BACKGROUND Hemophilia B, an X-linked disorder, is ideally suited for gene therapy. We investigated the use of a new gene therapy in patients with the disorder. METHODS We infused a single dose of a serotype-8–pseudotyped, self-complementary adenovirus-associated virus (AAV) vector expressing a codon-optimized human factor IX (FIX) transgene (scAAV2/8-LP1-hFIXco) in a peripheral vein in six patients with severe hemophilia B (FIX activity, <1% of normal values). Study participants were enrolled sequentially in one of three cohorts (given a high, intermediate, or low dose of vector), with two participants in each group. Vector was administered without immunosuppressive therapy, and participants were followed for 6 to 16 months. RESULTS AAV-mediated expression of FIX at 2 to 11% of normal levels was observed in all participants. Four of the six discontinued FIX prophylaxis and remained free of spontaneous hemorrhage; in the other two, the interval between prophylactic injections was increased. Of the two participants who received the high dose of vector, one had a transient, asymptomatic elevation of serum aminotransferase levels, which was associated with the detection of AAV8-capsid–specific T cells in the peripheral blood; the other had a slight increase in liver-enzyme levels, the cause of which was less clear. Each of these two participants received a short course of glucocorticoid therapy, which rapidly normalized aminotransferase levels and maintained FIX levels in the range of 3 to 11% of normal values. CONCLUSIONS Peripheral-vein infusion of scAAV2/8-LP1-hFIXco resulted in FIX transgene expression at levels sufficient to improve the bleeding phenotype, with few side effects. Although immune-mediated clearance of AAV-transduced hepatocytes remains a concern, this process may be controlled with a short course of glucocorticoids without loss of transgene expression. (Funded by the Medical Research Council and others; ClinicalTrials.gov number, NCT00979238.)

Key points• The recently identified TMEM16/anoctamin protein family includes Ca 2+ -activated Cl − channels (TMEM16A and TMEM16B), a Ca 2+ -activated non-selective cation channel (TMEM16F) and proteins for which the function remains unclear.• TMEM16 channel proteins consist of eight putative transmembrane domains (TMs) with the 5th and 6th TMs flanking a loop predicted to protrude deep into the membrane. Recent studies suggest that this re-entrant loop may compose part of the pore of TMEM16A channels while also containing residues involved in Ca 2+ binding.• Here, we investigate the functional role of the putative pore-loop by examining the electrophysiological properties of chimeras produced by transplanting this region between TMEM16 family members with different conduction properties and Ca 2+ sensitivities.• We revealed that the putative pore-loop of TMEM16 channels has an unexpected role in controlling the whole-cell Ca 2+ -activated Cl − conductance by regulating the number of functional channels present on the plasma membrane. AbstractThe recently identified TMEM16/anoctamin protein family includes Ca 2+ -activated anion channels (TMEM16A, TMEM16B), a cation channel (TMEM16F) and proteins with unclear function. TMEM16 channels consist of eight putative transmembrane domains (TMs) with TM5-TM6 flanking a re-entrant loop thought to form the pore. In TMEM16A this region has also been suggested to contain residues involved in Ca 2+ binding. The role of the putative pore-loop of TMEM16 channels was investigated using a chimeric approach. Heterologous expression of either TMEM16A or TMEM16B resulted in whole-cell anion currents with very similar conduction properties but distinct kinetics and degrees of sensitivity to Ca 2+ . Furthermore, whole-cell currents mediated by TMEM16A channels were ∼six times larger than TMEM16B-mediated currents. Replacement of the putative pore-loop of TMEM16A with that of TMEM16B (TMEM16A-B channels) reduced the currents by ∼six-fold, while the opposite modification (TMEM16B-A channels) produced a ∼six-fold increase in the currents. Unexpectedly, these changes were not secondary to variations in channel gating by Ca 2+ or voltage, nor were they due to changes in single-channel conductance. Instead, they depended on the number of functional channels present on the plasma membrane. Generation of additional, smaller chimeras within the putative pore-loop of TMEM16A and TMEM16B led to the identification of a region containing a non-canonical trafficking motif. Chimeras composed of the putative pore-loop of TMEM16F transplanted into the TMEM16A protein scaffold did not conduct anions or cations. These data suggest that the putative pore-loop does not form a complete, transferable pore domain. Furthermore, our data reveal an unexpected role for

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5 Background: Hemophilia B (HB), an X-linked bleeding disorder, is ideally suited for gene therapy. We investigated a novel approach using peripheral vein infusion of a single dose of a serotype-8 pseudotyped self-complementary adeno-associated virus (AAV) vector expressing a codon-optimized coagulation factor IX (FIX) transgene (scAAV2/8-LP1-hFIXco). Methods: Six severe HB subjects (FIX ≤1%) were enrolled sequentially into one of three dose cohorts with two subjects in each group. Vector was administered without immunosuppression. The subjects were followed for 6–16 months post treatment. Results: AAV-mediated expression of FIX at 2–11% of normal was observed in all subjects. Four of the six have discontinued prophylaxis and remain free of spontaneous hemorrhage. The other two have increased the interval between FIX prophylaxes. A high-dose subject developed asymptomatic, transient elevation of serum transaminases associated with detection of AAV8 capsid specific T cells in peripheral blood. The second high-dose subject experienced a slight increase of liver enzymes, of less clear etiology. Treatment of each with a short course of steroids led to rapid normalization of the transaminases and maintenance of FIX levels in the 3–11% range. Conclusion: Peripheral vein administration of scAAV2/8-LP1-hFIXco was well tolerated and resulted in FIX transgene expression at levels sufficient to improve the bleeding phenotype. Immune-mediated clearance of AAV-transduced hepatocytes remains a concern but our data suggest that this process may be controlled with a short course of steroids without loss of transgene expression. Hence, our novel approach shows promise for gene therapy of HB and other protein deficiencies. (ClinicalTrials.gov number, NCT00979238) Disclosures: Nathwani: Amsterdam Molecular Therapeutics: Patents & Royalties. Gray:Amsterdam Molecular Therapeutics: Patents & Royalties. Davidoff:Amsterdam Molecular Therapeutics: Patents & Royalties.

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