Rb1 loss modifies but does not initiate alveolar rhabdomyosarcoma

Abstract

BackgroundAlveolar rhabdomyosarcoma (aRMS) is a myogenic childhood sarcoma frequently associated with a translocation-mediated fusion gene, Pax3:Foxo1a.MethodsWe investigated the complementary role of Rb1 loss in aRMS tumor initiation and progression using conditional mouse models.ResultsRb1 loss was not a necessary and sufficient mutational event for rhabdomyosarcomagenesis, nor a strong cooperative initiating mutation. Instead, Rb1 loss was a modifier of progression and increased anaplasia and pleomorphism. Whereas Pax3:Foxo1a expression was unaltered, biomarkers of aRMS versus embryonal rhabdomyosarcoma were both increased, questioning whether these diagnostic markers are reliable in the context of Rb1 loss. Genome-wide gene expression in Pax3:Foxo1a,Rb1 tumors more closely approximated aRMS than embryonal rhabdomyosarcoma. Intrinsic loss of pRb function in aRMS was evidenced by insensitivity to a Cdk4/6 inhibitor regardless of whether Rb1 was intact or null. This loss of function could be attributed to low baseline Rb1, pRb and phospho-pRb expression in aRMS tumors for which the Rb1 locus was intact. Pax3:Foxo1a RNA interference did not increase pRb or improve Cdk inhibitor sensitivity. Human aRMS shared the feature of low and/or heterogeneous tumor cell pRb expression.ConclusionsRb1 loss from an already low pRb baseline is a significant disease modifier, raising the possibility that some cases of pleomorphic rhabdomyosarcoma may in fact be Pax3:Foxo1a-expressing aRMS with Rb1 or pRb loss of function.

New insights into signalling-pathway alterations in rhabdomyosarcoma

“…Recent work has shown that the loss of RB1 does not appear to initiate RMS, but is a disease modifier. The loss of RB1 in an ERMS or an ARMS model modifies the tumour phenotype to mimic an undifferentiated pleomorphic sarcoma ( Rubin et al , 2011 ) or a pleomorphic RMS identity ( Kikuchi et al , 2013 ), respectively. The interplay between pRB and TBX2 in RMS or normal skeletal muscle is unknown.…”

Genomic Characterization of a Metastatic Alveolar Rhabdomyosarcoma Case Using FISH Studies and CGH+SNP Microarray Revealing <b><i>FOXO1-PAX7</i></b> Rearrangement with <b><i>MYCN </i></b>and<b><i> MDM2</i></b> Amplification and <b><i>RB1</i></b> Region Loss

“…Similarly, alterations in RB1 pathways and in genes encoding proteins that regulate RB1 function such as CDK4 (amplification) and CDKN2A and CDKN2B (deletions) have been identified in ARMS [Kohashi et al, 2008;Kikuchi et al, 2013;Shern et al, 2014]. Our case demonstrated a genomic loss at chromosome 13q14.11q14.3 containing the RB1 locus that is situated close to the amplified FOXO1 region ( Fig.…”

Defining the Extracellular Matrix of Rhabdomyosarcoma

“…Four samples of formalin-xed para n embedded (FFPE) human RMS were available to include in a custom mouse model tissue microarray (TMA) (1 x aRMS, 3 x eRMS). Forty-eight murine model sarcomas were also included, representing developmental stages and genotypes including early myoblast (origin), postnatal stem cell (origin), maturing myo ber (origin), Pax3:Foxo1-expressing, Trp53 wild type or mutated and Rb1wildtype or mutated (5,7,10).…”

“…Lymphatic and hematogenous (pulmonary) metastasis is a predominant feature(4)and a primary cause of mortality in these models (8). We characterized these soft tissue sarcoma models and demonstrate them to be representative of the human diseases by histopathology, gene expression and other features (3,4,9,10). Furthermore, these conditional models have the special features of knowing the cell-of-origin as well as the mutational pro le making them a valuable tool to study RMS ( Figure 1).…”