[Mutation analysis of MFN2 gene in Chinese patients with Charcot-Marie-Tooth disease]

OBJECTIVE

To analyze MFN2 gene mutation in Chinese patients Charcot-Marie-Tooth disease (CMT) and to establish a quick and effective diagnostic method.

METHODS

Through denaturing high-performance liquid chromatography (DHPLC) combined with DNA sequencing, MFN2 gene mutation analysis was carried out in 35 Chinese CMT2 patients including 9 probands of CMT2 pedigree and 26 sporadic CMT2 patients.

RESULTS

The investigators found three abnormal sequence variations in MFN2 gene: c.281G-->A, c.395G-->A and c.408A-->T. c.395G-->A (C132T) was a novel causative missense mutation firstly reported while c.281G-->A (R94Q) a hotspot mutation and c.408A-->T (V136V) a single nucleotide polymorphism (SNP). The accuracy and specificity of DHPLC detection reached up to 100%.

CONCLUSION

Through DHPLC combined with DNA sequencing, MFN2 mutations are detected in Chinese CMT2 patients. There are two causative missense mutations: c.395G-->A (C132T) and c.281G-->A (R94Q) and one SNP c.408A-->T (V136V). Such a method is an effective and economic diagnostic screening tool of MFN2 gene in CMT patients on a large scale.

Adenine nucleotide translocase is involved in a mitochondrial coupling defect in MFN2-related Charcot-Marie-Tooth type 2A disease

Charcot-Marie-Tooth type 2A disease (CMT2A), a dominantly inherited peripheral neuropathy, is caused by mutations in MFN2, a mitochondrial fusion protein. Having previously demonstrated a mitochondrial coupling defect in CMT2A patients' fibroblasts, we here investigate mitochondrial oxygen consumption and the expression of adenine nucleotide translocase (ANT) and uncoupling proteins from eight other patients with the disease. The mitochondrial uncoupling was associated with a higher respiratory rate, essentially involving complex II proteins. Furthermore, a twofold increase in the expression of ANT led to the reduced efficiency of oxidative phosphorylation in CMT2A cells, suggesting that MFN2 plays a role in controlling ATP/ADP exchanges.

[tMfn2 gene transfer promotes vascular smooth-muscle cells apoptosis via activation of the mitochondrial cell death pathway]

OBJECTIVE

To investigate the effect of tMfn2 gene transfer on promoting the apoptosis of vascular smooth-muscle cells (VSMCs).

METHODS

VSMCs were infected by adenovirus-mediated tMfn2 (Adv-tMfn2) or adenovirus-mediated Mfn2 (Adv-Mfn2). FACS analysis, cell death ELISA and TUNEL staining were used to investigate the role of tMfn2 and Adv-Mfn2 gene transfer on VSMCs apoptosis. Western blot was used to analyze the protein expression of p-Akt, Bcl-2, Bax and cleaved caspase-9.

RESULTS

FACS and ELISA results showed that tMfn2 was superior to Mfn2 in promoting VSMCs apoptosis and tMfn2 gene transfer induced VSMCs apoptosis in a time-dependent manner (P < 0.01). TUNEL staining evidenced that there were more positive-apoptotic VSMCs in tMfn2 group than that in Mfn2 group (P < 0.01). The protein expressions of phosphorylated Akt and Bcl-2 were significantly decreased, whereas Bax and cleaved caspase-9 protein expressions were significantly upregulated in tMfn2-transfected VSMCs.

CONCLUSIONS

tMfn2 transfer promoted apoptosis of VSMCs in a time dependent manner via the PI3K-Akt signaling pathway and mitochondrial apoptotic pathway.

Simultaneous mutation and copy number variation (CNV) detection by multiplex PCR-based GS-FLX sequencing

We evaluated multiplex PCR amplification as a front-end for high-throughput sequencing, to widen the applicability of massive parallel sequencers for the detailed analysis of complex genomes. Using multiplex PCR reactions, we sequenced the complete coding regions of seven genes implicated in peripheral neuropathies in 40 individuals on a GS-FLX genome sequencer (Roche). The resulting dataset showed highly specific and uniform amplification. Comparison of the GS-FLX sequencing data with the dataset generated by Sanger sequencing confirmed the detection of all variants present and proved the sensitivity of the method for mutation detection. In addition, we showed that we could exploit the multiplexed PCR amplicons to determine individual copy number variation (CNV), increasing the spectrum of detected variations to both genetic and genomic variants. We conclude that our straightforward procedure substantially expands the applicability of the massive parallel sequencers for sequencing projects of a moderate number of amplicons (50-500) with typical applications in resequencing exons in positional or functional candidate regions and molecular genetic diagnostics.

Mitochondrial dynamics as a bridge between mitochondrial dysfunction and insulin resistance

Muscle from obese subjects or from type 2 diabetic patients show mitochondrial dysfunction, and this may participate in the insulin resistance in those conditions. The mechanisms involved in mitochondrial dysfunction are not completely understood. Dynamic mitochondrial filaments or networks form by mitochondrial fusion and fission events. There is substantial evidence that proteins participating in mitochondrial fusion or fission also have a role in metabolism. Thus, mitofusin-2 (Mfn2) a mitochondrial fusion protein, stimulates respiration, substrate oxidation and OXPHOS subunits expression. In this regard, muscle from obese subjects, or from type 2 diabetic patients, show a reduced expression of Mfn2 and, amelioration of insulin sensitivity by bariatric surgery is associated with an increased Mfn2 expression in muscle. Here, we propose the hypothesis that mitochondrial dynamics proteins play a role in mitochondrial dysfunction in obesity or in type 2 diabetes and that it may also participate in the development of insulin resistance.

[Ultrastructural lesions of axonal mitochondria in patients with childhood-onset Charcot-Marie-Tooth disease due to MFN2 mutations]

We present neuropathological findings based on sural nerve biopsy in six children with mutations of the mitofusin 2 gene (MFN2). All six children had severe axonal neuropathies (mild or severe hereditary motor and sensory neuropathy, HMSN), with onset in early childhood. All had a marked decrease in the density of mainly large myelinated fibers. Although neurophysiological findings were suggestive of axonal degeneration, some onion bulbs were present in each case. Unequivocal mitochondrial changes were apparent only on longitudinal sections. Many axonal mitochondria appeared smaller than normal and round or spherical instead of tubular. These mitochondria were abnormally aggregated, accumulating primarily at the axon periphery. This peripheral distribution was clearest in residual large myelinated fibers. The inner and outer mitochondrial membranes were irregular, and the cristae were quite often disrupted. These changes were observed in both myelinated and unmyelinated fibers. Mitofusin 2 is a large mitochondrial transmembrane GTPase, with two coiled coil domains and two transmembrane spans. It is targeted to the outer mitochondrial membrane, where it interacts with mitofusin 1 to regulate the mitochondrial network architecture by stimulating mitochondrialfusion. The mitochondrial changes we observed could thus result from abnormal mitochondrial fusion and fission. Neuropathologic abnormalities can be sufficiently characteristic to suggest the genetic basis of some hereditary neuropathies such as those associated with mutations in MPZ, GJB1, GDAP1, MTMR2, SH3TC2, PRX, FGD4 and LMNA. This may also be true of MFN2-related neuropathies.

[Transfection of human hyperplasia suppressor gene and hyperthermia have a synergistically apoptotic effect on colon cancer cell HT-29 in vitro]

OBJECTIVE

To investigate the suppression effect and apoptosis of the colon cell line HT-29 treated with adenovirus-expressed human hyperplasia suppressor gene (hHSG) and hyperthermia.

METHODS

Replication-defective recombinant adenovirus containing hHSG was constructed, and then infected into HT-29 cell line to detect its overexpression by western blot. The synergic effect with hyperthermia was observed, cell survival was assessed using Vi-CELL TMXR Cell Viability Analyzer, and cell apoptosis and cell cycle distribution were determined by flow cytometry.

RESULTS

After the cells were treated with 100 MOI (multiplicity of infection) Ad5-hHSG and heated at 42 degree C for 1 h, the number of viable cells decreased sharply, compared with single treatment with Ad5-hHSG or 42 degree C for 1 h (P < 0.01). The apoptotic rate of combined groups was significantly higher than that of Ad5-hHSG group or the heated group respectively (P < 0.01). Moreover, the combination treatment of Ad5-hHSG and 42 degree C for 1 h induced G2/M arrest.

CONCLUSION

Ad5-hHSG could increase the sensitivity of HT-29 to hyperthermia.

[Inhibition of cardiac hypertrophy by mitofusin 2 in neonatal rat cardiomyocytes]

OBJECTIVE

To study the expression and function of Mitofusin 2 (Mfn2) in cultured neonatal rat cardiomyocytes during PE-induced hypertrophy.

METHODS

The hypertrophy neonatal rat cardiomyocytes model was induced by 0.01 mol/L PE. RT-PCR and Western blot were applied to assess Mfn2 mRNA expression and protein level respectively. Cultured neonatal rat cardiomyocytes were treated by PE after Ad GFP or Ad Mfn2 infection, the protein synthesis was determined by 3H-leucine incorporation assay.

RESULTS

PE led to ANF mRNA level (by approximately 1 fold, P < 0.01) elevation and cell surface area (by approximately 1 fold, P < 0.01) increasing. Mfn2 mRNA (by approximatelt 50%, P < 0.01) and protein (by approximately 50%, P < 0.01) decreased remarkably in PE treated cardiomyocytes compared with those in control group. Compared with cells infected by Ad GFP (1.72+/-0.12 vs 2.47+/-0.06, P < 0.05, cell area (1.530+/-0.008 vs 0.830+/-0.009, P <0.01) and protein synthesis (0.98+/-0.10 vs 2.47+/-0.06, P < 0.01) were also largely abrogated in neonatal rat cardiomyocytes infected by Ad Mfn2.

CONCLUSION

These results indicate that the expression of Mfn2 mRNA and Mfn2 protein decreased in PE-induced neonatal rat cardiomyocytes hypertrophy model. Overexpression of Mfn2 in cultured neonatal rat cardiomyocytes could attenuate the protein synthesis and cell surface area increase after PE treatment. Accordingly, Mfn2 is an important regulator in cardiomyocytes hypertrophy.

Effects of mitofusin-2 gene on cell proliferation and chemotherapy sensitivity of MCF-7

In order to evaluate the effect of mitofusin-2 gene (mfn2) on proliferation and chemotherapy sensitivity of human breast carcinoma cell line MCF-7 in vitro, pEGFPmfn2 plasmid carrying full length of mitofusin-2 gene was transfected, by using sofast, into MCF-7 cells. Mitofusin-2 gene expression in MCF-7 cells transfected by sofast after 48 h was detected by PCR and Western blotting, and the stable expression of GFP protein in MCF-7 cells by Western blot analysis. The proliferation of MCF-7 cells was assayed by MTT and cell counting. By using PI method, the effects of mfn2 on the cell cycle distribution of MCF-7 were measured. Annexin-V/PI double labeling method was employed to detect the changes in apoptosis induced by chemotherapeutics before and after transfection. The results showed that the MCF-7 cells transfected with mfn2 gene could stably and highly express GFP protein. MTT assay revealed that after transfection of mfn2 cDNA, the proliferation of MCF-7 cells was significantly inhibited. DNA histogram showed that cells arrested in S phase, and the percentage of S phase cells was 42.7, 17.2 and 19.6 in mfn2 cDNA transfection group, blank plasmid transfection group and blank control group, respectively (P<0.05). The apoptosis ratio of the cells transfected with mfn2 gene was increased from 3.56% to 15.95%, that of the cells treated with camptothecin (CAMP) followed by mfn2 gene transfection was 69.6%, and that in blank plasmid transfection group and blank control group was 31.0% and 23.4% respectively (P<0.05). It was suggested that transfection of mfn2 gene could significantly inhibit the proliferation of MCF-7 cells and promote their sensitivity to CAMP with a synergic effect.

Cerebral involvement in axonal Charcot-Marie-Tooth neuropathy caused by mitofusin2 mutations

Mutations in the mitofusin 2 (MFN2) gene are a major cause of primary axonal Charcot- Marie-Tooth (CMT) neuropathy. This study aims at further characterization of cerebral white matter alterations observed in patients with MFN2 mutations. Molecular genetic, magnetic resonance imaging (MRI), magnetic resonance spectroscopy (MRS), and diffusion tensor imaging (DTI) investigations were performed in four unrelated patients aged 7 to 38 years with early onset axonal CMT neuropathy. Three distinct and so far undescribed MFN2 mutations were detected. Two patients had secondary macrocephaly and mild diffuse predominantly periventricular white matter alterations on MRI. In addition, one boy had symmetrical T2-hyperintensities in both thalami. Two patients had optic atrophy, one of them with normal MRI. In three patients proton MRS revealed elevated concentrations of total N-acetyl compounds (neuronal marker), total creatine (found in all cells) and myo-inositol (astrocytic marker) in cerebral white and gray matter though with regional variation. These alterations were most pronounced in the two patients with abnormal MRI. DTI of these patients revealed mild reductions of fractional anisotropy and mild increase of mean diffusivity in white matter. The present findings indicate an enhanced cellular density in cerebral white matter of MFN2 neuropathy which is primarily due to a reactive gliosis without axonal damage and possibly accompanied by mild demyelination.

Mitofusin 2 Triggers Vascular Smooth Muscle Cell Apoptosis via Mitochondrial Death Pathway

Previous studies have shown that mitofusin 2 (Mfn-2) (or hyperplasia suppressor gene [HSG]) inhibits vascular smooth muscle cell (VSMC) proliferation. Here, we demonstrate that Mfn-2 is a primary determinant of VSMC apoptosis. First, oxidative stress with H2O2, inhibition of protein kinase C with staurosporine, activation of protein kinase A with forskolin, and serum deprivation concurrently elevate Mfn-2 expression and induce VSMC apoptosis. Second, overexpression of Mfn-2 also triggers apoptosis of VSMCs in culture and in balloon-injured rat carotid arteries, thus contributing to Mfn-2-mediated prevention of neointima formation after angioplasty. Third, Mfn-2 silencing protects VSMCs against H2O2 or Mfn-2 overexpression-induced apoptosis, indicating that upregulation of Mfn-2 is necessary and sufficient for oxidative stress-mediated VSMC apoptosis. The Mfn-2 proapoptotic effect is independent of its role in mitochondrial fusion but mainly mediated by inhibition of Akt signaling and the resultant activation of the mitochondrial apoptotic pathway, as manifested by decreased Akt phosphorylation, increased mitochondrial Bax/Bcl-2 ratio, cytochrome c release, and activation of caspases-9 and caspase-3. Furthermore, Mfn-2-induced apoptosis was blocked by overexpression of an active phosphoinositide 3-kinase mutant or Bcl-xL or inhibition of caspase-9 but not caspases-8. Thus, in addition to its antiproliferative effects, Mfn-2 constitutes a primary determinant of VSMC apoptosis.

Upregulation of peroxisome proliferator-activated receptor gamma coactivator gene (PGC1A) during weight loss is related to insulin sensitivity but not to energy expenditure

AIMS/HYPOTHESIS

We investigated whether skeletal muscle peroxisome proliferator-activated receptor gamma coactivator-1 (PGC1A; also known as PPARGC1A) and its target mitofusin-2 (MFN2), as well as carnitine palmitoyltransferase-1 (CPT1; also known as carnitine palmitoyltransferase 1A [liver] [CPT1A]) and uncoupling protein (UCP)3, are involved in the improvement of insulin resistance and/or in the modification of energy expenditure during surgically induced massive weight loss.

MATERIALS AND METHODS

Seventeen morbidly obese women (mean BMI: 45.9 +/- 4 kg/m(2)) were investigated before, and 3 and 12 months after, Roux-en-Y gastric bypass (RYGB). We evaluated insulin sensitivity by the euglycaemic-hyperinsulinaemic clamp, energy expenditure and substrate oxidation by indirect calorimetry, and muscle mRNA expression by PCR.

RESULTS

Post-operatively, PGC1A was enhanced at 3 (p = 0.02) and 12 months (p = 0.03) as was MFN2 (p = 0.008 and p = 0.03 at 3 and 12 months respectively), whereas UCP3 was reduced (p = 0.03) at 12 months. CPT1 did not change. The expression of PGC1A and MFN2 were strongly (p < 0.0001) related. Insulin sensitivity, which increased after surgery (p = 0.002 at 3, p = 0.003 at 12 months), was significantly related to PGC1A and MFN2, but only MFN2 showed an independent influence in a multiple regression analysis. Energy expenditure was reduced at 3 months post-operatively (p = 0.001 vs before RYGB), remaining unchanged thereafter until 12 months. CPT1 and UCP3 were not significantly related to the modifications of energy expenditure or of lipid oxidation rate.

CONCLUSIONS/INTERPRETATION

Weight loss upregulates PGC1A, which in turn stimulates MFN2 expression. MFN2 expression significantly and independently contributes to the improvement of insulin sensitivity. UCP3 and CPT1 do not seem to influence energy expenditure after RYGB.

Assembly factors Rpf2 and Rrs1 recruit 5S rRNA and ribosomal proteins rpL5 and rpL11 into nascent ribosomes

More than 170 proteins are necessary for assembly of ribosomes in eukaryotes. However, cofactors that function with each of these proteins, substrates on which they act, and the precise functions of assembly factors--e.g., recruiting other molecules into preribosomes or triggering structural rearrangements of pre-rRNPs--remain mostly unknown. Here we investigated the recruitment of two ribosomal proteins and 5S ribosomal RNA (rRNA) into nascent ribosomes. We identified a ribonucleoprotein neighborhood in preribosomes that contains two yeast ribosome assembly factors, Rpf2 and Rrs1, two ribosomal proteins, rpL5 and rpL11, and 5S rRNA. Interactions between each of these four proteins have been confirmed by binding assays in vitro. These molecules assemble into 90S preribosomal particles containing 35S rRNA precursor (pre-rRNA). Rpf2 and Rrs1 are required for recruiting rpL5, rpL11, and 5S rRNA into preribosomes. In the absence of association of these molecules with pre-rRNPs, processing of 27SB pre-rRNA is blocked. Consequently, the abortive 66S pre-rRNPs are prematurely released from the nucleolus to the nucleoplasm, and cannot be exported to the cytoplasm.

Overexpression of Mitofusin 2 inhibited oxidized low-density lipoprotein induced vascular smooth muscle cell proliferation and reduced atherosclerotic lesion formation in rabbit

Our previous studies have implies that Mitofusin 2 (Mfn2), which was progressively reduced in arteries from ApoE(-/-) mice during the development of atherosclerosis, may take part in pathogenesis of atherosclerosis. In this study, we found that overexpression of Mfn2 inhibited oxidized low-density lipoprotein or serum induced vascular smooth muscle cell proliferation by down-regulation of Akt and ERK phosphorylation. Then we investigated the in vivo role of Mfn2 on the development of atherosclerosis in rabbits using adenovirus expressing Mitofusin 2 gene (AdMfn2). By morphometric analysis we found overexpression of Mfn2 inhibited atherosclerotic lesion formation and intima/media ratio by 66.7% and 74.6%, respectively, compared with control group. These results suggest that local Mfn2 treatment suppresses the development of atherosclerosis in vivo in part by attenuating the smooth muscle cell proliferation induced by lipid deposition and vascular injury.

[Effect of mitofusin-2 gene in apoptosis of human breast carcinoma cell line in vitro]

OBJECTIVE

To investigate the role of mitofusin-2 gene (mfn2) in apoptosis in human breast carcinoma cell line MCF-7 cells after in vitro transfection.

METHODS

pEGFP mfn2 was transfected by sofast in vitro. Expression of GFP was observed by Western blot, and the MCF-7 cell proliferation was measured by MTT and cell counting. Apoptosis in MCF-7 cells was observed in annexin-V/PI and chondrosome transmembrane potential of MCF-7 marked in JC-1 by FCM. The Ultrastructure of cells was observed by transmission electron microscopy.

RESULTS

The stable expression of GFP in MCF-7 cells was confirmed by Western blot. Mfn2 significantly inhibited cell proliferation, revealed by MTT, and decrease chondrosome transmembrane potential. Exogenous mfn2 gene significantly induced apoptosis. The apoptotic rate was increased from 3.6% to 16.0% (P < 0.05). Mfn2 gene induced break down and loss of mitochondrial cristae, and rarefaction of mitochondrial ground substance. Swollen mitochondria intensely aggregated around the cell nuclei.

CONCLUSION

Mfn2 can strongly induce apoptosis in MCF-7 cells, which may be associated with decrease of mitochondrial transmembrane potential.

[Hyperplasia suppressor gene induces vascular smooth muscle cell apoptosis]

OBJECTIVE

To study the effect of hyperplasia suppressor gene (HSG) in inducing vascular smooth muscle cell apoptosis and the underlying mechanisms.

METHODS

The cultured VSMCs were transfected with an adenoviral vector containing rat HSG gene. Effects of HSG on VSMC apoptosis were investigated by fluorescent dye staining to detect the tact of nuclei, and by flow cytometry to define the content of DNA and to detect the levels of caspase-3. The expressions of Bcl-2 and Bax were also performed by Western blot analysis.

RESULTS

The increased expression of HSG in VSMCs infected with AdHSG induced apoptotic cell death detected by flow cytometry assay and nucleic staining. Compared with control groups, HSG induced vascular smooth muscle cell apoptosis 72 h after infected with adenoviral vector (39.6%+/-3.2% vs. 2.6%+/-0.9%, P< 0.01). Overexpression of HSG also increased the activity of caspase-3 (0.354+/-0.104 vs. 0.064+/-0.022,P<0.01)and the release of cytochrome c. The Bcl-2 protein level was decreased ( 0.26+/-0.03 vs. 1.06+/-0.07,P<0.01)in AdHSG infected cells.

CONCLUSION

Hyperplasia induced vascular smooth muscle cell apoptosis. HSG downregulated the expression of Bcl-2/Bax and activated caspase-3, and therefore promoted the release of cytochrome c and induced cell apoptosis.

Mitofusin 2 Protects Cerebellar Granule Neurons against Injury-induced Cell Death

Of the GTPases involved in the regulation of the fusion machinery, mitofusin 2 (Mfn2) plays an important role in the nervous system as point mutations of this isoform are associated with Charcot Marie Tooth neuropathy. Here, we investigate whether Mfn2 plays a role in the regulation of neuronal injury. We first examine mitochondrial dynamics following different modes of injury in cerebellar granule neurons. We demonstrate that neurons exposed to DNA damage or oxidative stress exhibit extensive mitochondrial fission, an early event preceding neuronal loss. The extent of mitochondrial fragmentation and remodeling is variable and depends on the mode and the severity of the death stimuli. Interestingly, whereas mitofusin 2 loss of function significantly induces cell death in the absence of any cell death stimuli, expression of mitofusin 2 prevents cell death following DNA damage, oxidative stress, and K+ deprivation induced apoptosis. More importantly, whereas wild-type Mfn2 and the hydrolysis-deficient mutant of Mfn2 (Mfn2(RasG12V)) function equally to promote fusion and lengthening of mitochondria, the activated Mfn2(RasG12V) mutant shows a significant increase in the protection of neurons against cell death and release of proapoptotic factor cytochrome c. These findings highlight a signaling role for Mfn2 in the regulation of apoptosis that extends beyond its role in mitochondrial fusion.

[Effects of mitofusin-2 gene on proliferation and chemosensitivity of human breast carcinoma cell line MCF-7]

BACKGROUND & OBJECTIVE

Mitofusin-2(mfn2), a proliferation-inhibiting gene, targets to the outer membrane of mitochondria. Its overexpression suppresses the proliferation of vascular smooth muscle cells. This study was to explore the effects of mfn2 gene on the proliferation and chemosensitivity of human breast carcinoma cell line MCF-7.

METHODS

Plasmid pEGFP-mfn2 containing mfn2 cDNA was constructed and transfected into MCF-7 cells by sofast. The expression of green fluorescent protein (GFP) in MCF-7 cells was detected by Western blot. Cell proliferation was measured by MTT assay and cell counting. Cell cycle and chemosensitivity of MCF-7 cells to camptothecin (CAM) was observed by flow cytometry (FCM).

RESULTS

After transfection of pEGFP-mfn2, the stable expression of GFP protein was detected in MCF-7 cells, and cell cycle was arrested: the S phase proportion was significantly higher in pEGFP-mfn2-transfected cells than in pEGFP-transfected and untransfected cells [(42.7+/-1.3)% vs. (17.2+/-2.0)% and (19.6+/-1.7)%, P<0.05]. The apoptosis rate were significantly higher in pEGFP-mfn2-transfected cells than in pEGFP-transfected and untransfected cells [(16.0+/-0.3)% vs. (4.5+/-0.9)% and (3.6+/-0.6)% before treatment of CAM, P<0.05; (69.6+/-4.3)% vs. (31.0+/-1.8)% and (23.4+/-2.8)% after 4-hour treatment of CAM, P<0.05].

CONCLUSION

mfn2 gene can inhibit the proliferation of MCF-7 cells and increase their chemosensitivity to CAM.

[Mutations in the mitofusin 2 gene are the most common cause of Charcot-Marie-Tooth type 2 disease]

In contrast to Charcot-Marie-Tooth type 1 disease (CMT1), which is most commonly caused by 17p11.2-p12 duplication (in 70% of CMT1 cases), the axonal form of hereditary motor and sensory neuropathy (CMT2) seemed to be a genetically heterogeneous disease group, with no single gene playing a major pathogenetic role. In 2004, 10 mutations were identified in CMT2A families in the MFN2 gene coding for the mitochondrial protein mitofusin-2, previously mapped to the 1p35-36 locus. In the last two years, MFN2 gene mutations were shown to be the most common cause of autosomal dominant hereditary axonopathy. In addition, MFN2 gene mutations were also identified in CMT type 6 (axonal neuropathy with optic nerve atrophy). Recent reports indicate that some MFN2 gene mutations may by inherited as autosomal recessive traits. As MFN2 gene mutations are the most common cause of autosomal dominant CMT2 disease (33% of cases), MFN2 gene testing may be considered a diagnostic test for CMT2.

Mitochondrial coupling defect in Charcot-Marie-Tooth type 2A disease

OBJECTIVE

Mutations of the mitofusin 2 gene (MFN2) may account for at least a third of the cases of Charcot-Marie-Tooth disease type 2 (CMT2). This study investigates mitochondrial cellular bioenergetics in MFN2-related CMT2A.

METHODS

Mitochondrial network morphology and metabolism were studied in cultures of skin fibroblasts obtained from four CMT2A patients harboring novel missense mutations of the MFN2 gene.

RESULTS

Although the mitochondrial network appeared morphologically unaltered, there was a significant defect of mitochondrial coupling associated with a reduction of the mitochondrial membrane potential.

INTERPRETATION

Our results suggest that the sharply reduced efficacy of oxidative phosphorylation in MFN2-related CMT2A may contribute to the pathophysiology of the axonal neuropathy.

Mitofusin-2 is a major determinant of oxidative stress-mediated heart muscle cell apoptosis

An inexorable loss of terminally differentiated heart muscle cells is a crucial causal factor for heart failure. Here, we have provided several lines of evidence to demonstrate that mitofusin-2 (Mfn-2; also called hyperplasia suppressor gene), a member of the mitofusin family, is a major determinant of oxidative stress-mediated cardiomyocyte apoptosis. First, oxidative stress with H(2)O(2) led to concurrent increases in Mfn-2 expression and apoptosis in cultured neonatal rat cardiomyocytes. Second, overexpression of Mfn-2 to a level similar to that induced by H(2)O(2) was sufficient to trigger myocyte apoptosis, which is associated with profound inhibition of Akt activation without altering ERK1/2 signaling. Third, Mfn-2 silencing inhibited oxidative stress-induced apoptosis in H9C2 cells, a cardiac muscle cell line. Furthermore, Mfn-2-induced myocyte apoptosis was abrogated by inhibition of caspase-9 (but not caspase-8) and by overexpression of Bcl-x(L) or enhanced activation of phosphatidylinositol 3-kinase-Akt, suggesting that inhibition of Akt signaling and activation of the mitochondrial death pathway are essentially involved in Mfn-2-induced heart muscle cell apoptosis. These results indicate that increased cardiac Mfn-2 expression is both necessary and sufficient for oxidative stress-induced heart muscle cell apoptosis, suggesting that Mfn-2 deregulation may be a crucial pathogenic element and a potential therapeutic target for heart failure.

Mitofusin 2 gene mutation (R94Q) causing severe early-onset axonal polyneuropathy (CMT2A)

Charcot-Marie-Tooth disease (CMT) has been classified into two types: demyelinating forms (CMT1) and axonal forms (CMT2). Mutations in the CMT2A locus have been linked to the KIF1B and the mitofusin 2 (MFN2) genes. Here, we report a German patient with CMT2 with an underlying spontaneous mutation (c.281G-->A) in the MFN2 gene. Clinically, the patient presented with early-onset CMT that was not associated with additional central nervous system pathology. The disease course was rapidly progressive in the first years and slowed afterwards. We also suggest that single patients with early-onset axonal polyneuropathies should be screened for MFN2 mutations.

Phenotype of Charcot-Marie-Tooth disease Type 2

OBJECTIVE

To investigate the clinical and electrophysiologic phenotype of Charcot-Marie-Tooth disease (CMT) Type 2 in a large number of affected families.

METHODS

We excluded CMT Type 1, hereditary neuropathy with liability to pressure palsies, and CMT due to Cx32 gene mutations by DNA analysis. We performed genetic analysis of the presently known CMT Type 2 genes.

RESULTS

Sixty-one persons from 18 families were affected. Ninety percent of patients were able to walk with or without the help of aids. Proximal leg muscle weakness was present in 13%. Asymmetrical features were present in 15%. Normal or brisk knee reflexes were present in 36%. Extensor plantar responses without associated spasticity occurred in 10 patients from eight families. Only three causative mutations were identified in the MFN2, BSCL2, and RAB7 genes. No mutations were found in the NEFL, HSPB1, HSPB8, GARS, DNM2, and GDAP1 genes.

CONCLUSIONS

At group level, the clinical phenotype of Charcot-Marie-Tooth disease (CMT) Type 2 is uniform, with symmetric, distal weakness, atrophy and sensory disturbances, more pronounced in the legs than in the arms, notwithstanding the genetic heterogeneity. Brisk reflexes, extensor plantar responses, and asymmetrical muscle involvement can be considered part of the CMT Type 2 phenotype. The causative gene mutation was found in only 17% of the families we studied.