The DcpS inhibitor RG3039 improves survival, function and motor unit pathologies in two SMA mouse models.

Spinal muscular atrophy (SMA) is caused by insufficient levels of the survival motor neuron (SMN) protein due to the functional loss of the SMN1 gene and the inability of its paralog, SMN2, to fully compensate due to reduced exon 7 splicing efficiency. Since SMA patients have at least one copy of SMN2, drug discovery campaigns have sought to identify SMN2 inducers. C5-substituted quinazolines increase SMN2 promoter activity in cell-based assays and a derivative, RG3039, has progressed to clinical testing. It is orally bioavailable, brain-penetrant and has been shown to be an inhibitor of the mRNA decapping enzyme, DcpS. Our pharmacological characterization of RG3039, reported here, demonstrates that RG3039 can extend survival and improve function in two SMA mouse models of varying disease severity (Taiwanese 5058 Hemi and 2B/- SMA mice), and positively impacts neuromuscular pathologies. In 2B/- SMA mice, RG3039 provided a >600% survival benefit (median 18 days to >112 days) when dosing began at P4, highlighting the importance of early intervention. We determined the minimum effective dose and the associated pharmacokinetic (PK) and exposure relationship of RG3039 and DcpS inhibition ex vivo. These data support the long PK half-life with extended pharmacodynamic outcome of RG3039 in 2B/- SMA mice. In motor neurons, RG3039 significantly increased both the average number of cells with gems and average number of gems per cell, which is used as an indirect measure of SMN levels. These studies contribute to dose selection and exposure estimates for the first studies with RG3039 in human subjects.

Human mutations in NDE1 cause extreme microcephaly with lissencephaly [corrected].

Genes disrupted in human microcephaly (meaning "small brain") define key regulators of neural progenitor proliferation and cell-fate specification. In comparison, genes mutated in human lissencephaly (lissos means smooth and cephalos means brain) highlight critical regulators of neuronal migration. Here, we report two families with extreme microcephaly and grossly simplified cortical gyral structure, a condition referred to as microlissencephaly, and show that they carry homozygous frameshift mutations in NDE1, which encodes a multidomain protein that localizes to the centrosome and mitotic spindle poles. Both human mutations in NDE1 truncate the C-terminal NDE1domains, which are essential for interactions with cytoplasmic dynein and thus for regulation of cytoskeletal dynamics in mitosis and for cell-cycle-dependent phosphorylation of NDE1 by Cdk1. We show that the patient NDE1 proteins are unstable, cannot bind cytoplasmic dynein, and do not localize properly to the centrosome. Additionally, we show that CDK1 phosphorylation at T246, which is within the C-terminal region disrupted by the mutations, is required for cell-cycle progression from the G2 to the M phase. The role of NDE1 in cell-cycle progression probably contributes to the profound neuronal proliferation defects evident in Nde1-null mice and patients with NDE1 mutations, demonstrating the essential role of NDE1 in human cerebral cortical neurogenesis.