Bien-Willner and Mitra Acta Neuropathologica Communications 2014, 2:74 http://www.actaneurocomms.org/content/2/1/74 RESEARCH Open Access Mutation and expression analysis in medulloblastoma yields prognostic variants and a putative mechanism of disease for i17q tumors Gabriel A Bien-Willner1,2* and Robi D Mitra2 Abstract Current consensus identifies four molecular subtypes of medulloblastoma (MB): WNT, sonic hedgehog (SHH), and groups “3/C” and “4/D”. Group 4 is not well characterized, but harbors the most frequently observed chromosomal abnormality in MB, i17q, whose presence may confer a worse outcome. Recent publications have identified mutations in chromatin remodeling genes that may be overrepresented in this group, suggesting a biological role for these genes in i17q. This work seeks to explore the pathology that underlies i17q in MB. Specifically, we examine the prognostic significance of the previously-identified gene mutations in an independent set of MBs as well as to examine biological relevance of these genes and related pathways by gene expression profiling. The previously-implicated p53 signaling pathway is also examined as a putative driver of i17q tumor oncogenesis. The data show gene mutations associated with i17q tumors in previous studies (KMD6A, ZMYM3, MLL3 and GPS2) were correlated with significantly worse outcomes despite not being specific to i17q in this set. Expression of these genes did not appear to underlie the biology of the molecular variants. TP53 expression was significantly reduced in i17q/ group 4 tumors; this could not be accounted for by dosage effects alone. Expression of regulators and mediators of p53 signaling were significantly altered in i17q tumors. Our findings support that chromatin remodeling gene mutations are associated with significantly worse outcomes in MB but cannot explain outcomes or pathogenesis of i17q tumors. However, expression analyses of the p53 signaling pathway shows alterations in i17q tumors that cannot be explained by dosage effects and is strongly suggestive of an oncogenic role. Keywords: Medulloblastoma, Group 4, i17q, Expression, Outcomes, Fluidigm, Next-generation sequencing Introduction In an attempt to more accurately stratify patients, ef- Medulloblastoma (MB) is the most common malignant forts have utilized the molecular genetics and genomics brain tumor in children [1,2]. Risk stratification strat- of these tumors to identify both prognostic markers and egies that place patients into standard-risk (SR) and biological processes that may underlie the cause or pro- high-risk (HR) groups show 5-year survival rates of gression of disease. Both the sonic hedgehog (SHH) and roughly 86(+/−9)% and 40% respectively [3-5]. However, WNT signaling pathways have been known to be in- clinical staging does not always accurately predict tumor volved in subsets of MBs [7-9]. Other relevant prognos- behavior in the individual patient. Current treatment tic markers in MB include anaplastic/large cell histology strategies, while often curative, frequently result in sig- and MYC amplification, although these are of limited nificant lifelong debilitation, and the limited predictive clinical utility due to their relative rarity [10-12]. The power of the two-tier stratification system likely signifi- most common genetic abnormality seen in MB is i17q cantly contributes to overall patient morbidity [6]. (or more specifically, idic(17)(p11.2)), a chromosomal re- arrangement created by non-allelic homologous recom- bination at 17p11.2 that is relatively specific to MB in * Correspondence: [email protected] brain tumors [13-18]. Patients with this abnormality 1Department of Pathology and Immunology, Washington University, Box have shown to have early recurrence and worse out- 8118, 660 S, Euclid Ave, St. Louis, MO 63110, USA comes in several studies [8,14,19-24]. Interestingly, this 2Deptartment of Genetics, Washington University, St. Louis, MO, USA © 2014 Bien-Willner and Mitra; licensee BioMed Central Ltd. This is an Open Access article distributed under the terms of the Creative Commons Attribution License (http://creativecommons.org/licenses/by/4.0), which permits unrestricted use, distribution, and reproduction in any medium, provided the original work is properly credited. The Creative Commons Public Domain Dedication waiver (http://creativecommons.org/publicdomain/zero/1.0/) applies to the data made available in this article, unless otherwise stated. Bien-Willner and Mitra Acta Neuropathologica Communications 2014, 2:74 Page 2 of 12 http://www.actaneurocomms.org/content/2/1/74 rearrangement is much more frequent in male patients. see if these correlate with i17q status or the established Due to the nature of this rearrangement, tumors with molecular MB variants, and may thus underlie the biology i17q are hemizygous for chromosome 17p telomeric to or oncogenesis of these tumors. 17p11.2 while the remainder of the chromosome is du- plicated. To date no specific mechanism has been pro- Materials and methods posed to explain pathogenesis of these tumors, although Patients hemizygosity of genes and tumor suppressors on 17p Patients for gene sequencing were selected based on a such as TP53 have been suspected. Anticipated “second- diagnosis of medulloblastoma from the archives of hit” TP53 mutations have not been described in i17q- WUSM in accordance with an approved institutional re- positive MBs and these hypotheses have been largely view board protocol (# 201104083) (Washington University overshadowed recently by genomics and expression pro- HRPO) and their details have been previously published filing of tumors [25-27]. However, these initial studies [19]. SR patients were identified by a documented lack of identified other p53 signal modifiers such as WIP1 that residual disease as well as a lack of drop metastases in brain have not been completely explored and warrant further and spinal magnetic resonance imaging (MRI) status post- study. surgical resection, as well as negative CSF cytology. HR Recent attempts to sub-classify tumors based on patients typically had either residual disease or drop metas- microarray gene expression profiling have led to a con- tases (as reported in post-surgical or radiological reports). sensus that there are likely four MB molecular variants Patients with MYC amplifications were also placed in the [28,29]. These studies have supported the previously high-risk group. Patient age was not used as an identifier of described SHH and WNT variants, and have split high risk given conflicting literature [35-37]. The data remaining cases into “group 3/C” and “group 4/D”. used for expression analyses are publicly available and the Currently it is thought that group 3 tumors are driven details of those patients, as well as the methods of expres- by MYC expression and may have worse outcomes sion profiling, have been published elsewhere [28]. A table [28,30,31]. However, to date there has been relatively summarizing clinical and molecular information for each little molecular investigation into the pathogenesis of patient can be found in Additional file 1: Table S1. group 4, the most common subtype. Depending on the data sets used for the hierarchical clustering analysis of Sample preparation the expression data, i17q cases fall either primarily into Formalin-fixed, paraffin embedded (FFPE) blocks of group 4 (with the remaining cases falling into group 3) patient tumors and control (non-tumor) material were or entirely within group 4, while making up a majority evaluated for tumor cell content by a pathologist and of its cases. This suggests that this chromosomal aberra- cored (G.A.B.). Multiple 2 mm cores of both tumor and tion may be critical to the pathogenesis of these tumors. control tissue were obtained per case when available. A recent cytogenetic study of over 1000 tumors identi- Tumor cores were homogeneous with tumor comprising fies i17q in ¾ of all group 4 tumors [24]. Other rare >90% of the sampled area. DNA was extracted with the chromosomal rearrangements such as MYCN/CDK6 Gentra Purgene Kit (Qiagen, Valencia, CA). Samples amplifications and more recently SNCAIP duplication were treated with proteinase K until all tissue was have also been associated with group 4 tumors [29,32]. digested (up to four days). DNA yields ranged from 1– Attempts to identify MB variant-specific mutations 3100 μg (mean 593 μg, median 218 μg; 93% of samples have been recently undertaken with both whole-genome >10 μg). For sequencing, tumor samples were prepared and exome sequencing in two large MB sample sets to at a concentration of ~250 ng/μl. further define the disease [33,34]. These studies identi- fied several mutations that were overrepresented or ex- Gene amplification and sequencing clusive to MB variants, including those harboring i17q. Genes of interest (MLL3, GPS2, KDM6A, and ZMYM3) Many of the identified mutations in i17q tumors are in were selected based on whole-genome and exome-based chromatin remodeling genes, including histone methyl- studies as being specific to, or over-represented in i17q + transferases (MLL3), histone demethylases (KDM6A), and MB samples and having a role in chromatin remodeling histone de-acetylases (ZMYM3 and GPS2), suggesting that [33,34]. Amplification of the coding regions was under- these pathways may be key to their pathogenesis. taken with the Fluidigm microfluidics-based Access Array In this work we explore the specificity and prognostic system (San Francisco, CA). Primer sets were designed by significance of mutations