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  • Suzzi, Stefano; Ahrendt, Reiner; Hans, Stefan; Semenova, Svetlana A.; Chekuru, Avinash; Wirsching, Paul; Kroehne, Volker; Bilican, Saygin; Sayed, Shady; Winkler, Sylke; Spiess, Sandra; Machate, Anja; Kaslin, Jan; Panula, Pertti; Brand, Michael (2021)
    LRRK2 gain-of-function is considered a major cause of Parkinson's disease (PD) in humans. However, pathogenicity of LRRK2 loss-of-function in animal models is controversial. Here we show that deletion of the entire zebrafish lrrk2 locus elicits a pleomorphic transient brain phenotype in maternal-zygotic mutant embryos (mzLrrk2). In contrast to lrrk2, the paralog gene lrrk1 is virtually not expressed in the brain of both wild-type and mzLrrk2 fish at different developmental stages. Notably, we found reduced catecholaminergic neurons, the main target of PD, in specific cell populations in the brains of mzLrrk2 larvae, but not adult fish. Strikingly, age-dependent accumulation of monoamine oxidase (MAO)-dependent catabolic signatures within mzLrrk2 brains revealed a previously undescribed interaction between LRRK2 and MAO biological activities. Our results highlight mzLrrk2 zebrafish as a tractable tool to study LRRK2 loss-of-function in vivo, and suggest a link between LRRK2 and MAO, potentially of relevance in the prodromic stages of PD. Author summary Parkinson's disease is the second most common degenerative disorder of the brain. Mutations of the LRRK2 gene are the most recurrent genetic cause of pathology, and are thought to result in a more active LRRK2 protein, a large enzyme whose biological function is unclear. Therefore, LRRK2 inhibitors are regarded as promising therapeutics. However, mouse models do not reproduce human pathology unless they also lack LRRK1, and there is evidence of dominant negative effects of LRRK2 mutations. Therefore, the characterization of reliable LRRK2 knockout models might provide insights. In our study, we used the zebrafish as a tractable model to study both early developmental and adult phenotypes resulting from the loss of the entire endogenous lrrk2 gene. We found that mutant embryos displayed subtle brain phenotypes, including reduction of catecholaminergic neurons, the main target of human disease, that spontaneously resolved with development, and a late-onset and progressive increase of dopamine and serotonin degradation consistent with increased MAO enzyme activity. Our results suggest that similar defects might occur in the pre-symptomatic stage of the disease in humans, and warrant further evaluation of LRRK2 inhibition in a therapeutic perspective.
  • Genome Aggregation Database Prod T; Genome Aggregation Database Consor; 23andMe Res Team; Whiffin, Nicola; Armean, Irina M.; Kleinman, Aaron; Havulinna, Aki S.; Daly, Mark; Palotie, Aarno; MacArthur, Daniel G.; Ripatti, Samuli (2020)
    Analysis of large genomic datasets, including gnomAD, reveals that partial LRRK2 loss of function is not strongly associated with diseases, serving as an example of how human genetics can be leveraged for target validation in drug discovery. Human genetic variants predicted to cause loss-of-function of protein-coding genes (pLoF variants) provide natural in vivo models of human gene inactivation and can be valuable indicators of gene function and the potential toxicity of therapeutic inhibitors targeting these genes(1,2). Gain-of-kinase-function variants in LRRK2 are known to significantly increase the risk of Parkinson's disease(3,4), suggesting that inhibition of LRRK2 kinase activity is a promising therapeutic strategy. While preclinical studies in model organisms have raised some on-target toxicity concerns(5-8), the biological consequences of LRRK2 inhibition have not been well characterized in humans. Here, we systematically analyze pLoF variants in LRRK2 observed across 141,456 individuals sequenced in the Genome Aggregation Database (gnomAD)(9), 49,960 exome-sequenced individuals from the UK Biobank and over 4 million participants in the 23andMe genotyped dataset. After stringent variant curation, we identify 1,455 individuals with high-confidence pLoF variants in LRRK2. Experimental validation of three variants, combined with previous work(10), confirmed reduced protein levels in 82.5% of our cohort. We show that heterozygous pLoF variants in LRRK2 reduce LRRK2 protein levels but that these are not strongly associated with any specific phenotype or disease state. Our results demonstrate the value of large-scale genomic databases and phenotyping of human loss-of-function carriers for target validation in drug discovery.