*Article* **SDHB and SDHA Immunohistochemistry in Canine Pheochromocytomas**

#### **Firas M. Abed 1, Melissa A. Brown 2, Omar A. Al-Mahmood 3 and Michael J. Dark 1,2,4,\***


Received: 27 August 2020; Accepted: 15 September 2020; Published: 17 September 2020

**Simple Summary:** Pheochromocytomas are adrenal tumors that occur in both dogs and people. One of the more common gene families involved in the development of this tumor in people is succinate dehydrogenase (SDH). In people, immunohistochemistry can be used with biopsy samples to predict gene pathways that may be involved in the development of the tumor. This is faster and cheaper than performing extensive sequencing to determine if genes are involved. We tested 35 dog tumors to determine how likely SDH mutations were. While our data sugges<sup>t</sup> significant numbers of SDH mutations, these mutations do not appear to be associated with tumor aggression.

**Abstract:** Pheochromocytomas (PCs) are tumors arising from the chroma ffin cells of the adrenal glands and are the most common tumors of the adrenal medulla in animals. In people, these are highly correlated to inherited gene mutations in the succinate dehydrogenase (SDH) pathway; however, to date, little work has been done on the genetic basis of these tumors in animals. In humans, immunohistochemistry has proven valuable as a screening technique for SDH mutations. Human PCs that lack succinate dehydrogenase B (SDHB) immunoreactivity have a high rate of mutation in the SDH family of genes, while human PCs lacking succinate dehydrogenase A (SDHA) immunoreactivity have mutations in the SDHA gene. To determine if these results are similar for dogs, we performed SDHA and SDHB immunohistochemistry on 35 canine formalin-fixed, para ffin-embedded (FFPE) PCs. Interestingly, there was a loss of immunoreactivity for both SDHA and SDHB in four samples (11%), suggesting a mutation in SDHx including SDHA. An additional 25 (71%) lacked immunoreactivity for SDHB, while retaining SDHA immunoreactivity. These data sugges<sup>t</sup> that 29 out of the 35 (82%) may have an SDH family mutation other than SDHA. Further work is needed to determine if canine SDH immunohistochemistry on PCs correlates to genetic mutations that are similar to human PCs.

**Keywords:** dog; pheochromocytoma; SDH; Immunohistochemistry
