Blood biodistribution and vector shedding of valoctocogene roxaparvovec in people with severe hemophilia A.

Following systemically-administered adeno-associated virus (AAV) gene therapy, vector particles are widely distributed, which has raised concerns about horizontal or germline transmission of vector. Characterization of biodistribution and kinetics of vector DNA in body fluids can address these concerns and provide insights into vector behavior in accessible samples. We investigated the biodistribution and vector shedding profile of valoctocogene roxaparvovec in men with severe hemophilia A enrolled in the phase 3 GENEr8-1 trial. Participants (n=134) received a single 6E13 vg/kg infusion and were assessed over 3 years. Vector DNA was measured with 4 different assays. Total vector DNA was evaluated in blood, saliva, stool, semen, and urine by quantitative (q)PCR. Encapsidated vector DNA was measured in plasma and semen with immunocapture-based qPCR. Contiguity of vector genomes and assembly of inverted terminal repeat fusions were measured in whole blood and peripheral blood mononuclear cells (PBMCs) using multi-color digital PCR. Median peak vector DNA levels observed 1 to 8 days after dosing were highest in blood, followed by saliva, semen, stool, and urine. Concentrations then declined steadily. Encapsidated vector DNA cleared faster than total vector DNA, achieving clearance by <=12 weeks in plasma and semen. Predominant vector genome forms transitioned from non-contiguous to full-length over time in whole blood and PBMCs, indicating formulation of stable circularized episomes within nucleated cells. The replication-incompetent nature of valoctocogene roxaparvovec, coupled with the steady clearance of total and encapsidated vector DNA from shedding matrices, indicates risk of transmission is low. This trial is registered at www.clinicaltrials.gov as NCT03370913.

Structural and mechanistic insights into a lysosomal membrane enzyme HGSNAT involved in Sanfilippo syndrome.

Heparan sulfate (HS) is degraded in lysosome by a series of glycosidases. Before the glycosidases can act, the terminal glucosamine of HS must be acetylated by the integral lysosomal membrane enzyme heparan-α-glucosaminide N-acetyltransferase (HGSNAT). Mutations of HGSNAT cause HS accumulation and consequently mucopolysaccharidosis IIIC, a devastating lysosomal storage disease characterized by progressive neurological deterioration and early death where no treatment is available. HGSNAT catalyzes a unique transmembrane acetylation reaction where the acetyl group of cytosolic acetyl-CoA is transported across the lysosomal membrane and attached to HS in one reaction. However, the reaction mechanism remains elusive. Here we report six cryo-EM structures of HGSNAT along the reaction pathway. These structures reveal a dimer arrangement and a unique structural fold, which enables the elucidation of the reaction mechanism. We find that a central pore within each monomer traverses the membrane and controls access of cytosolic acetyl-CoA to the active site at its luminal mouth where glucosamine binds. A histidine-aspartic acid catalytic dyad catalyzes the transfer reaction via a ternary complex mechanism. Furthermore, the structures allow the mapping of disease-causing variants and reveal their potential impact on the function, thus creating a framework to guide structure-based drug discovery efforts.

Ion transport and regulation in a synaptic vesicle glutamate transporter.

Synaptic vesicles accumulate neurotransmitters, enabling the quantal release by exocytosis that underlies synaptic transmission. Specific neurotransmitter transporters are responsible for this activity and therefore are essential for brain function. The vesicular glutamate transporters (VGLUTs) concentrate the principal excitatory neurotransmitter glutamate into synaptic vesicles, driven by membrane potential. However, the mechanism by which they do so remains poorly understood owing to a lack of structural information. We report the cryo-electron microscopy structure of rat VGLUT2 at 3.8-angstrom resolution and propose structure-based mechanisms for substrate recognition and allosteric activation by low pH and chloride. A potential permeation pathway for chloride intersects with the glutamate binding site. These results demonstrate how the activity of VGLUTs can be coordinated with large shifts in proton and chloride concentrations during the synaptic vesicle cycle to ensure normal synaptic transmission.