Cancer of the urinary bladder is a neoplasm with considerable importance in veterinary medicine, given its high incidence in several domestic animal species and its life-threatening character. Bladder cancer in companion animals shows a complex and still poorly understood biopathology, and this lack of knowledge has limited therapeutic progress over the years. The development and validation of Transitional cell carcinoma (TCC) molecular markers is of great importance for scientists and clinicians alike. Somatic and hereditary BRAF mutations received much attention and can now be detected via multiple types of tests, sometimes in useful combinations with CNA tests. Urine-based tests for detecting BRAF may allow the early detection of post-treatment relapse.
1. Prevalence and Aetiology
Urinary bladder cancer represents about 2 percent of all reported canine malign neoplasms
[1][2], and it is rare in cats
[3][4]. The majority of canine bladder tumours are malignant and of epithelial origin
[5]. Transitional cell carcinoma (TCC), also referred to as urothelial carcinoma (UC) is the most frequent canine urinary bladder tumour
[6]. The aetiology of the canine disease is thought to be multifactorial. Several risk factors have been proposed to play a role, such as exposure to older topical insecticides for flea and tick control, obesity, female sex, herbicides and breed predisposition (e.g., Scottish Terrier, West Highland White Terrier, Shetland Sheepdog, Beagle and others)
[7][8][9].
Urinary bladder tumours are also frequently observed in cattle grazing on pastures infested by toxic ferns (mainly
Pteridium spp.) (reviewed by Gil da Costa et al., 2012
[10]) and have been reported in other ruminant species
[11]. The aetiology of bladder cancer in ruminants is much clearer than in companion animals. Grazing on poisonous ferns has been identified as a decisive risk factor since the mid-1900s
[12][13]. The occurrence of bladder tumours in cattle is closely related to the geographical distribution of toxic ferns and originates in a syndrome known as bovine enzootic haematuria. Bladder lesions have also been reproduced experimentally in multiple laboratory animal models, by administering the fern or its toxin ptaquiloside
[14][15][16][17][18]. Ptaquiloside is a DNA-alkylating agent, which causes point mutations, as well as structural and numeric chromosomal aberrations
[19][20][21]. This toxin also has immunotoxic properties, contributing to reducing immune surveillance against newly arising neoplasms
[22][23][24]. Other ferns containing ptaquiloside (e.g.,
Pteris spp. and
Dryopteris spp.) or structurally related illudane toxins (for a review of illudane toxins, see Gil da Costa et al., 2013
[25]) do occur and have been reported to cause bladder cancer in cattle in various locations worldwide
[26][27]. Bracken consumption has been proposed to facilitate a persistent abortive infection of the bovine bladder by bovine papillomavirus (BPV) types 1, 2, 13 and 14
[28][29][30][31]. These Delta BPV types are hypothesised to contribute to bladder carcinogenesis, by activating the platelet-derived growth factor receptor beta (PDGFR-β) through their oncoprotein E5
[32][33].
2. Histology and Grading
The urothelium is a hierarchically organised tissue, comprising basal, intermediate and umbrella cells, and the development of urothelial cancers progressively subverts this normal hierarchical structure
[34]. Data obtained from human patients and from laboratory animals have helped trace different types of urothelial carcinoma to specific cell populations of origin and different differentiation pathways
[35][36][37][38]. Although the pathogenesis of canine bladder cancer is less clear, it seems that numerous genetic changes involving key genes are shared between human and dogs, reflecting a conserved mechanism of pathogenesis
[39]. Current laboratory models of bladder cancer, based on rats and mice, are out of the scope of the present work, but several comprehensive reviews have been recently published
[40][41].
About 90 percent of all urinary bladder tumours in dogs are epithelial and malignant, and 50 to 90 percent of these will metastasise. Urinary bladder carcinomas include transitional cell carcinoma, squamous cell carcinoma, adenocarcinoma and undifferentiated carcinoma (
Table 1)
[2]. Among primary epithelial neoplasms of the urinary bladder, TCC represents 75 to 90 percent in dogs
[2]. More than 50 percent of overall malignant tumours show involvement of both the bladder and the urethra
[5]. Benign epithelial tumours are rare, and only 10 percent of canine urinary bladder tumours are of mesenchymal origin, with smooth muscle neoplasms being the most common
[2].
Table 1. Primary bladder tumours in dogs, types and percentages (the most common tumours reported are shown; adapted from Meuten and Meuten, 2016
[2]).
Canine transitional cell carcinomas are classified based on their growth patterns (
Table 2). They can be divided into papillary (papillary or cauliflower exophytic growths projected into the lumen) and non-papillary (plaques, flat nodules or masses), and into infiltrating or non-infiltrating tumours
[2]. The consistent observation is that the majority (90 percent) of canine TCC shows an infiltrating growth pattern
[2]. Papillary infiltrating TCC is one of the most common variants and likely to metastasise. The non-papillary and infiltrating type is the second or the most common variant in dogs, depending on the study, and shows a high tendency to metastasise
[2].
Table 2. Classification of canine TCC subtypes based on growth pattern (adapted from Meuten and Meuten, 2016
[2]).
In cattle, the histological features of bladder tumours are quite different, with 51.2% of purely epithelial tumours, 17.4% of purely mesenchymal tumours and 31.4% of coexisting epithelial and mesenchymal tumours, and numerous benign tumours (papillomas, haemangiomas, etc.)
[42].
Over the years, several different grading systems for urothelial carcinomas in humans have been proposed and applied to veterinary tumours, looking for a better approach on the evaluation of the tumours’ biologic behaviour
[2][43]. Meuten and Meuten (2016) proposed a more simplified classification of TCC into low or high grade (
Figure S1, Supplementary Material could be found in
https://www.mdpi.com/2306-7381/9/3/107#supplementary). The majority of canine TCC are invasive, high grade and at an advanced stage when diagnosed
[2]. In affected dogs, high-grade tumours seem to be more common in terriers than in non-terrier breeds
[44].
3. Diagnosis
3.1. Clinical Signs and Differential Diagnosis
Clinical signs in dogs with TCC are usually nonspecific, many of which, such as dysuria, haematuria and pollakiuria, are commonly observed with urinary tract disease
[2][5]. Concurrent urinary tract infections (UTI) are often present
[2]. Tumour growth can lead to obstruction of the ureters or urethra and invasion and disruption of the normal functioning of the urethral sphincter
[45]. On physical examination, a thickening of the urethra and of the trigone region of the bladder and enlargement of iliac lymph nodes may be found and, occasionally, a mass in the bladder or a distended bladder
[1]. Urinary tract obstruction can occur prior to the development of lethal metastasis and is a common cause of death in dogs with TCC
[6]. However, a normal physical examination does not exclude the presence of a TCC
[46]. Differential diagnoses of canine TCC comprise other neoplasia, chronic cystitis, polypoid cystitis, fibroepithelial polyps, granulomatous cystitis/urethritis, calculi, among others
[1].
In cattle, haematuria and weight loss are the main symptoms of bladder cancer and often present as part of the previously mentioned syndrome, known as bovine enzootic haematuria
[26][47].
3.2. Diagnostic Procedures and Staging
Diagnostic procedures for TCC should include a complete blood cell count, serum biochemistry profile, urinalysis, urine culture (to rule out lower urinary tract infection) and cancer staging
[1][46].
Definitive diagnosis of TCC can be established via histopathologic examination of tumour tissue and/or cytology of a representative sample
[2]. Biopsies can be collected by means of cystotomy, cystoscopy and traumatic catheterisation, invasive procedures that usually involve general anaesthesia
[1][39]. Cytological samples may be obtained by direct or ultrasound-guided percutaneous mass fine-needle aspirate or traumatic catheterisation
[48].
The risk of tumour implantation or seeding/dissemination throughout other tissues following diagnostic or therapeutic procedures has been reported, especially after surgical manipulation of the tumour
[49][50][51][52]. Even though reports are scarce, these should be carefully interpreted. Where possible, less invasive techniques should be favoured.
A less invasive technique consists of performing a cytology from urine sediment. If tumour cells are present, a diagnosis can be achieved
[2]. However, negative results do not rule out TCC. In one study, malignant cells were seen in only 30% of dogs with lower urinary tract tumours
[5]. Thus, cytological results must be interpreted with caution, especially upon the presence of inflammation of the urinary tract, and correlation with clinical data is essential for reaching a diagnosis.
Clinical staging of canine TCC includes thoracic and abdominal radiography, abdominal ultrasonography and specific urinary tract imaging
[1][46]. Computer tomography (CT) has increasingly been used to aid in diagnostics and staging, particularly for more accurately evaluating the urethra and to detect metastases
[53].
Figure S2 (Supplementary Material, could be found in
https://www.mdpi.com/2306-7381/9/3/107#supplementary) shows the TNM (tumour, node, metastasis) classification for clinical staging of canine bladder cancer
[54]. The TNM stage at diagnosis for TCC has shown to be strongly related to prognosis. More advanced TNM stage at the time of diagnosis was significantly associated with shorter survival
[7][53]. Tumours located in the urethra were also associated with shorter survival time than ones in the bladder
[53]. TCC has rarely been curable; however, with current therapies, many dogs will achieve stable disease for several months after diagnosis
[1].
3.3. Recent Advances in Diagnostic Techniques for UC
As mentioned above, clinical presentation of canine TCC is comparable to several other (and far more common) urinary tract disorders. Consequently, the diagnosis of TCC is often delayed, allowing the tumour to grow, infiltrate and metastasise
[2]. In fact, most TCCs are currently not diagnosed until they reach an advanced stage and thus present poor prognosis
[55]. As such, effective (and preferably less invasive) methods for the early identification of UC are needed, which could improve responses to treatment and survival rates among affected dogs
[56]. In particular, because TCC tumour cells and metabolites may be shed into urine, this body fluid is likely to present tumour-specific molecules that could be used as biomarkers for tumour detection using easily accessible samples collected through non-invasive techniques
[46][57]. Over recent years, several potential biomarkers for canine TCC have been investigated for diagnostic/screening or prognostic purposes, and a few of them are currently available for commercial use.
These and other potential biomarkers are summarised in Table 3 and detailed below.
Table 3. Current and potential markers for clinical applications in canine transitional cell carcinoma.