Bilateral Basal Nuclei Vacuolar Lesions: A Novel and Emerging Potential Background Finding in Beagle Dogs

Abstract

In this case presentation, the speaker and co-authors represented a group of scientists engaged in a cross-institutional precompetitive working group focused on elucidating a novel background change in the basal nuclei of Beagle dogs. The group’s ongoing efforts since first publication of the lesion in 2024 enabled further characterization of the lesion and revealed additional incidences in control animals. The characterization, including newly discovered lesion features, and terminology of the condition were outlined and suggestions for interpretation in nonclinical toxicity studies were given.

A condition characterized mainly by vacuolation and gliosis of the caudate nucleus has recently emerged as a potential background finding in Beagle dogs. As previously described, ¹ the lesion affected, as a minimum, the caudate nucleus bilaterally (typically discovered on a level 7 or 9 H&E-stained slide [Figure 3 in Palazzi et al ² ]). However, because these findings were in several cases not limited to the caudate nuclei but extended to the putamen (Figure 1A), and/or globus pallidus, “basal nuclei” were selected as a locator of these findings. As described before, ¹ the change was characterized by vacuolation of the neuropil and white matter tracts (Figure 1B), variable (astro)gliosis with the presence of gemistocytic astrocytes (Figure 1C), and demyelination. To confirm that the lesion belongs to this entity, it was recommended to establish bilaterality of the lesion, characterize the associated gliosis with appropriate immunohistochemistry (IHC) markers for astrocytes and microglia (eg, GFAP, Iba-1) and demonstrate the demyelinating component of the lesion using relevant stains (eg, Luxol Fast Blue, Myelin Basic Protein-IHC). The term suggested for documenting this change in a nonclinical study setting was “Degeneration neuropil, basal nuclei, bilateral.”

Figure 1.

(A) Subgross demonstration of pallor in the dorsolateral aspect of the caudate nucleus (black flag) and putamen (gray flag), HE, 1X, courtesy of Madhu Babu Ravi; (B) vacuoles are present in the neuropil and white matter tracts (flags) with evidence of gliosis, HE, 20X; and (C) gemistocytic astrocytes (arrows), vacuoles in the neuropil (asterisk), and vacuoles in white matter tract (flag), HE, 40X.

To date, this lesion has been observed in a total of 26 male and female young adult dogs (Marshall BioResources) used in nonclinical safety studies by various institutions worldwide, affecting animals across all dose groups, including Controls, in approximately similar proportions (nine in High Dose; six in Mid Dose; six in Low Dose; five in Control). Newly identified cases displayed a spectrum of additional lesion features to what was described before, including involvement of the putamen and caudal medulla; presence of apoptotic cells/apoptotic bodies in vacuolar lesions without signs of coagulative necrosis; and coagulative necrosis and cavitation with the presence of gitter cells.

Given the presence of this basal nucleus lesion in several Control animals, the evidence suggests that this change should be regarded as a novel background finding in the Beagle brain. However, a slightly higher occurrence was noted in High Dose animals, along with an overrepresentation of the lesion in animals treated with compounds exhibiting central nervous system tissue exposure. Therefore, it was pointed out that test item-related exacerbation of the lesion is a possibility to be taken into account. Furthermore, it was noted that, given the likelihood of this lesion appearing in a test item-treated animal by chance, misinterpreting it as a test item-related change could significantly impact the risk-benefit assessment and decision-making regarding the further development of a compound.

The etiology remains unknown. In general, genetic alterations leading to toxic-metabolic disturbances in the brain, intoxications, alterations in blood supply, nutrient deficiencies, or viral infection can be discussed as possible causes. Investigations, including genealogy and whole genome sequencing with substantial support from the breeder, are currently underway. An association with steroid-responsive meningitis-arteritis (Beagle Pain Syndrome) is unlikely due to the lack of relevant vascular inflammation in the affected animals.

During the question-and-answer session, it was suggested that the lesion might have a hemodynamic origin. As is typical for nonclinical drug safety studies, the myocardium, aorta, and tissue-bound blood vessels were examined in the cases described, but no evidence of vascular defects was found. However, as not all hemodynamic changes will result in morphologic changes visible by light microscopy, their lack may only serve as an indication toward a different etiology. Another question concerned sequential sectioning of an affected brain. It was explained that sequential sectioning had been performed in two of the cases. ¹ In one animal, this revealed a central area of necrosis that had not been visible on the original standard slide containing the caudate nucleus. In contrast, in the other animal, serial sectioning revealed that the lesion had a similar appearance in all sections, both caudally and cranially, and across both hemispheres. This suggests that the timing of the euthanasia, as part of the planned study protocol, following the primary insult that caused tissue necrosis, might have influenced the histopathological findings, with vacuolation and gliosis representing remnants of an earlier necrotizing lesion. This hypothesis, along with others, is currently being explored by the aforementioned working group.