In vivo monitoring of viral gene injection therapy in ovine batten disease

Abstract

Recent progress in gene therapy for CLN5 and CLN6 ovine Batten disease (neuronal ceroid-lipofuscinosis, NCL) has highlighted the need for in vivo monitoring to provide compelling data for translation to human therapies while surveying the long term impacts of these therapies. Sheep have brains similar to those of non-human primates, have similar cognitive ability and are social domesticated animals. Neurological examinations are carried out monthly. Simple maze tests have proved to be informative and data are now being collected via a precision GPS system. Affected sheep can be distinguished from normal sheep from 6 months of age. Routine computer tomography (CT) scanning is simple and economic. This provides 3-D images of the intracranial space, which is decreased in affected animals by 9 months following cranial thickening into the space left by neurodegeneration. It also allows modelling of the increased volume of the lateral and third ventricles in affected sheep compared to unaffected controls and the identification of cranial markers, ideal for developing co-ordinates for stereotactic injections. CT scans are benchmarked against more sophisticated magnetic resonance structural imaging (MRI) at specific times in disease development. While observations of the sheep in the field and in maze studies indicate a relatively early visual impairment that correlates with the atrophy of the visual cortex, electroretinography (ERG) shows a later onset and slower development of retinal degeneration. Differences are seen in dark-adapted electroretinography between controls and CLN5 affected sheep at 7 months and later in CLN6 affected sheep. All these changes are progressive, differences becoming stark with time. The success of our gene therapies beyond the natural endpoint of disease highlights the desirability of being able to gather in vivo data over the long term, to determine the long term efficacy of treatment and to monitor for any other symptoms that may develop, particularly more mild ones that may be amendable to secondary treatments.