Genomics Underlying Familial Thyroid Carcinoma in Dogs

*Yun Yu and Richard R.P.A. Crooijmans*

### **Abstract**

Thyroid cancer is the most common endocrine neoplasm occurring in dogs. We reported familial thyroid follicular cell carcinomas (FCCs) in 54 Dutch German longhaired pointer (GLP) dogs. We investigated the genetics of the FCC in these dogs, including the germline risk mutations and somatic driver mutations. We identified the germline risk factor locating in the *TPO* gene for these hereditary FCCs through a combination of genome-wide association study (GWAS) and homozygosity mapping analyses using SNP array genotype data and whole-genome sequencing data. We further investigated the somatic mutation landscape of these FCCs using high-depth whole-genome sequencing technology of the tumors. A recurrent missense mutation in the *GNAS* gene was identified as a very promising driver mutation. We validated this somatic mutation using Sanger sequencing and revealed a prevalence of 62.5% among thyroid tumors identified in the Dutch GLPs. In addition, we can also review the findings in genetics of other canine thyroid tumors in recent years.

**Keywords:** thyroid carcinoma, dog, animal, germline risk factor, somatic mutation

### **1. Introduction**

Dogs have a pair of thyroid glands that are located on each side of windpipe in the neck. Dogs, like humans, develop thyroid cancers in these glands. Thyroid cancer is the most common endocrine neoplasm, accounting for 1%–4% of all canine neoplasms [1]. Thyroid neoplasms can be classified as adenomas or carcinomas. Adenoma is a benign neoplasm, and carcinoma is malignant. Carcinomas are distinguished from adenomas by capsular and/or vascular invasion. Approximately 60%–90% of canine thyroid neoplasms are carcinomas [1]. The thyroid cancer in dogs can be unilateral or bilateral, which account for 67%–75% and 25%–35%, respectively [2]. The typical clinical sign of a thyroid tumor in dogs is the palpable mass in the neck. Other clinical complaints related to thyroid carcinoma include intermittent cough, alopecia, polyuria, polydipsia, weight loss [3].

A canine thyroid tumor can originate from either follicular cells or C-cells (parafollicular cells) in thyroid gland. Thyroid tumors in dogs consist of mainly eight histological subtypes according to a classification of World Health Organization [4], and they are highly similar to corresponding types in humans in histological growth pattern. There are seven subtypes of thyroid tumor originating from follicular cells, including follicular thyroid carcinoma (FTC), papillary thyroid carcinoma (PTC), compact

thyroid carcinoma (CTC), follicular-compact thyroid carcinoma (FCTC), poorly differentiated thyroid carcinoma, undifferentiated (anaplastic) thyroid carcinoma, and carcinosarcoma. The thyroid cancer originating from C-cells is called medullary thyroid carcinoma (MTC). Most of canine thyroid tumor originates from follicular cells, accounting for 64%–71%, which is also called non-medullary thyroid carcinoma (NMTC). Thyroglobulin is a protein made by the follicular cells of the thyroid gland, and calcitonin is made by C-cells [5, 6]. Thyroglobulin and calcitonin immunohistochemistry experiments are useful to differentiate thyroid cancer originating from follicular cells or C-cells. Thyroid carcinoma originating from follicular cells exhibits positive immunolabeling for thyroglobulin, and MTC exhibits strong immunoreactivity for calcitonin [1].

Thyroid neoplasms in dogs are usually non-functional. Clinical or biochemical evidence of hyperthyroidism can be observed in less than 25% of affected dogs. Meanwhile, hypothyroidism is also possible because of destruction of normal thyroid tissue, suppression of thyroid-stimulating hormone (TSH) secretion, and subsequent atrophy of normal thyroid tissue [2].

Metastasis is common in canine thyroid carcinoma. Approximately one out of three dog patients have metastasis by the time of diagnosis [1, 7], and 65%–90% of untreated dogs are diagnosed with regional or distant metastasis at necropsy [2]. The lung and regional lymph nodes are the most common organs where canine thyroid carcinoma metastasizes to. Metastasis to other organs is occasionally seen, such as the adrenal glands, liver, heart, brain, kidneys, and bone [1]. In dogs, risk of thyroid tumor increases with age. The average age at diagnosis is between 9 and 10 years. Dogs at age > 10 years have a significantly higher risk of thyroid cancer than younger dogs [1, 8].

Canine thyroid tumors show also some differences if compared with humans. For instance, in humans, females have approximately 2–3 times higher risk for nonmedullary thyroid cancer than males [9]. While, in dogs, most studies reported equal incidence of thyroid carcinoma in both sexes [3, 8]. The mechanism underlying the sex bias in incidence of thyroid cancer in humans is still unclear. Another difference is that the most prevalent subtype of thyroid cancer is different between humans and dogs. In humans, PTC is the most frequently diagnosed thyroid cancer, accounting for approximately 85%–90% [10]. However, in dogs, PTC is relatively rare, and FTC is the most common type of thyroid carcinoma [1].
