title: Early genotype- and sex-dependent differences in the F344tgHD rat model for Huntington’s Disease
Creators: name: Ratz-Wirsching, Veronika
Publisher: name: Friedrich-Alexander-Universität Erlangen-Nürnberg (FAU)
Year: 2024

description: To this day, Huntington Disease (HD) is an uncurable disease with detrimental symptomatology leading to death within 10 - 15 years after diagnosis. Despite ongoing efforts and promising new targets, current therapy focuses on mere alleviation of symptoms. Regarding interventional therapeutic approaches, the prodromal phase of HD is coming into focus, because severe neuropathological damage and distinguishable motor impairments are still absent but close monitoring allows for reliable detection of subtle changes in cognition and personality. Nonetheless, defining this phase and predicting its course is difficult. Another open topic in HD research is the impact of sex on disease severity and progression. A comprehensive investigation of the impact of sex in HD patients is impeded through the genetic disparity and the non standardizable environment. The usage of animal models enables circumvention of these restrictions by high standardization of both genetic and environmental factors. However, a common malpractice in basic and preclinical research with animals, is the omission of sex as biological variable. Not only does this pose a neglection of the 3R principles, but it also weakens the translatability of results. In HD research, differences have been described between female and males in various rodent models, whenever sex was included as factor, making a clear separation of and control for sex obligate. One of the few rat models for HD is the F344/HanSvh-Tg(tmHTT51CAG) (F344tgHD) model, with a truncated Htt construct comprising 51 CAG repeats. A comprehensive sex comparison of this model as well as in-depth analysis of potential gene-dosage (zygosity) effects at early age were still owing. Hence, this study presents a comprehensive, longitudinal phenotyping with classical as well as automated behavioral assays in combination with investigation of molecular HD hallmarks, namely striatal medium spony neurons (MSN) loss, striatal volume reduction and presence of mutant HTT aggregates in female and male F344tgHD rats aged 1 to 8 months. Additionally, HTT most abundant interactor HAP40 protein level and distribution as well as its interplay with (m)HTT was investigated to determine its status quo in this HD model. This study describes a prodromal-like phase in F344tgHD rats, where subtle behavioral impairments are already detectable before occurrence of striatal cell loss. Additionally, differences between female and male rats were observed beginning with middle age especially in the motor and the sensorimotor domains. Male individuals were generally less active, demonstrated different gait characteristics than females and displayed anxiety reduced behavior. Investigation of HAP40 revealed ubiquitous expression on level of the striatum and an age dependent increase in male but not female animals. The animals’ genotype had no impact on protein levels or cellular distribution and no direct interaction between mHTT and HAP40 could be detected in the F344tgHD model. Our findings confirm that the congenic F344tgHD line is of highest face-validity, with close resemblance of the human genetics, i.e., a relatively low number of CAG repeats, a slowly progressing pathology with a prodromal-like phase and a subtle phenotype. By differentiation of the sexes regarding HD-related changes and by specifying the prodromal-like phase in this model, we pave the way for future treatment studies.