Medulloblastoma, WNT-activated/SHH-activated: clinical impact of molecular analysis and histogenetic evaluation
Medulloblastoma (MDB) is a small cell poorly differentiated embryonal tumor of the cerebellum, which more frequently compromises children. Overall prognosis is favorable, but dependent of stage, histopathological pattern and molecular group. Approximately 30% of the affected patients will die from the disease. WHO 2016 Classification of Tumors of the Central Nervous System (CNS) has been classified MDB into four principal groups: WNT-activated MDB, SHH-activated MDB, group 3 MDB, and group 4 MDB. WNT-activated MDB is associated to monosomy 6, CTNNB1, DDX3X and TP53 mutations, beta-catenin nuclear immunoexpression, and a better prognosis than SHH-activated MDB.
WNT-activated tumors account approximately for 10% of cases of MDBs, and are thought to arise from cells in the dorsal brain stem/lower rhombic lip progenitor cells. SHH-activated MDB more frequently arises in the lateral hemispheres of the cerebellum, and clinical outcome in this group is variable. TP53-mutant SHHactivated MDB usually shows the large cell/anaplastic pattern, and can be related to MYCN amplification, GLI2 amplification and 17p loss. TP53-wildtype SHH-activated MDB is more commonly of desmoplastic/nodular morphology, and can be related to PTCH1 deletion and 10q loss. Gene expression and methylation profiling is the gold standard for defining molecular groups of MDB. In immunohistochemistry assays, anti-GAB1 antibody expression is positive in tumors showing SHH pathway activation or PTCH mutation, while positive immunoexpression for YAP1 antibody can be only found in WNT-activated and SHH-activated MDB.
KeywordsMedulloblastoma WNT-activated SHH-activated Embryonal neuroepithelial tumor Central nervous system tumors Prognosis
WNT-activated and SHHactivated medulloblastomas: general findings
The role of WNT and SHH pathways in the neoplastic transformation of cerebellum/dorsal stem cells and development of MDB
Gene expression and methylation profiling is the gold standard for defining molecular groups of MDB. WNT-activated MDB (group 1 of WHO 2016 Classification of Tumors of the Central Nervous System) is typically found in children between 7 and 14 years and has an excellent prognosis with standard therapeutic approaches [1, 2, 4, 6, 7, 16, 19, 22, 28]. Nearly all WNT-activated MDB cases are classic tumors. Frequently mutated genes in WNT-activated MDB are CTNNB1 (90% of cases), TP53 (12.5% of cases), SMARCA4 (27% of cases), KMT2D (12.5% of cases), and DDX3X (50% of cases). Around 85% MDB that are characterized by WNT pathway activation show monosomy 6 and/or harbor a CTNNB1 mutation in exon 3, and these genetic alterations determine the positive immunoexpression for beta-catenin antibodies in tumor cell nuclei [1, 2, 7, 12, 16, 22, 29, 30]. TP53 mutations in WNT-activated MDB are not related to a worse prognosis, and the presence of APC germline mutations rare in this group of tumors [4, 5, 7, 8, 9, 10, 22, 31]. WNT pathway is fundamental for the normal development and organogenesis, including neural cells. This pathway regulates intracellular localization of the beta-catenin protein. Inactive WNT pathway determines the association between beta-catenin with a multimeric protein complex, which include AXIN1 protein, glycogen synthase kinase-3-beta, and APC gene [4, 8, 9, 11, 16, 17, 18, 28, 31, 32]. With this ligation, beta-catenin is phosphorylated and targeted for degradation via ubiquitin-dependent proteasomal pathways. Activation of WNT pathway determines glycogen synthase kinase-3-beta inhibition, destabilization of the APC/glycogen synthase kinase-3-beta/AXIN1 complex, and accumulation of beta-catenin in the nucleus. Translocation of beta-catenin to the nucleus leads to upregulation of cyclin D1 and MYC (promitotic genes) [7, 8, 9, 11, 16, 18, 26, 28, 31, 32]. CTNNB1 mutation compromises specifically the glycogen synthase kinase-3-beta phosphorylation domain of beta-catenin and promotes upregulation and nuclear accumulation of aberrant TCF/LEF target genes and tumorigenesis. CTNNB1 stimulates the producing of inhibitors such as Frizzled-related protein (sFRP) and WNT inhibitor factor 1 (WIF-1). Positive nuclear beta-catenin immunoexpression is strongly associated to CTNNB1 mutations in more than 80% of cases of MDB. The presence of APC mutations in sporadic cases of MDB does not determine upregulation of the WNT pathway [4, 7, 8, 9, 11, 16, 17, 18, 26, 28, 31, 32].
Clinical outcomes in SHH-activated MDB (group 2 of WHO 2016 Classification of Tumors of the Central Nervous System) are variable. SHH-activated MDB less frequently determines metastatic lesions than group 3 MDB [1, 5, 7, 22, 23, 28, 33]. Spread within the neuroaxis is a common feature of SHH-activated and TP53 mutant MDB. The analysis of PTHC mutations in sporadic MDBs identified the SHH signaling pathway in MDB tumorigenesis. Positive immunoexpression for YAP1 antibody can be only found in WNT-activated and SHH-activated MDB. Positive immunoexpression for GAB1 and YAP1 is widespread and strong in the great majority of cases of non-desmoplastic SHH-activated MDB. In nodular/desmoplastic MDB, strong expression for BAG1 and YAP1 is found within internodular regions [4, 6, 7, 22, 26, 28, 34]. SHH-activated MDB with cytological anaplasia is more prone to exhibit strong p53 expression, which is related to germline TP53 mutations. Desmoplastic/nodular MDB shows pathological activation of the SHH pathway, which is related to PTCH1, SMO, SHH, GLI2, MYCN, and SUFU gene mutations. Desmoplastic/nodular MDB correspond approximately for 20% of cases of this tumor and are not related to isochromosome 17q [1, 4, 6, 7, 10, 28, 31, 32, 35]. Activation of SHH pathway can be evaluated by GAB1 and TNFRSF16 in immunohistochemistry technique, in special in internodular areas [4, 8, 10, 11, 14, 26, 36]. MDB with extensive nodularity is usually SHH-activated tumor with an excellent prognosis, with overall survival rates of 95%. Large cell/anaplastic MDB is most frequent between SHH-activated and group 3 MDB, and are associated to GLI2 and MYCN amplification, TP53 mutations, a massive genomic rearrangement called chromothripsis, and a poor outcome with standard therapies. Five-year progression-free survival for large cell/anaplastic MDB is 30–40% [4, 5, 7, 9, 19, 31, 32, 37].
MDB, SHH-activated, and TP53 mutant are rare tumors with poor prognosis that compromise patients between 4 and 17 years and are defined as embryonal tumors of the cerebellum with evidence of SHH pathway activation and either germline or somatic TP53 mutation [1, 4, 7, 12, 28, 30, 33]. SHH pathway activation in TP53-mutant tumors is related to MYCN amplification, 17p loss, and GLI2, MYCN, or SHH gene amplification, and mutations in PTCH, SUFU, and SMO are uncommon. TP53-mutant SHH-activated MDB usually shows the large cell/anaplastic pattern. GLI2 amplification denotes a high-risk tumor [1, 4, 7, 12, 28, 33].
MDB, SHH-activated, and TP53-wildtype are related to germline or somatic mutations in the negative regulators PTCH or SUFU and somatic mutations in SMO, and generally compromise children aged four or less years [1, 7, 12, 22, 24, 25, 38]. Mutations DD3X or KMT2D genes, and amplification of MYCN or MYCL also can be found between these lesions. Some cases are associated to deletions in 9q and 14q chromosomes. TP53-wildtype SHH-activated MDB is more commonly of desmoplastic/nodular morphology, and is associated to PTCH deletion and 10q loss, which denote a low-risk tumor in infants [1, 4, 7, 12, 22, 24, 25, 37].
The SHH pathway plays a critical role in normal cerebellar development. SHH ligand is secreted by Purkinje neurons, which promote mitogenesis in external granular layer progenitor cells [4, 6, 10, 24, 28]. The response to SHH signal is controlled by PTCH and SMO, which are transmembrane proteins. PTCH suppresses SMO activity in the absence of SHH ligand. If there is SHH stimulation, this inhibition is released and promotes SMO-induced transcriptional response, which is regulated by GLI-1, GLI-2, and GLI-3, a family of transcription factors. SUFU (suppressor of fused) cooperate with Slimb (BTRCP, F-box protein) to inhibit GLI-1 mediated transcription [1, 24, 25, 37, 39, 40, 41, 42]. SHH-activated MDB (20% of cases) arise through multiple alternative components, such as PTCH mutations (around 10% of cases), SMO-activating mutations (around 5% of cases), and SUFU mutations (0–10% of cases) [1, 24, 25, 26, 37, 39, 40, 41]. Genetic alterations of the SHH pathway determines an inadequate constitutive activation of the signaling cascade, downstream mitotic effects regulated by overexpression of GLI proteins and PTCH gene (a GLI-dependent target gene), and downstream MYCN, cyclin D1, and BMI-1 function. Common genetic alterations in MDB also include MYC amplifications (5–15%), MYCN amplifications (5–15%), PIK3CA mutation (5%), CASP8 (35%), HIC-1 (35%), and RASSF1A (90%) [1, 24, 26, 37, 39, 40, 43].
Medulloblastomas subtypes: most common molecular, histopathological, and clinical findings
Final considerations and therapies targeting WNT and/or SHH pathways in cases of medullobastoma
Molecular indicators in MDB with favorable outcome include WNT-activated tumors, monosomy 6, CTNNB1 mutation, and beta-catenin nuclear immunoexpression. Molecular indicators indicative of poor outcome include MYC/MYCN amplifications, loss of chromosome 17p, and gain of chromosome 17q [4, 6, 7, 30, 45, 46]. Patients who developed MDB can be treated with a combination of surgery and/or radiotherapy/chemotherapy regimens. Surgery is considered a standard part of treatment for histologic confirmation of tumor type and as a means to improve outcome [1, 42, 43, 45, 46, 47, 48]. Standard-risk medulloblastoma can be defined as total or near-total surgical resection with less than or equal to 1.5 cm2 (measured on axial plane) of residual tumor on early postoperative MRI, no CNS metastasis on MRI, no tumor cells on the cytospin of lumbar CSF, and no clinical evidence of extra-CNS metastasis. Low-risk MDB includes the WNT subgroup, which exhibits ß-catenin mutation (mandatory testing), or ß-catenin nuclear immuno-positivity by IHC (mandatory testing) ß-catenin nuclear immuno-positivity by IHC and monosomy 6 (optional testing). MDB can be grouped as a low-risk tumor if the patient has undergone total/near-total tumor resection. These cases can receive conventionally fractionated radiotherapy (once a day) with a dose of 54 Gy to the primary tumor and 18.0 Gy to the craniospinal axis. Chemotherapy is also a standard element in the treatment of MDB. Chemotherapy can be used to delay the need for radiation therapy in 20 to 40% of children younger than 3 to 4 years with non-disseminated MDB. Distinct chemotherapeutic regimens have been used, including the use of cisplatin, lomustine, vincristine, cyclophosphamide, etoposide, or even concomitant high-dose intravenous methotrexate and/or intrathecal methotrexate or mafosfamide, and/or intraventricular methotrexate [1, 42, 43, 45, 46, 47, 48, 49].
Prognosis in children is dependent on age, metastatic status at presentation, postoperative Karnofsky Performance Scale (KPS) score, molecular subtype, and completeness of surgical resection. Histopathologic subclassification of MDBs can modify therapeutic planning [4, 6, 7, 9, 19, 22, 30, 50, 51]. A higher prevalence of PTCH and SMO mutations in adult SHH-activated MDBs can predict responsiveness to inhibitors of the receptor SMO, and SHH-inhibiting drugs like vismodegib that act downstream SMO activity are currently in development. Investigation of target drugs able to suppress the WNT pathway can be also future adjuvant therapeutic modality [7, 13, 19, 41, 43, 45, 47, 52].
Compliance with ethical standards
Conflict of interest
The author declares that there is no conflict of interests.
This article does not contain any studies with human participants or animals performed by the author.
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