transcriptional profiling has revealed that Group 3 medulloblastomas are characterized by elevated expression of a photoreceptor program, which has not been described in the normal cerebellar development but is well characterized in the developing pineal gland and retinal. By investigating and comparing brain and tumor development between our previously developed medulloblastoma mice model (GMYC), where mice spontaneously develop Group 3 medulloblastoma after 4-6 months of age, and their control counterparts, we found that tumor cells emerged from progenitor cells where MYC overexpression drove the transformation of immature progenitor cells expressing a photoreceptor program. Our data suggest that MYC-driven Group 3 medulloblastoma originates from progenitor cells expressing a photoreceptor program, which has implications for future research and the development of novel treatments targeting this devastating childhood malignancy.

MBRS-10. QUIESCENT SOX9-POSITIVE CELLS BEHIND MYC DRIVEN MEDULLOBLASTOMA RECURRENCE

Tumor recurrence is the leading cause of death in medulloblastoma, the most frequent malignant pediatric brain tumor. Recurrence occurs when subpopulations of cancer cells evade standard therapy by acquiring features of immune escape, metastatic spread, and treatment resistance. The transcription factor SOX9 correlated with treatment resistance and dissemination in aggressive Group 3 medulloblastoma. By studying paired primary-recurrent medulloblastoma samples and patient-derived xenograft models, we identified rare SOX9-positive slow-cycling, therapy-resistant tumor cells that accumulate in relapses and in metastases. In an inducible transgenic Group 3 tumor model, doxycycline treatment kills all tumor cells by turning MYC off. However, when MYC expression was redirected to the SOX9 promoter, recurrences from rare, dormant SOX9-positive cells developed with 100% penetrance. Expression profiling revealed that recurrences were more inflammatory, metastatic, and showed elevated MGMT methyltransferase levels which depleted recurrent cells when selectively inhibited. Our model explains how recurrences develop from SOX9-induced quiescence in MYC-driven brain cancer.

MEDU-26. LATENT SOX9-POSITIVE CELLS RESPONSIBLE FOR MYC-DRIVEN MEDULLOBLASTOMA RECURRENCE

Tumor recurrence is the leading cause of death among children with medulloblastoma, the most common type of malignant pediatric brain tumors. The mechanisms behind medulloblastoma recurrence are not fully understood. We previously showed that the transcription factor SOX9 promotes cisplatin treatment resistance in medulloblastoma. Here we show that SOX9 levels correlate with poor prognosis in Group 3 tumors. By studying paired primary-recurrent medulloblastoma samples and patient-derived xenograft (PDX) models we further identified rare SOX9-positive slow-cycling, therapy-resistant tumor cells that accumulate in relapses and in leptomenigeal metastases of Group 3 and Group 4 patients. By using an inducible Tet-OFF transgenic (GTML) mouse model for malignant MYCN-driven Group 3 tumors we identified rare SOX9-positive, quiescent brain tumor cells that are more resistant to cisplatin. Dox treatment normally cures GTML transgenic animals that developed aggressive medulloblastoma by turning MYCN off. However, when crossing the Tet-OFF GTML model with a Tet-ON rtTA-Sox9 model we can redirect MYCN expression to the Sox9 promoter ultimately driving brain tumor recurrence from rare SOX9-positive cells with 100% penetrance. In this novel animal model, recurrent tumors were actively disseminating from the hindbrain to the spinal cord and into the forebrain similar to distant relapses found in patients. By overexpressing SOX9 in human Group 3 tumor cells, MYC was directly inhibited and cell proliferation was decreased. PDX models of Group 3 tumors further showed increased levels of SOX9-positivity and less proliferative cells in metastatic compartments. Expression profiling revealed that recurrences were more inflammatory, metastatic, immune evasive and showed elevated MGMT methyltransferase levels which depleted recurrent cells and sensitized them for chemotherapy when using the MGMT inhibitor lomeguatrib. To summarize, our data clarify important and complex mechanisms by which latent medulloblastoma cells fail to respond to standard therapy and generate relapses.

Macronutrient intake and utilization by rats: interactions with type I adrenocorticoid receptor stimulation

Corticosterone-free (adrenalectomized, ADX) and intact rats were offered experimentally compounded diets in which 65% of available calories were supplied by a single macronutrient (single-diet study). ADX impaired the intake, weight gain (especially as body fat), and efficient utilization of high-protein and high-fat diets. In contrast, no behavioral, metabolic, or compositional changes could be found among ADX rats maintained on a diet high in carbohydrates. When ADX rats were given separate sources of macronutrients (self-selection study) they did not self-select a high-carbohydrate diet. Instead, they displayed a strong fat avoidance and a relative increase in protein intake, the macronutrient they utilize least efficiently. Separate groups of ADX animals were continuously infused with 25 or 125 micrograms.kg-1.day-1 aldosterone, a specific type I adrenocorticoid receptor agonist. Type I receptor stimulation eliminated all ADX-related deficiencies found in the single-diet and self-selection studies: caloric intake, feeding efficiency, carcass composition, and macronutrient preferences were restored to or beyond the corresponding values of adrenal-intact rats. The normal rat's ability to ingest and utilize macronutrients optimally is dependent on corticosterone's stimulation of type I receptors.

Corticosterone's dual metabolic actions

Corticosterone possesses two distinctly opposite metabolic actions. The actions are strictly dose-dependent and are linked to type I and type II corticosteroid receptor binding. These conclusions are drawn from continuous infusion studies where corticosterone yields a bitonic dose-response curve for body weight gain and feeding efficiency. Anabolic at low serum levels, corticosterone concentrations above 2 micrograms/dl bring about an opponent catabolic process that intensifies and eventually masks the anabolic action. Relatively pure type I (aldosterone) and type II (RU28362 and dexamethasone) corticosterone receptor agonists produce opposite monotonic functions that respectively mimic the ascending and descending arms of the corticosterone dose-response curve. Stimulation of either receptor increases the proportion of carcass fat to lean body mass by either increasing carcass lipids (type I) or by reducing protein (type II).