We tested whether proteins implicated in Huntington's and other polyglutamine (polyQ) expansion diseases can cause axonal transport defects. Reduction of Drosophila huntingtin and expression of proteins containing pathogenic polyQ repeats disrupt axonal transport. Pathogenic polyQ proteins accumulate in axonal and nuclear inclusions, titrate soluble motor proteins, and cause neuronal apoptosis and organismal death. Expression of a cytoplasmic polyQ repeat protein causes adult retinal degeneration, axonal blockages in larval neurons, and larval lethality, but not neuronal apoptosis or nuclear inclusions. A nuclear polyQ repeat protein induces neuronal apoptosis and larval lethality but no axonal blockages. We suggest that pathogenic polyQ proteins cause neuronal dysfunction and organismal death by two non-mutually exclusive mechanisms. One mechanism requires nuclear accumulation and induces apoptosis; the other interferes with axonal transport. Thus, disruption of axonal transport by pathogenic polyQ proteins could contribute to early neuropathology in Huntington's and other polyQ expansion diseases.

Although the process of mammary tumorigenesis requires multiple genetic events, it is unclear to what extent carcinogenesis proceeds through preferred secondary pathways following a specific initiating oncogenic event. Similarly, the extent to which established mammary tumors remain dependent on individual mutations for maintenance of the transformed state is unknown. Here we use the tetracycline regulatory system to conditionally express the human c-MYC oncogene in the mammary epithelium of transgenic mice. MYC encodes a transcription factor implicated in multiple human cancers. In particular, amplification and overexpression of c-MYC in human breast cancers is associated with poor prognosis, although the genetic mechanisms by which c-MYC promotes tumor progression are poorly understood. We show that deregulated c-MYC expression in this inducible system results in the formation of invasive mammary adenocarcinomas, many of which fully regress following c-MYC deinduction. Approximately half of these tumors harbor spontaneous activating point mutations in the ras family of proto-oncogenes with a strong preference for Kras2 compared with Hras1. Nearly all tumors lacking activating ras mutations fully regressed following c-MYC deinduction, whereas tumors bearing ras mutations did not, suggesting that secondary mutations in ras contribute to tumor progression. These findings demonstrate that c-MYC-induced mammary tumorigenesis proceeds through a preferred secondary oncogenic pathway involving Kras2.

Cystic kidney disease represents a major cause of end-stage renal disease, yet the molecular mechanisms of pathogenesis remain largely unclear. Recent emphasis has been placed on a potential role for canonical Wnt signaling, but investigation of this pathway in adult renal homeostasis is lacking. Here we provide evidence of a previously unidentified canonical Wnt activity in adult mammalian kidney homeostasis, the loss of which leads to cystic kidney disease. Loss of the Jouberin (Jbn) protein in mouse leads to the cystic kidney disease nephronophthisis, owing to an unexpected decrease in endogenous Wnt activity. Jbn interacts with and facilitates β-catenin nuclear accumulation, resulting in positive modulation of downstream transcription. Finally, we show that Jbn is required in vivo for a Wnt response to injury and renal tubule repair, the absence of which triggers cystogenesis.Cystic kidney disorders include autosomal recessive and dominant polycystic kidney diseases as well as nephronophthisis 1,2 . Although the exact causes of these related disorders are not clear, various signaling pathways have been implicated. Specifically, several of the proteins encoded by the nephronophthisis-associated genes have been identified as negative modulators of the canonical Wnt pathway while activating the noncanonical Wnt pathway (planar cell polarity (PCP)) 3,4 , suggesting a specific link between cystic kidney disease and Wnt signaling. The emerging model is that cystogenesis is at least partly due to overactivation of canonical Wnt signaling. This is supported by work with mutants of negative Wnt regulators, which show embryonic kidney cysts 5,6 . However, similar embryonic kidney cysts have also been Correspondence should be addressed to J.G.G. (jogleeson@ucsd.edu). Note: Supplementary information is available on the Nature Medicine website.Author Contributions: M.A.L. designed the experimental approach, conducted the experiments and wrote the manuscript. J.G.G. supervised the project and experimental approach, interpreted data and contributed to manuscript preparation. C.M.L. designed and generated the Ahi1 −/− mouse mutant and provided feedback. J.L.S. generated mutant constructs and assisted in microscopy. L.S. contributed to in vitro localization experiments. M.D. contributed to IRI experiments. S.K.N. provided feedback regarding renal characterization and manuscript preparation. K.W. provided feedback and reagents for in vitro Wnt assays.Published online at http://www.nature.com/naturemedicine/. Reprints and permissions information is available online at http://npg.nature.com/reprintsandpermissions/. NIH Public Access Author ManuscriptNat Med. Author manuscript; available in PMC 2010 July 2. Results Ahi1 −/− mice show pathology consistent with nephronophthisisJbn is the protein product of the AHI1 gene that is mutated in Joubert syndrome 9,10 , a disorder associated with cerebellar hypoplasia, retinitis pigmentosa and nephronophthisis 11 . We used Ahi1-null mice generated by homologous recombination, whi...

Recent studies of oncogene dependence in conditional transgenic mice have suggested the exciting possibility that transient or prolonged MYC inactivation may be sufficient for sustained reversal of the tumorigenic process. In contrast, we report here that following oncogene downregulation, the majority of c-MYC-induced mammary adenocarcinomas grow in the absence of MYC overexpression. In addition, residual neoplastic cells persist from virtually all tumors that do regress to a nonpalpable state and these residual cells rapidly recover their malignant properties following MYC reactivation or spontaneously recur in a MYC-independent manner. Thus, MYC-induced mammary tumor cells subjected to either brief or prolonged MYC inactivation remain exquisitely sensitive to its oncogenic effects and characteristically progress to a state in which growth is MYC-independent.