Michael Steven Hershfield

Molecular Basis and Therapy of Inherited Disorders of Purine Metabolism
ABSTRACT
We have a longstanding interest in inherited disorders of purine metabolism. Our primary focus has been the combined immunodeficiency disease caused by inherited deficiency of adenosine deaminase (ADA1) and purine nucleoside phosphorylase (PNP), an more recently the deficiency of the enzyme ADA2. In addition to these rare recessive disorders, we have maintained an interest in gout, the most common purine metabolic disease. We have also investigated the biochemistry, metabolism, and biological effects of nucleoside analogs, including their use for treating neoplastic and viral diseases.

During our first decade at Duke we studied the biochemical mechanisms responsible for immune deficiency caused by ADA and PNP deficiency. We subsequently investigated the operation of these mechanisms in vivo in collaboarative studies of ADA knockout mice and in ADA-deficient patients. We defined the molecular basis for the interaction between human ADA and CD26/Dipeptidyl Peptidase IV (DPPIV), a cell membrane associated multifunctional glycoprotein, also known as the adenosine deaminase complexing protein. This work has cast doubt on the postulated role of the ADA-DPPIV complex, or "ecto-ADA", in normal immune function.

About 40 years ago, in collaboration with Dr. Rebecca Buckley, we initiated, and subsequently played a central role in the clinical development of polyethylene glycol (PEG)-modified adenosine deaminase (PEG-ADA) as replacement therapy for severe combined immunodeficiency disease (SCID) due to ADA1 deficiency. PEG-ADA was the first PEG-modified therapeutic agent to receive USFDA approval (in 1990), and the first effective form of enzyme replacement therapy for an inherited metabolic disease. We have also collaborated in evaluating the metabolic efficacy of stem cell transplantation and stem cell gene therapy for treating ADA- SCID. We have systematically investigated the mutational basis for ADA deficiency, and the relationship between genotype and phenotype, both clinical and metabolic. For this purpose we developed a system for scoring ADA missense variants based on quantitating their effects on expression of ADA activity in an ADA-deleted strain of E. coli. 

Over the past 4+ decades we have served as a worldwide resource for establishing the diagnosis of ADA1 and PNP deficiency, and to monitor the metabolic effects of PEG-ADA therapy as well as ADA gene therapy and stem cell transplantation in patients with ADA deficiency in the US and over 20 other countries. Over the past 15 years we have also served as a resource for functional testing to diagnose ADA2 deficiency (DADA2). Our laboratory is one of the few that has maintained CLIA certification for the above testing (for which we have never charged).

In addition to our work on rare inherited disorders we have conducted groundbreaking translational research related to gout, the most common disorder of purine metabolism in humans. We developed a PEGylated recombinant urate oxidase (Pegloticase, Krystexxa) as an Orphan Drug for treating patients with refractory gout and poorly controlled hyperuricemia.  After demonstrating the effectiveness of Pegloticase in preventing uric acid nephropathy in a urate oxidase knockout mouse model, we participated with John Sundy and other members of the Duke Rheumatology division in the first in-human phase 1 clinical trials of Pegloticase in patients with refractory gout. We subsequently obtained support from the USFDA Office of Orphan Products Development to conduct a Phase II clinical study of Pegloticase in order to optimize dosing and assess the potential effects of profoundly reducing serum uric acid levels on oxidative stress status. We discovered the ability of treatment with pegloticase to induce antibodies to the PEG polymer, an observation relvant to the immune response to other PEGylated proteins and lipid nanaoparticles that contain PEG. In 2010 Pegloticase was one of 21 new drugs to receive FDA approval. 



Keywords: human genetic disease; enzyme replacement therapy; polyethylene glycol modified enzymes; mutation; immune deficiency disease; ADA deficiency; purine nucleoside phosphorylase deficiency; gout