Neoplastic diseaseIsocitrate Dehydrogenase 1 Expression in Canine Gliomas
Introduction
The incidence of canine intracranial tumours is 14.5–20 per 100,000, with the prevalence of primary brain tumours reported to be 2.3% (Solleveld et al., 1986, Dobson et al., 2002, Song et al., 2013). This is comparable to the 25.48 per 100,000 person-years incidence of primary brain tumours reported in people (de Robles et al., 2015). Of the primary brain tumours described in dogs, 45.1–51.5% are of meningeal origin and 31.4–36.6% are of glial (astrocytic or oligodendroglial) cell origin (Snyder et al., 2006, Song et al., 2013). In veterinary medicine there is an increasing demand from dog owners to definitively treat pets diagnosed with these tumours. In people, accurate differentiation between the different glial tumour types is essential to enable appropriate treatment, as the clinical course, response to therapy and survival differs between neoplasm types. For example, the median survival time for people diagnosed with grade II oligodendrogliomas is 139.1 months compared with grade II oligoastrocytomas, which have a median survival time of 79.2 months, or glioblastoma multiforme having a median survival time of only 9.7 months (Okamoto et al., 2004, Johnson and O'Neill, 2012). Furthermore, glioblastomas are generally considered resistant to chemotherapeutic treatment compared with oligodendrogliomas with 1p/19q deletions, for example, which are more sensitive to chemotherapy and radiotherapy (Stupp et al., 2006).
Differentiating between the different glial tumours can be challenging both ante mortem and post mortem. Accurate histological classification and grading of glial tumours is jeopardized by the subjective criteria currently utilized to classify and grade these tumours. In people the most challenging problem associated with evaluation of glioma is differentiating between oligodendroglioma, oligoastrocytoma and astrocytoma, as they often have morphological similarities, which can make them difficult to differentiate (Coons et al., 1997). Furthermore, it can be difficult to distinguish between benign gliosis and diffuse gliomas (Tanboon et al., 2016). Therefore, a need exists, both in human and veterinary medicine, to improve diagnostic accuracy. This may be achieved by identifying molecular markers or genetic events specific to the histological tumour type and grade, which can aid the histological classification and grading of these tumours, in addition to acting as therapeutic targets and predictors of prognosis. Recently, antigen expression was investigated in a series of 97 canine central nervous system tumours utilizing a panel of 28 antibodies, with an aim being to identify biomarkers that may aid in the diagnosis and differentiation of these tumours (Spitzbarth et al., 2017). Results of the profiling demonstrated an overlap in the antigen expression profiles across the tumour types, concluding that immunohistochemistry (IHC) does not yet provide sufficient information to provide a definitive diagnosis and that examination of sections stained by haematoxylin and eosin (HE) remains the gold standard (Spitzbarth et al., 2017).
Isocitrate dehydrogenase 1 (IDH1) is a cytoplasmic and peroxisomal enzyme that catalyses the oxidative decarboxylation of isocitrate to α-ketoglutamate in the citric acid cycle (Bleeker et al., 2010, Mondesir et al., 2016, Waitkus et al., 2016). IDH1 is also the main producer of nicotinamide adenine dinucleotide phosphate (NADPH) in the brain (Bleeker et al., 2010). A missense mutation in the IDH1 gene at codon 132 (IDH1 R132H) results in substitution of the amino acid arginine with histidine. Consequently, inhibition of IDH1 wild-type activity occurs, in addition to a reduction in α-ketogluterate and NADPH production and the production of NADP+ and D-2-hydroxygluterate, the latter being an oncometabolite, which causes epigenetic alterations (Bleeker et al., 2010, Mondesir et al., 2016, Tanboon et al., 2016, Waitkus et al., 2016). In people the IDH1 R132H point mutation occurs in >70% of low-grade gliomas and has been demonstrated to be a significant independent prognostic factor for both progression-free survival and overall survival (Hartmann et al., 2009, van den Bent et al., 2010). Furthermore, it aids in the differentiation between benign gliosis and diffuse gliomas (Capper et al., 2010a). Gliomas are therefore routinely assessed for IDH1 R132H, either immunohistochemically or via direct DNA sequencing, to aid glial tumour diagnosis and molecular classification (Mondesir et al., 2016, Tanboon et al., 2016).
Since canine gliomas resemble their human counterpart both morphologically and immunohistochemically, it is likely they share similar genetic abnormalities such as the IDH1 R132H point mutation; this may be useful in the ante-mortem diagnosis of canine glial tumours (Lipsitz et al., 2003, Stoica et al., 2004, Higgins et al., 2010, Reitman et al., 2010). The canine IDH1 gene is also comparable with the human gene, having 96.0–96.9% similarity at the protein level (Reitman et al., 2010, Kawakami et al., 2018). If the IDH1 R132H point mutation is demonstrated in canine gliomas, the prognostic significance and therapies utilized for human glioma patients with this mutation may be transferable to the dog. It would also further support a comparative approach to molecular medicine. Therefore, the aims of this study were: (1) to investigate a series of canine glial tumours for the IDH1 R132H point mutation using immunohistochemical analysis; and (2) to determine if the IDH1 R132H point mutation is associated with glial tumour type and histological grade.
Section snippets
Materials and Methods
A retrospective search for canine glial tumour specimens obtained at necropsy examination or by surgical biopsy between 1978 and 2014 was performed using the anatomical pathology database at The University of Melbourne Veterinary Teaching Hospital, Werribee, Victoria, Australia. Tumour specimens retrieved from the archive included formalin-fixed and paraffin wax-embedded astrocytomas, oligodendrogliomas, oligoastrocytomas and gliomatosis cerebri. Included as control tissues were two
Results
Thirty-one tumour specimens from dogs of variable age, breed and sex (Supplementary Table 2) were examined. Thirty tumour specimens were obtained at necropsy examination and one from surgical biopsy. Two tumours were located in the spinal cord (one each of anaplastic oligodendroglioma and anaplastic oligoastrocytoma), the remaining 29 tumours were intracranial. Tumour samples included seven astrocytomas (five diffuse astrocytomas, two anaplastic astrocytomas), 16 oligodendroglial tumours (nine
Discussion
While the IDH1 R132H point mutation occurs commonly in low-grade human gliomas, the mutation was not identified in this population of canine glial tumours. The presence of the IDH1 R132H point mutation was therefore not found to be associated with tumour type or histological grade. As expected, however, pan-IDH1 (IDH1 wild-type and mutated IDH1) was identified in all tumour specimens.
It is assumed the pan-IDH1 immunoreactivity identified in the tumour specimens represents IDH1 wild-type
Acknowledgments
The authors acknowledge the Anatomical Pathology Department, Melbourne Health, Parkville, Victoria, Australia, for providing the non-identifiable human control tissues, in addition to Professor B. Summers and Dr A. Stent for their contribution in reviewing the histopathological slides. The authors also wish to acknowledge P. Benham for his assistance with the IHC. This work was supported by the University of Melbourne Veterinary Hospital Research Grant, Werribee, Victoria, Australia. The
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