Brett H. Graham, MD, PhD

Our laboratory is interested in studying the genetics of metabolic function and disease through the manipulation of genetic model systems, particularly the mouse and the fruit fly, Drosophila melanogaster.  Specifically, we are interested in the function of the mitochondrion in normal cellular biology and disease.  By taking advantage of the strengths of each model system, we intend to dissect the pathophysiology of mitochondrial dysfunction to progress towards the ultimate goal of developing novel therapeutic strategies for diseases that exhibit mitochondrial dysfunction.

From a cell-based forward genetic screen for mutant mitochondrial phenotypes, we identified a mutant gene trap mouse embryonic stem (ES) cell clone for Sucla2, which encodes the ADP-specific b subunit of succinyl-CoA synthetase (SCS), a component of the TCA cycle.  In humans, SUCLA2 mutations have been demonstrated to cause mitochondrial encephalomyopathy with mitochondrial DNA (mtDNA) depletion.  We have used the Sucla2 mutant ES clone to generate a transgenic line and mutant embryos exhibit mtDNA depletion and mitochondrial dysfunction in brain and muscle.  We are currently using conditional knockout and conditional genetic rescue strategies to bypass late embryonic lethality and to study SCS deficiency in adult animals.  We are also developing and studying gene trap mutants for select components of respiratory chain complexes I, II, and V.

Voltage-Dependent Anion Channels (VDACs or mitochondrial porins) are a family of proteins present in the mitochondrial outer membrane that play a critical role in the regulation of outer membrane permeability.  porin is the predominant VDAC in Drosophila.  We have generated and been studying hypomorphic mutants of porin.  These mutants exhibit defects in energy metabolism, male fertility, and neuromuscular and synaptic function.  We have performed genetic screens for suppressors of mutant porin phenotypes and are working to identify candidate suppressor loci.

Another project in the lab concerns the development of Drosophila models of mitochondrial disease.  We have generated and are characterizing null alleles for genes encoding subunits of various complexes of the mitochondrial electron transport chain.  In particular, we have been studying flies mutant for NDUFS3, a structural subunit of mitochondrial complex I.  Mutant animals exhibit complex I deficiency and neurological dysfunction manifested by locomotor defects, increased sensitivity to mechanical stress (“bang sensitivity”), and progressive deterioration of retinal function as measured by electroretinogram (ERG).  These animals also exhibit increased oxidative stress and mutant phenotypes are partially ameliorated by supplementation with antioxidants.  We are currently performing a pilot drug suppressor screen designed to detect improvements in the ERG.  The long-term goal of this project is to use these mutant alleles in both genetic and drug modifier screens to identify suppressors of mutant phenotypes that will provide insight into the pathophysiology of mitochondrial disease and potentially provide novel therapeutic strategies and targets.

We have also begun studying the role of citrate metabolism in neurometabolism.  This is based on the recent identification of patients with early infantile epileptic encephalopathy caused by pathogenic variants in the plasma citrate transporter (SLC13A5).  We have recently obtained a transgenic mouse line deficient for Slc13a5.  The goal of this project is to determine how perturbation of cellular citrate import disrupts metabolic compartmentation between astrocytes and neurons, thereby causing seizures.  

Mitochondrial Disorders; Inborn Errors of Metabolism; Clinical Genetics