United Mitochondrial Disease Foundation
Research Grant
07/01/07 - 6/30/09
PI: Patrice Hamel
Molecular genetic dissection of mitochondrial complex I assembly.

Lay summary:
Many human pathologies such as myopathies and neurodegenerative disorders like Parkinson's disease are associated with dysfunction of the mitochondria, an essential compartment devoted to the production of energy and present in all living cells. Among the most prevalent forms of mitochondria dysfunction (with an estimated incidence of 1 in 10,000 live births) are human deficiencies in a mitochondrial protein complex called complex I. The investigation of complex I deficiencies is difficult because direct studies of human material are subjected to strict regulations and because many complex I patients die young. The development of non human models of study is very desirable for the understanding of the disease and the design of an appropriate cure. We propose to study complex I assembly in the single-celled green alga Chlamydomonas reinhardtii that is much simpler than humans but yet displays a very similar mitochondrial complex I. Chlamydomonas mutants lacking complex I are amenable to studies because they are viable and display a slow growth in the dark. Our goal is to identify Chlamydomonas mutants that are specifically deficient in complex I assembly via molecular genetics and clone novel genes that control the formation of this complex. The proposed research is not clinically-oriented but its relevance to the human health is unquestionable because there is still no molecular explanation for 60% of complex I linked human diseases. It is believed that mutations in yet-to-be discovered genes controlling the assembly of complex I are responsible for these diseases with no molecular explanation.

Abstract:
This proposal is concerned with the molecular genetic dissection of complex I, a multimeric enzyme of the respiratory chain in the mitochondrial inner membrane. Complex I defects are a very common cause of mitochondrial diseases in humans. The fact that about 60% of human patients with complex I deficiency carry no mutations in the nuclear genes coding for complex I subunits suggests that mutations in yet-to-be discovered assembly factors may account for complex I deficiencies. At present, there is no genetically-tractable model system to investigate the molecular mechanisms of complex I assembly due to the fact that complex I is of dual genetic origin with only a few subunits encoded in the mitochondrial genome while the majority of the subunits is encoded in the nucleus. Our experimental system of investigation is the green alga Chlamydomonas reinhardtii, a unique unicellular organism amenable to both genetic and biochemical studies of complex I. Chlamydomonas is ideally suited for our purpose because a) its complex I is highly similar to the one in humans, b) it is amenable to manipulation of both the nuclear and mitochondrial genomes and b) nuclear and mitochondrial homoplasmic complex I mutants are viable if maintained phototrophically. In this research project, we propose to address the question of complex I assembly using Chlamydomonas, as a model of study. Our specific aims are to 1) clone the AMC1 and AMC2 genes (for Assembly of Mitochondrial Complex I) from existing amc mutants that display a complex I assembly defect and carry out the functional analysis of their gene products and 2) isolate nuclear mutants specifically deficient for complex I assembly via insertional mutagenesis with the objective to discover novel assembly factors. Our long-term goal is to develop a model system to unravel the assembly mechanism of this enzyme in order to elucidate the molecular basis of complex I deficiencies in humans.