Medulloblastoma: recurrence and metastasis

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    377ISSN 2045-090710.2217/CNS.13.30 2013 Future Medicine Ltd CNS Oncol. (2013) 2(4), 377385

    1University Health Network Pathology, Arthur & Sonia Labatt Brain Tumour Research Centre, Department of Laboratory Medicine & Pathobiology, Toronto, ON, Canada *Author for correspondence: sidney.croul@uhn.on.ca

    Medulloblastoma is the most common malignant brain tumor of childhood, with the rate of long-term survival or cure presently at 7080%.

    There is a significant burden of complications in survivors, probably due to intense therapies.

    Currently there is a risk-adapted scheme for prognosis of medulloblastoma: High risk: children 1.5cm2 of residual disease and/or evidence of metastasis; Low risk: children >3years of age with minimal residual tumor and no metastases.

    Pathological stratification falls into five groups: classic, desmoplastic/nodular, extensively nodular, large cell and anaplastic. The large-cell and anaplastic variants carry poor prognoses and high frequencies of metastatic disease.

    Current genetic consensus classification involves four main subgroups (WNT, SHH, group 3 and 4): WNT tumors generally have classic histology and very good long-term prognoses; SHH tumor prognosis is intermediate between WNT and group 4 tumors; Group 3 tumors are recognized by their transcriptional profile. There is a close association between

    group 3 tumors and high levels of Myc expression; Group 4 tumors are also recognized by transcriptional profiling. Isochromosome 17q is most common

    in group 4. Group 4 tumors have an intermediate prognosis.

    Combined molecular/clinical grading may be more informative than either modality alone.

    Recent genome-wide association studies have identified many more potential molecular abnormalities than previously appreciated.

    Metastasis research indicates that primary and metastatic tumors may be phenotypically different.

    Owing to good prognosis, trials are contemplated for WNT tumors with reduced primary therapy.Preliminary reports for SHH pathway inhibitors show dramatic but short-lived shrinkage of advanced tumors. The are now several clinical trials with this class of agent to determine if long-term tumor control can be obtained.

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    Medulloblastoma: recurrence and metastasis

    REVIEW

    Donya Aref1 & Sidney Croul*1

    For reprint orders, please contact: reprints@futuremedicine.com

  • CNS Oncol. (2013) 2(4) future science group378

    Review Aref & Croul

    SUMMARY Medulloblastoma is the most common malignant brain tumor of childhood. Although there is now long-term survival or cure for the majority of children, the survivors bear a significant burden of complications due, at least in part, to the intense therapies given to ensure eradication of the tumor. Significant efforts have been made over the years to be able to distinguish between patients who do and do not need intensive therapies. This review summarizes the history and current state of clinical risk stratification, pathologic diagnosis and genetics. Recent developments in correlation between genetics and pathology, genome-wide association studies and the biology of medulloblastoma metastasis are discussed in detail. The current state of clinical treatment trials are reviewed and placed into the perspective of potential novel therapies in the near term.

    Medulloblastoma: historical overviewMedulloblastoma is the most common malig-nant brain tumor of childhood. It is estimated that the annual incidence rate is 0.5 per 100,000 in children younger than 15 years of age [1]. There is a bimodal peak of incidence between ages 34 and 89 years [2]. Adult cases account for less than 1% of the total and rarely occur in individuals beyond the age of 40 years.

    Prior to the 1970s, the 5-year survival for pat-ients with this tumor was 30% with deaths due to tumor recurrence and leptomeningeal dis-semination of tumor (across the surface of the brain and spinal cord) [3]. In the past 30 years, improvements in surgery, imaging, pathologic stratification, radiotherapy and chemotherapy have changed that dramatically. The rate of long-term survival or cure now stands at 7080% for standard-risk older children. In infants, adults and high-risk cases the outcome is less favorable [4]. Patients with recurrent disease following ini-tial therapy bear the worst prognosis: median survival of 3 years of age, with minimal residual tumor and no metastases are low risk [4,5].

    PathologyMedulloblastomas are stratified pathologically into classic, desmoplastic/nodular, extensively nodular, anaplastic, large-cell, myogenic differen-tiation and melanocytic differentiation variants.

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    future science group www.futuremedicine.com 379

    Of these, the anaplastic and large-cell variants carry poor prognoses [912] and are characterized by high frequencies of metastatic disease [9,10,13]. Owing to the shared poor prognosis and the recognition that the pathologic features of these two variants are often intermixed in the same tumor, it has been proposed that anaplastic and large-cell medulloblastomas are a continuum and should be combined into a large-cell/anaplastic category [9,12,14].

    There is disagreement between studies as to whether the desmoplastic variant bears a bet-ter prognosis than the classic medulloblastoma [1518]. It has been proposed that the variable results are due to the use of different diagnos-tic criteria for the two entities [18,19]. In infants, several studies have shown better survival for the group of patients with desmoplastic/nodular as well extensively nodular medullo blastomas in comparison with the group with classic medulloblastomas [20,21].

    GeneticsSince different morphological variants of medul-loblastoma exist and, to some extent, these phenotypes can predict biological behavior and outcome, the idea that these represent dif-ferent disease entities with diverging pathway perturbations and underlying mechanisms of tumorigenesis has long been suspected [22].

    Traditional chromosomal analysis revealed that the most common cytogenetic abnormal-ity in medulloblastomas is isochromosome 17q, which is present in 3040% of tumors [23,24]. Isochromosome 17q results in loss of 17p and gain of 17q. The loss of 17p, which can also occur via interstitial deletion or monosomy of 17, occurs in as many as 50% of medullo blastomas and is associated with poor prognosis [11,2528]. Loss of chromosome 6 is observed indepen-dently of isochromosome 7q and is associated with good prognosis. There are also abnormali-ties of chromosomes 7, 8, 9 and 11 [29], as well as double minutes. Although traditional cyto-genetics has been progressively supplanted by comparative genomic hybridization, FISH, spec-tral karyotyping and high-throughput genom-ics, many of these early findings point the way toward our current understanding of genetic risk stratification in this disease.

    The current consensus classification of medul-loblastomas, based primarily on transcriptomic data recognizes four principal subgroups (WNT, SHH, group 3 and 4) [30].

    The WNT pathway was initially recognized to be involved in medulloblastoma pathogenesis via the rare association with familial adenoma-tous polyposis in Turcot syndrome [31]. Germline mutations of the APC gene in that syndrome ablate the normal negative regulation of APC on the WNT pathway. Somatic APC mutations in sporadic medulloblastoma are relatively rare. Only 34% of tumors contain sequence changes [32,33]. In addition, while the APC mutations in familial adenomatous poly posis patients result in a truncated, nonfunctional protein, those in spo-radic tumors are missense mutations of undeter-mined functional significance. A more common route of WNT involvement in medulloblastoma occurs via b-catenin mutations, which occur in 510% of tumors and activate WNT pathway signaling [3235]. Using immunohistochemistry for b-catenin as a marker, between 18 and 25% of medulloblastomas show evidence of WNT activation [34,36,37]. This correlates fairly well with the results of transcriptomic studies [38]. Virtually all WNT tumors have classic histol-ogy. The promising long-term prognosis of this subgroup [36,3844] correlates well with the obser-vation that there is frequent deletion of chromo-some 6. Recently, it has been recognized that experimental WNT medulloblastomas arise from a distinct subset of developing cells in the developing rhombic lip and dorsal midbrain. This accounts for the presentation of the human tumors in the dorsal midbrain. Both the cell of origin and the involvement of the WNT path-way are integral to the good long-term prognosis of these patients [45].

    The SHH subgroup historically derives from the recognition that PTCH, the human homo-log of the Drosophila segment polarity gene is involved in medulloblastoma pathogenesis. Located on chromosome 9, where allelic losses are found in 1018% of medulloblastomas [46,47], PTCH is mutated in Gorlins syndrome patients who develop nevi, basal cell cancers and desmoplastic medulloblastomas [4648]. Inactivating Ptch mutations are also found in approximately 8% of sporadic medullo blastomas [4953]. SHH signaling plays an important role in cerebellar development since SHH is a mitogen for cerebellar granule cell precursors, one of the putative sources of medulloblastoma [54]. Path-way activation involves the secretion of SHH by Purkinje cells, which binds to Ptch at the cer-ebellar granule cell membrane, releases the nor-mal Ptch inhibition of SMO and activates GLI

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    Review Aref & Croul

    transcription factors, resulting in granule cell proliferation [55]. Besides Ptch mutation