ABSTRACT
Medulloblastoma (MB) is the most common malignant brain tumor of childhood. Very little is known about aggressive forms of this disease, such as metastatic or recurrent MBs. In order to identify pathways involved in aggressive MB pathophysiology, we performed unbiased, whole genome microarrays on MB tumors at both the human and murine levels. Primary human MBs were compared, transcriptomically, to their patient-matched recurrent or metastatic tumors. Expression profiling was also performed on murine tumors from two spontaneously developing MB mouse models (Ptch+/- and Smo/Smo) that present with differing clinical severities. At both the human and murine levels we identified transforming growth factor-beta (TGF-ß) as a potential contributor to MB progression/metastasis. Smad3, a major downstream component of the TGF-ß pathway, was also evaluated using immunohistochemistry in malignant human tissues and was shown to correlate with MB metastasis and survival. Similarly, Smad3 expression during development identified a subset of cerebellar neuronal precursors as putative cells of origin for the Smad3-positive MBs. To our knowledge, this is the first study that links TGF-ß to MB pathogenesis. Our research suggests that canonical activation of this pathway leads to better prognosis for patients.
Subject(s)
Cerebellar Neoplasms/metabolism , Cerebellum/metabolism , Hedgehog Proteins/metabolism , Medulloblastoma/metabolism , Signal Transduction/physiology , Transforming Growth Factor beta/metabolism , Adult , Animals , Cerebellar Neoplasms/genetics , Cerebellar Neoplasms/pathology , Cerebellum/pathology , Disease Models, Animal , Disease Progression , Female , Gene Regulatory Networks , Hedgehog Proteins/genetics , Humans , Male , Medulloblastoma/genetics , Medulloblastoma/pathology , Mice , Neurons/metabolism , Phosphorylation , Smad3 Protein/genetics , Smad3 Protein/metabolism , Transforming Growth Factor beta/geneticsABSTRACT
Ribonucleotide reductase (RNR) is a critical enzyme of nucleotide metabolism, synthesizing precursors for DNA replication and repair. In prokaryotic genomes, RNR genes are commonly targeted by mobile genetic elements, including free standing and intron-encoded homing endonucleases and inteins. Here, we describe a unique molecular solution to assemble a functional product from the RNR large subunit gene, nrdA that has been fragmented into two smaller genes by the insertion of mobE, a mobile endonuclease. We show that unique sequences that originated during the mobE insertion and that are present as C- and N-terminal tails on the split NrdA-a and NrdA-b polypeptides, are absolutely essential for enzymatic activity. Our data are consistent with the tails functioning as protein interaction domains to assemble the tetrameric (NrdA-a/NrdA-b)(2) large subunit necessary for a functional RNR holoenzyme. The tails represent a solution distinct from RNA and protein splicing or programmed DNA rearrangements to restore function from a fragmented coding region and may represent a general mechanism to neutralize fragmentation of essential genes by mobile genetic elements.
