# Case Report: Atypical Nodular Dermatofibrosis and Renal Cysts in a Bichon Frise with a BRCA2 Mutation and No FLCN Mutation

## TL;DR

A Bichon Frise dog with skin nodules and kidney cysts was found to have a BRCA2 gene mutation, not the usual FLCN mutation, suggesting new genetic causes for this rare condition.

## Contribution

This is the first report linking BRCA2 mutations to nodular dermatofibrosis and renal cysts in a Bichon Frise.

## Key findings

- The dog had non-cancerous skin nodules with abnormal collagen accumulation.
- Multiple kidney cysts were detected, but their nature could not be confirmed.
- BRCA2 gene mutations were identified, not the typical FLCN mutation.

## Abstract

Nodular dermatofibrosis (ND) is a rare disease causing multiple firm skin nodules, often linked to kidney cysts or tumors. It typically affects specific breeds like the German Shepherd and is associated with mutations in the folliculin (FLCN) gene. This case describes a Bichon Frise, a breed not previously linked to the disease and lacking the usual FLCN mutation. Instead, the dog had a BRCA2 gene mutation, known for roles in DNA repair and tumor suppression. The dog showed both skin nodules and kidney cysts—a combination not previously reported in this breed. Tests confirmed the nodules were not cancerous but showed abnormal collagen accumulation. The kidney cysts’ type could not be confirmed. This case suggests ND may occur in other breeds and result from genetic changes beyond FLCN. It highlights the need to explore diverse genetic factors in the disease’s development. Greater understanding can help veterinarians diagnose and manage the condition more effectively, improving the welfare of affected dogs.

A 10-year-old intact female Bichon Frise presented with multiple firm skin nodules on all four limbs. The nodules progressively increased in number and size over seven months. Diagnostic tests included cytology of fine-needle aspirates, histopathology of skin biopsies, radiography, and abdominal ultrasonography. Cytology revealed spindle-shaped mesenchymal cells and extracellular matrix components, and histopathology confirmed ND characterized by mature collagen deposition without evidence of malignancy. Ultrasonography detected multiple kidney cysts bilaterally, although their exact nature (benign or malignant) could not be confirmed histologically. Genetic analysis was performed, revealing no mutation in the traditionally implicated FLCN gene but multiple nonsynonymous mutations in the BRCA2 gene. This case suggests a potential association between BRCA2 gene mutations and the development of ND with renal cystic lesions, broadening the known genetic causes beyond the commonly reported FLCN mutation. Regular genetic screening and close monitoring of dermatological and renal conditions in atypical breeds are recommended. To the best of current knowledge, this is the first case report demonstrating ND and renal cysts associated with BRCA2 mutations in a Bichon Frise.