Novel Genetic and Therapeutic Approaches Focusing on Siglec-8 for the Diagnosis and Treatment of Human Idiopathic Eosinophilic Disorders

Bruce S. Bochner, M.D.

Johns Hopkins University School of Medicine

Funded in September, 2007: $600000 for 3 years
LAY SUMMARY . ABSTRACT . BIOGRAPHY .

LAY SUMMARY

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Genetic Mutations May Allow Excess Lung Inflammation Associated with Asthma and Esophagitis

Researchers will determine whether asthma and related diseases are associated with mutations in a molecule that ordinarily keeps levels of certain immune white blood cells (called “eosinophils”) low, and instead allows them to accumulate in inflamed airways.

Levels of immune eosinophils in the blood are normally quite low, but excess levels are found in tissues in a number of human diseases, including asthma and esophagitis. Therapies that could control or reduce eosinophil levels, therefore, might prevent permanent tissue damage. The researchers have identified a molecule that is selectively found on the surface of eosinophils, called “Siglec-8.”  Based on animal studies, they hypothesize that Siglec-8 molecules ordinarily bind to certain sugar-based molecules in tissues, which eventually leads to the death of the eosinophils. They further hypothesize that in asthma and esophagitis, genetic mutations occur in the signaling or binding abilities of the Siglec-8 molecules.  The eosinophils, therefore, do not die, but instead build up in inflamed airway tissues.  Mutations or “polymorphisms” in the Siglec gene are very common and the mutation could provide a pathway to either increased resistance or susceptibility to asthma.

The Johns Hopkins and Cincinnati Children’s Hospital Medical Center investigators will, in the aggregate, study human tissues from 500 patients with asthma, 500 patients with esophagitis, and 1,000 healthy study participants.  The Cincinnati collaborators will conduct the genetic analyses to try to identify the gene mutation(s) that are involved, and the markers for these mutations that could be used for diagnosis.

Significance:  The findings may result in the development of new diagnostic tests to identify patients with mutations, and also lead to new approaches to treating asthma, esophagitis, and other immune eosinophil-related diseases.  

ABSTRACT

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Novel Genetic and Therapeutic Approaches Focusing on Siglec-8 for the Diagnosis and Treatment of Human Idiopathic Eosinophilic Disorders

Siglec-8 is a member of the CD33 sialic acid-bind immunoglobulin-like lectin (Siglec) family found predominantly on human eosinophils. This transmembrane protein has an extracellular domain that recognizes specific carbohydrate molecules (glycans), and intracellular tyrosine-based inhibition motifs that putatively converts receptor binding into immune suppression. Mouse Siglec-F and human Siglec-8 are functionally convergent paralogs. Mouse Siglec-F is also expressed predominantly on mouse eosinophils. Incubation of human eosinophils with specific Siglec-8 antibodies, or mouse eosinophils with Siglec-F antibodies, induces apoptosis, and dosing of mice with Siglec-F antibodies reduces eosinophil numbers. Mice deficient in Siglec-F have exaggerated allergic lung eosinophilic responses. Both Siglec-8 and Siglec-F bind a unique glycan ligand referred to as 6’-sulfo-sLex, or NeuAcα2-3(6-O-sulfo)Galβ1-4[Fucα1-3]GlcNAc.

Therefore, activation of Siglec-8/Siglec-F through its natural glycan ligand may provide a novel means to specifically inhibit and/or deplete eosinophils, thereby reducing eosinophilic inflammatory responses. We hypothesize that Siglec-8 on human eosinophils binds to its natural glycan ligands, expressed on tissues, to limit their survival by activating their apoptosis. We also hypothesize that some human hypereosinophilic disorders, including eosinophilic esophagitis, are due to genetic abnormalities in Siglec-8 function that occur, for example, as a result of altered or reduced tissue expression of 6’-sulfo-sLex glycans, or mutations in the ligand binding or signaling domains of Siglec-8.

To test these hypotheses, we will 1) Localize and characterize natural Siglec-8 glycan ligands and determine their tissue expression in normal and eosinophilic human tissues; 2) Determine whether genetic mutations in Siglec-8 are associated with human eosinophilic disorders including asthma, nasal polyposis, eosinophilic esophagitis and idiopathic hypereosinophilic syndromes; and 3) determine whether genetic polymorphisms in Siglec-8 result in functional aberrations in Siglec-8 function.

Our proposed work on Siglec-8 should identify new mechanistic and genetic causes of eosinophilic disorders.  This would result in the development of new diagnostic tests to identify subsets of patients with loss-of-function abnormalities in Siglec-8 or its glycan ligand. Therapeutic approaches targeting Siglec-8 could also be developed for the treatment of diseases characterized by pathological eosinophilia.

INVESTIGATOR BIOGRAPHIES

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Bruce S. Bochner, M.D.

Bruce S. Bochner, M.D., is Professor of Medicine and a Cosner Scholar in Translational Research at the Johns Hopkins University School of Medicine in Baltimore, Maryland.  After receiving his medical training at the University of Illinois College of Medicine at Chicago, he began his postdoctoral training in the laboratory of Dr. Robert Schleimer here at Johns Hopkins in the Division of Allergy and Clinical Immunology of the Department of Medicine in 1985.  In 1988 he joined the faculty and in 1999 was promoted to full professor.  In January 2003 he succeeded Dr. Lawrence Lichtenstein as Division Director. 

Dr. Bochner is a Fellow of the American Academy of Allergy Asthma and Immunology and the American College of Allergy Asthma and Immunology.  He is a member of the American Association of Immunologists, the American Society for Clinical Investigation, the Association of American Physicians and the Collegium Internationale Allergologicum.  He has been an Associate Editor for the Journal of Allergy and Clinical Immunology since 1993, and is an editor of the Middleton's Allergy: Principles and Practice textbook.  He previously served on the Board of Directors of the American Board of Allergy and Immunology and is currently on the Board of Directors for the American Academy of Allergy Asthma and Immunology.  He is a former member of the Immunological Sciences Study Section of the National Institutes of Allergy and Infectious Diseases.  He is the author of more than 180 peer-reviewed publications, reviews, and book chapters, with an interest in the mechanisms of allergic cell recruitment, activation, and survival in human allergic inflammatory responses. 

Marc E. Rothenberg, M.D., Ph.D., is a professor of pediatrics and the director of the division of allergy and immunology at Cincinnati Children’s Hospital Medical Center.  He also serves as the Director for the Cincinnati Center for Eosinophilic Disorders and the program director of the Children’s Health Research Center grant from the NICHD at Cincinnati Children’s Hospital.  Dr. Rothenberg’s is internationally recognized as a leader in the allergy field.  His research is focused on molecular mechanisms of allergic inflammation and involves fundamental studies addressing the genes and molecules involved in allergic responses, the development and analysis of genetically engineered and antigen-driven models of allergic disease in mice, cellular and molecular immunology, and innovative translational clinical trials in patients.