Comparative oncology: The paradigmatic example of canine and human mast cell neoplasms

In humans, advanced mast cell (MC) neoplasms are rare malignancies with a poor prognosis. Only a few preclinical models are available, and current treatment options are limited. In dogs, MC neoplasms are the most frequent malignant skin tumours. Unlike low‐grade MC neoplasms, high‐grade MC disorders usually have a poor prognosis with short survival. In both species, neoplastic MCs display activating KIT mutations, which are considered to contribute to disease evolution. Therefore, tyrosine kinase inhibitors against KIT have been developed. Unfortunately, clinical responses are unpredictable and often transient, which remains a clinical challenge in both species. Therefore, current efforts focus on the development of new improved treatment strategies. The field of comparative oncology may assist in these efforts and accelerate human and canine research regarding diagnosis, prognostication, and novel therapies. In this article, we review the current status of comparative oncology approaches and perspectives in the field of MC neoplasms.

In the canine system, cutaneous MC tumours (MCTs) are frequently detected and can be divided into less aggressive and more aggressive variants. The aggressive MCT can involve regional lymph nodes and/or visceral organs. [5][6][7]14 It is worth noting that MCTs in dogs are the most frequent malignant skin tumours. [5][6][7] Whereas histological low-grade MCTs have a good-to-excellent prognosis, metastasized and/or high-grade canine MCTs have a poor prognosis with short STs. [5][6][7] In dogs as well as in humans, it is of considerable importance to establish the correct diagnosis and to define whether the patient has an indolent MC neoplasm or an advanced category of the disease.
Comparative oncology is an emerging field that is based on the assumption that the biochemical processes and pathogenesis contributing to the evolution and progression of spontaneous malignancies in human and animal species are often comparable and that these similarities can be exploited in basic and translational science. [41][42][43][44] Based on this assumption, many different projects in the field of comparative oncology have been initiated. [44][45][46] One emerging example with future potential might be research on human and canine MC neoplasms. 47,48 As mentioned before, these neoplasms have several aspects in common, such as KIT mutations and a poor outcome in advanced stages.
There is hope that the field of comparative oncology can assist in our efforts to accelerate human and canine research on MCs and to improve diagnosis, prognostication and, ultimately, therapy in MC neoplasms. However, a number of questions regarding classification, prognostication and therapy of advanced MC disorders in both species remain. For example, it remains to be explored whether the diagnostic criteria and prognostic parameters that have recently been established in human MC neoplasms, can be employed in a similar, analogous way in the canine system.
In the present article, we review the current status of comparative oncology approaches in the field of MC neoplasms, with special focus on diagnosis, prognostication and standard treatment of patients with MC neoplasm in humans and dogs.  55,56 In addition, it remains unknown whether elevated serum tryptase levels might serve as diagnostic and follow-up parameters in dogs with MCTs in the same way as in human MC neoplasms. If one or more of these markers were determined to be of diagnostic value, these parameters could be employed as additional diagnostic criteria in dogs in future proposals. For the moment, the diagnostic tools available to classify canine MCTs are morphological assessments, histochemical studies (such as toluidine blue) and immunohistochemical investigations of neoplastic MCs, which may provide a solid basis for forthcoming comparative oncology studies ( Figure 1). [57][58][59][60] While the diagnosis of canine MCT can be made by a fine-needle aspiration-based cytology in a majority of the cases, histological grading of MCT requires histological examinations of the primary tumour site. Occasionally, immunohistochemistry may be necessary to confirm the diagnosis, especially when MCT is undifferentiated. Potential diagnostic criteria for canine MCTs and a comparison with diagnostic criteria for human SM are shown in Table 1.

| CLINICAL AND HISTOLOGICAL PRESENTATION OF MC NEOPLASMS
Depending on the affected organ-system(s), human mastocytosis can be divided into CM, SM, and localized MCTs. 2,[49][50][51][52][53][54] In the human system, the classification of MC neoplasms proposed by the World Health Organization discriminates between several distinct subvariants of CM and SM (Table 2). CM is defined by typical features of cutaneous lesions, a diagnostic skin-histology, and the absence of criteria sufficient to establish the diagnosis SM. 2,8,[49][50][51][52]61 A positive Darier's sign supports the conclusion that the patient is suffering from CM. However, identical cutaneous lesions are also seen in SM (Table 1). Therefore, the lesion is described as mastocytosis in the skin (MIS), and only a BM examination can clarify the final diagnosis (CM or SM) in adults. 2,8,[49][50][51][52]61 It is noteworthy that a minimal infiltration of the BM with neoplastic MCs may be well detected in CM. Even if two minor SM criteria (but no major SM criterion) are found in these patients, the diagnosis remains CM. 2,8,[49][50][51][52]61 However, once three minor criteria are documented or at least one major and one minor criteria are found, the final diagnosis is SM. 2,49-52 SM variants that have a grave prognosis (SM-AHN, ASM, and MCL) are also referred to as advanced SM. The major criteria for advanced SM are the documented presence of SM-induced organ damage, also known as C-findings (Table 3). 2,49-52 Such C-findings include marked persistent cytopenia, hepatic disease with elevated liver enzymes and ascites, marked osteolyses (with or without pathologic fractures), or malabsorption with hypalbuminemia and weight loss. 2,[49][50][51][52] In veterinary oncology, the most frequent clinical presentation of MC neoplasms in dogs is a solitary cutaneous nodule ( Figure 1). [5][6][7] MCTs can have a very heterogeneous appearance, not always accompanied by a positive Darier's sign, and the diagnosis of MCT can usually be established by a cytological examination of a fine-needle aspiration. [5][6][7] However, fine-needle biopsy-derived cytology is not sufficient to determine the grade of the MCT. Therefore, an additional histological examination of the lesion is mandatory to determine the grade of the disease and thus to anticipate the behavior of the MCT in dogs. A number of clinical, molecular, and histopathological variables are considered to be of prognostic significance in canine MCTs (Table S1). The first grading system was established by Patnaik et al 57 in 1984 (Table S2). This classification system divides canine cutaneous MCTs into three grades, namely, grade 1 with well-differentiated morphology, grade 2 with intermediately differentiated cells, and grade 3 with poorly differentiated MCs. 57 Although this grading system correlates with the clinical outcome of patients, its practical application showed some inconsistencies because of inter-observer variations. 58,62,63 Therefore, a 2-tier histopathological grading system has been proposed by Kiupel et al 58 in 2011 (Table S3), with the aim to improve the Patnaik system regarding prognostication of patients.
Nonetheless, it has been shown that approximately 15% to 20% of dogs with Kiupel low-grade MCTs have metastatic disease at presentation, 64 suggesting that there is still a need for better prognostication and an improved grading system in cutaneous MCTs, and for the moment, many centres are using both the Patnaik and Kiupel prognostication model in individual canine MCT patients. In the   67 However, these studies are laborious and therefore are not regarded as standard practice.

| DETECTION OF KIT EXPRESSION AND EVALUATION OF THE KIT MUTATIONAL STATUS
In dogs, immunohistochemical staining for KIT is of prognostic value, as different staining patterns correlate with recurrence-rate and Expression of CD2 or CD25 in mast cells + -   Table 3. b B-findings and C-findings are listed in Table 3  In the canine system, clinical staging includes a complete blood count with differential counts, serum chemistry and cytological or histological and immunohistochemical studies of the primary organ site and of the secondary (cutaneous or extracutaneous, metastatic) lesion(s) (Figure 1). 57,58 Fine-needle aspiration biopsies of regional lymph nodes (even if normal in size), abdominal ultrasound (with or without fine-needle aspiration of liver and spleen regardless of the sonographic appearance) and thoracic radiography are usually performed. 64,73 In the majority of dogs, MCTs disseminate first into the regional lymph nodes, then to the spleen and liver, and finally into other visceral organs, whereas lung involvement is rare. 74     staging (investigations of BM, blood and/or other organs), and/or prognostication of canine MCTs remain to be determined in future studies.

| UTILITY OF TRYPTASE AND OTHER DISEASE PARAMETERS DURING FOLLOW-UP
In the human system, the basal serum tryptase level is a robust and widely used follow-up parameter for patients with SM. [77][78][79][80] In particular, tryptase is a serine protease that is produced and stored almost exclusively in MCs and is secreted from resting MCs into plasma in a constitutive manner. 81,82 As a result, the basal serum tryptase level reflects the total body burden of MCs in healthy individuals (normal physiologic baseline: 1-15 ng/mL) and in patients with mastocytosis. 82 In contrast, patients with CM generally have basal serum tryptase levels within the normal range, clearly elevated tryptase levels (>20 ng/mL) are almost invariably found in patients with SM and therefore also serve as a minor diagnostic criterion of SM. 49 In addition, the tryptase level is an important follow-up parameter in SM. 61 Likewise, whereas patients with ISM have stable tryptase levels, a steadily increasing basal serum tryptase is indicative of advanced SM and/or disease progression. Furthermore, effective therapy is usually accompanied by a decrease in the serum tryptase level. 31,[35][36][37]83 In addition, blood counts and (other) serum chemistry parameters, such as alkaline phosphatase levels, are employed in the follow-up of human patients with SM. 61 Furthermore, liver and spleen size (by ultrasound), lymph nodes, and the osteodensitometry (T score) are measured routinely in the follow-up of patients with SM. 61 In dogs, physical examination, in particular investigating the initial location(s) of the MCTs and regional lymph nodes, determination of blood counts and serum chemistry parameters, and abdominal ultrasound, are standard in the follow-up of MCT patients. 75 At present, no MC-specific serum-or plasma markers are available that could be employed as follow-up parameters for dogs with MCTs. Whether the inclusion of serologic MC-related follow-up parameters (like serum tryptase) would be of value for determining the course of canine MCT and for evaluation of responses to conventional or novel therapies remains to be determined in future studies.

| Treatment of Advanced SM and selection of anti-neoplastic drugs
In human patients with advanced MC neoplasm with slow progression, interferon-alpha (IFN-A) or cladribine (2CdA) have been considered as first-line therapy with response rates ranging between 10% and 30%. [83][84][85][86] In patients with rapidly progressing ASM and MCL, more intensive therapy is required. In patients who are fit and eligible, polychemotherapy containing fludarabine or 2CdA, often in combination with cytosine arabinoside (ARA-C), are recommended, and in case of a sufficient response (clear cytoreduction), haematopoietic stemcell transplantation (SCT) should be considered. 37,49,87,88 In patients with poor performance status, 2CdA or hydroxyurea can be used as palliate drugs. A more recent approach is to apply targeted drugs directed against KIT D816V. 34-37 Midostaurin has recently been described to exert major disease-modifying activity in patients with advanced SM including MCL. 35,37 In addition, midostaurin is useful for cytoreducing tumour load in patients with advanced SM prior to SCT. 37 In patients with MC sarcoma, the treatment recommendation is similar to that in MCL because most of these patients transform to ASM or MCL within weeks or months. 37,49,87 In addition to chemotherapy, radiation therapy may be applied in MCS patients. 37,49,87 However, most patients with MCS die within a short time-interval despite intensive therapy.
In dogs with resectable MCTs without distant metastasis, surgical intervention is the first-line therapy, especially for solitary grade I MCTs. 7 In cases of incomplete resection ("dirty margins"), re-excision is recommended whenever possible. 89 If re-excision is considered impossible because of diffuse infiltration or massive tumour expansion, post-surgical radiation therapy is required, and in high-grade and/or metastasized MCT, radiation is usually combined with systemic anti-neoplastic therapy. 90,91 In dogs with high-grade metastasized or unresectable MCTs, first-line treatment often consists of TKI therapy using masitinib (EU), toceranib (US, EU) or imatinib (Japan) or cytoreductive chemotherapeutics (eg, vinblastine, lomustine; often in combination with steroids). [38][39][40] More recently, the combination of toceranib with radiation therapy has been described to be an effective anti-neoplastic treatment approach for dogs with MCTs, including high-grade MCTs. 92 In case of primary or secondary resistance, different chemotherapy protocols using alternative cytoreductive drugs or combinations with TKI are prescribed. [93][94][95][96][97][98][99] However, although remissions can be achieved, responses are usually short-lived. [93][94][95][96][97][98][99] A summary of interventional treatment approaches in advanced SM in the human and canine system is depicted in Table 4.
Concomitant treatment with H1-and H2-blockers is recommended to prevent mediator-related side effects in all patients. 100,101 Whether the use of proton-pump inhibitors may improve the symptomatic treatment effect in canine MCT patients like in human SM patients remained to be evaluated in future clinical trials. 61

| Concluding remarks and future perspectives
Comparative oncology is an emerging field that supports the develop- Such comparative strategies require an interdisciplinary dialogue between human and veterinary medicine.