Objective: Huntington’s disease (HD) is caused by an abnormal expansion of a glutamine tract (polyQ) in huntingtin (HTT), a large scaffold protein with numerous reported binding partners and multiple roles in diverse cellular processes. Although disturbance of HTT’s physiological functions has long been implicated in the disease pathogenesis, little is known how HTT itself is regulated in vivo.

Background: Converging evidence support that HAP40 is an important interactor of HTT and a potential pathogenic factor in HD, potentially modulating HTT’s protein conformation and becoming aberrantly expressed in tissues from HD. There is no known functional evaluation of HAP40 in any physiological setting, its effects on HTT’s normal functions and on the toxicity of mutant HTT remains unclear.

Method: In a proteomic study for the HTT homolog (dHtt) in Drosophila, we isolated a ~40Kda protein, encoded by a novel cg8134 gene, as the strongest binding partner for endogenous dHtt.
To investigate the physiology functions of dhap40 in Drosophila, we generated dhap40 loss of function alleles. Using the CRISPR/Cas9 approach, we obtained multiple independent mutant lines that harbor various molecular lesions, in coding region of dhap40 gene.

Results: We demonstrated cg8134 is the functional ortholog of mammalian HAP40 with sequence homology to mammalian HAP40, and maintains conserved physical and functional interactions with HTT. Flies lacking cg8134, renamed as dhap40, manifest similar adult phenotypes as dhtt-null flies, including compromised mobility and shortened lifespan, while both flies mutated for dhap40 or dhtt, and human cells with HAP40- or HTT-knockout, share similar autophagic deficits. HAP40 and HTT control each other’s proteins stability in both flies and human cells. PolyQ expansion in HTT does not significantly affect its affinity for HAP40.  In Drosophila models of HD, HAP40 differentially affects the neuronal toxicity of full-length mutant HTT but not HTT exon 1, potentially through its effect on both the stability and conformation of HTT protein.

Conclusion: Our results uncover a conserved mechanism governing the stability and in vivo functions of HTT in evolutionarily distant species, and demonstrate HAP40 as a central regulator of HTT and a potential modulator of HD pathogenesis, thereby establishing HAP40 as a promising candidate for the “HTT-lowering” strategy against HD.

References: This abstract was previously presented at the CHDI meeting in Palm Springs, 2/28/2020 and will be presented at the AAN meeting in Toronto 4/2020

« Back to MDS Virtual Congress 2020

MDS Abstracts - https://www.mdsabstracts.org/abstract/the-roles-of-huntingtin-associated-protein-40-in-huntingtin-functions-and-huntingtons-disease-pathogenesis/