A new model to address the mechanism of neutropenia in patients with Barth syndrome
David Sykes, MD, PhD, Assistant Professor, Massachusetts General Hospital, Boston, MA

Award—US $50,000 over 1-year period

*This grant is made possible in part by support from Association Syndrome de Barth France

Abstract:

Barth syndrome is an X-linked hereditary disorder characterized by cardiomyopathy, skeletal muscle myopathy, and neutropenia. The syndrome arises because of inherited mutations in the gene TAZ, resulting in a loss of function of the protein tafazzin.

Tafazzin is an enzyme that processes cardiolipin, a phospholipid that is almost exclusively found in the inner membrane of the mitochondria. The lack of functional tafazzin abrogates one of the final steps in cardiolipin maturation, resulting in cardiolipin that contains more highly saturated lipid tails (i.e. fewer double bonds). It is this lack of an unsaturated cardiolipin pool that seems to contribute to the disorganized inner mitochondrial membrane.

The link between tafazzin-deficiency and myopathy is not entirely understood but seems logical given the dependence of muscle cells on mitochondrial function as well as oxidative respiration. The components of the electron transport chain are co-localized with cardiolipin in the inner mitochondrial membrane, and it is felt that their appropriate organization within the membrane lipid bilayer is dependent on the presence of mature cardiolipin which is lacking in those individuals with Barth syndrome.

The link between tafazzin-deficiency and neutropenia is less clear. Neutrophils are terminally-differentiated effector cells of the innate immune system. They are critical for protection against bacterial and fungal pathogens and patients without sufficient neutrophils are among the most immunocompromised and at risk of lethal infection. Neutrophils have few mitochondria at baseline and are generally believed to rely on glycolysis for energy production. It is not known if the mechanism of neutropenia in Barth syndrome is due to a lack of production or due to increased clearance (e.g. more prone to apoptosis).

This proposal seeks to elucidate the mechanism of neutropenia in patients with Barth syndrome. Understanding why tafazzin deficiency leads to neutropenia is the first step in developing a therapeutic intervention to reverse the neutropenia phenotype and avoid the resultant serious infectious complications.

Here we take advantage of an existing tafazzin knockout mouse for the study of hematopoietic progenitors and myeloid development in both the developing embryo and adult mouse. We have already been successful in establishing neutrophil progenitor cell lines both wild-type and tafazzin knockout mice, providing an unlimited source of progenitors and mature neutrophils for downstream analyses.

We will undertake experiments along two main lines of investigation: (1) is there a developmental defect in neutrophil production; and (2) is there a defect in the mature neutrophil immune function?

The developmental question will be addressed by the characterization of fetal liver hematopoiesis as well as adult hematopoiesis in tafazzin knockout mice. The functional question will be addressed using our progenitor lines in myriad experiments to assess neutrophil morphology, gene expression, metabolite profile, and their ability in assays of phagocytosis, migration, and cytokine production.

We anticipate that the answers to these foundational experiments will shed light on potential targets for therapeutic intervention. Once we understand why a defect in cardiolipin maturation due to tafazzin deficiency leads to neutropenia, we can rationally devise a small molecule or biologic therapy to reverse the defect.