Maze testing of sheep for early detection of Batten disease : A thesis submitted in fulfilment of the requirements for the Degree of Master of Science at Lincoln University

Abstract

Batten disease is a group fatal inherited neurodegenerative diseases typically manifesting in humans in childhood. The diseases are genetically heterogeneous and characterised by cognitive loss, psychomotor deterioration, retinal degeneration, brain atrophy, seizures and premature death. Treatments for Batten disease are at an early stage, but this study is part of the development of gene therapy for the disease caused by mutations in CLN5 and CLN6. Many of the forms of Batten disease that occur in humans have also occurred spontaneously in large animals, including ovine species. Two naturally occurring sheep models of NCL are maintained at Lincoln University, a CLN6 form in South Hampshire sheep and a CLN5 form in Borderdale sheep. Affected sheep are normal at birth, but develop clinical symptoms at around 10-14 months of age. These include progressive loss of sight, and psychomotor decline, as a result of severe cortical loss and loss of retinal photoreceptors. Premature death generally occurs at around 2 years of age. Cognitive decline is a common symptom of human Batten disease and also occurs in the CLN5 and CLN6 ovine forms of the disease. The purpose of this study was to investigate if a closed-field maze test could be a useful method for determining loss of cognition in sheep with Batten disease. It was hypothesised that cognitive decline may affect an affected sheep’s ability to transit a mazes, hence offering a potential method for early diagnosis of the disease. This would help speed up the optimisation of therapeutic interventions, by providing an early measure of their efficacy. The study consisted of three experiments. The first experiment involved the development of a maze that was navigable by the sheep, and complex enough to discern between normal and affected animals. Time to complete, and path length, determined by high precision GPS, were both used as measures of the sheep’s ability. The second experiment used the final iteration of the maze developed in experiment one, to determine when differences could be seen between normal and affected animals, and measure the effectiveness of the gene therapy treatment. The final experiment used a more cognitively complex maze that used visual cues to indicate the correct path through the maze. This study established that sheep were able to navigate a complex maze. The performance of the normal cohort of both breeds was the same. All animals were slowest and took their longest paths when first exposed to the maze, but were faster and took shorter paths in subsequent transits. The untreated affected cohorts of both genotypes were generally slower and took a longer path length through the maze than their unaffected counterparts. The loss of ability of individual affected (both untreated and treated) animals to negotiate the maze correlated well with other measures of disease progression. Due to the very variable performance of affected and treated animals, the experiment did not achieve the objective of being able to discern the onset of Batten disease at an earlier stage than other measures currently in use.