The mitofusin 1 and 2 (MFN and MFN2) proteins reside in the outer mitochondrial membrane and have been shown to regulate mitochondrial network architecture by mediating tethering and fusion of mitochondria. Mitochondria normally form a tubular and branched reticular network dynamically regulated by a balance of fusion and fission events. Absence of either Mfn1 or Mfn2 results in a fragmented mitochondrial network. Züchner et al. previously described mutations in the gene mitofusin 2 (MFN2) as the cause of the major autosomal-dominant, axonal form of Charcot-Marie-Tooth neuropathy (CMT2A). CMT type 2 (CMT2) is characterized by chronic axonal degeneration of peripheral nerves leading to the loss of functional nerve fibers. Mutations in MFN2 are the most common cause of CMT2, and in Chapter 2 we report the results from a genetic screen of MFN2 in a CMT2 patient cohort.

The original finding that mutations in MFN2 cause CMT2A led to investigations focused on deficiencies of mitochondrial fusion and transport, specifically in the context of long axonal processes affected in CMT. While some experimental work supports disrupted mitochondrial transport in the etiology of CMT2A, other studies on CMT2A patient fibroblasts and cell models suggest abnormal mitochondrial fusion and dynamics do not underlie the etiology of this. In the first half of Chapter 3, we present some of our initial investigations prior to de Brito and Scorrano's report published in 2008 regarding a novel role for Mfn2 in tethering the endoplasmic reticulum (ER) to mitochondria. In Mfn2 null mouse embryonic fibroblasts (MEFs) regions of contact between mitochondria and the endoplasmic reticulum (ER) are significantly reduced. These regions of contact are thought to form specialized subdomains of the ER, called mitochondrial associated membranes (MAM). Besides observing a fragmented ER network in Mfn2 knockout (KO) mouse embryonic (MEF) cells, de Brito and Scorrano presented several lines of evidence which suggest that the underlying pathogenic mechanism in CMT2A stems from disrupted ER-mitochondria. As this observation had not been replicated in the literature, we describe our attempts to replicate these finding in the last half of Chapter 3.

The MAM represents a sub-domain of the ER in close association with the mitochondrial outer membrane. The movement of phosphatidylserine (PS) from the MAM domains of the ER to mitochondria and its subsequent decarboxylation to phosphatidylethanolamine (PE) by the enzyme PS decarboxylase (Pisd) has been well characterized and is known to depend on the existence of an outer mitochondrial membrane protein. As PE has curvature inducing and fusogenic biophysical characteristics, a deficiency in PE would be an attractive mechanism contributing to the morphological and fusion defects observed in Mfn2 null cell models. We hypothesized that loss of Mfn2 would lead to specific decreases in mitochondrial and cellular levels of PE. Chapter 4 describes experiments designed to test this hypothesis. We observed significantly lower levels of PE in Mfn2 null cells, yet observe similar changes in Mfn1 null cells. Likewise, other lipid species such as ether linked PE (ePE) are decreased. To investigate how CMT2A mutations in MFN2 influence cellular phospholipid profiles, we then profiled cellular phospholipids of CMT2A patients and control lymphoblasts. We hypothesized that mutations in MFN2 would result in decreased levels of PE. In Chapter 5, we report the results of a phospholipid screen which reveal changes in ePE in CMT2A patient lymphoblasts, without the drastic decreases in PE previously observed in Mfn2 null lines.

In conclusion, our data indicates an important role for both mitofusins in the mitochondrial synthesis of PE. In the context of CMT2A mutations, ePE levels are specifically reduced. Future studies may reveal how deficiencies in ePE might have important functional consequences in the pathogenesis of CMT2A.