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Huntington disease (HD) is caused by the expansion of CAG triplet repeats in exon 1 of the huntingtin (HTT) gene, which also encodes the first 17 amino acids (N-17) that can modulate the toxicity of the expanded polyQ repeat. N-17 are conserved in a wide range of species and are found to influence the subcellular distribution of mutant Htt. Moreover, N-17 is subject to many posttranslational modifications that may regulate the function, stability, and distribution of HTT. However, the function of Htt exon 1 and its influence on the normal Htt remains to be fully investigated. By investigating a knock-in mouse model that lacks Htt exon1, we found that deletion of Htt exon1 does not affect the survival of mice and differentiation of cultured mouse neurons. Furthermore, the lack of Htt exon 1 does not alter the subcellular distribution of Htt, autophagy protein expression, and global gene transcription in the mouse brain. These results suggest that removing the entire exon 1 of Htt could be a therapeutic approach to eliminate expanded polyQ toxicity.

IntroductionHuntington’s disease (HD) is caused by expanded CAG repeats in the huntingtin gene (HTT) and is characterized by late-onset neurodegeneration that primarily affects the striatum. Several studies have shown that mutant HTT can also affect neuronal development, contributing to the late-onset neurodegeneration. However, it is currently unclear whether mutant HTT impairs the development of glial cells, which is important for understanding whether mutant HTT affects glial cells during early brain development.MethodsUsing HD knock-in mice that express full-length mutant HTT with a 140 glutamine repeat at the endogenous level, we analyzed the numbers of astrocytes and oligodendrocytes from postnatal day 1 to 3 months of age via Western blotting and immunocytochemistry. We also performed electron microscopy, RNAseq analysis, and quantitative RT-PCR.ResultsThe numbers of astrocytes and oligodendrocytes were not significantly altered in postnatal HD KI mice compared to wild type (WT) mice. Consistently, glial protein expression levels were not significantly different between HD KI and WT mice. However, at 3 months of age, myelin protein expression was reduced in HD KI mice, as evidenced by Western blotting and immunocytochemical results. Electron microscopy revealed a slight but significant reduction in myelin thickness of axons in the HD KI mouse brain at 3 months of age. RNAseq analysis did not show significant reductions in myelin-related genes in postnatal HD KI mice.ConclusionThese data suggest that cytoplasmic mutant HTT, rather than nuclear mutant HTT, mediates myelination defects in the early stages of the disease without impacting the differentiation and maturation of glial cells.