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Université Paris Descartes (J.B., F.A., F.D.), Centre National de la Recherche Scientifique Unité Propre de Recherche 9078, Faculté Necker-Enfants Malades, 75015 Paris, France; and Institut National de la Santé et de la Recherche Médicale U781 (A.R., A.M.), Hôpital Necker-Enfants Malades, 75015 Paris, France
Address all correspondence and requests for reprints to: Fatima Djouadi, Ph.D., Centre National de la Recherche Scientifique Unité Propre de Recherche 9078, Faculté Necker-Enfants Malades, 156 rue de Vaugirard, 75015 Paris, France. E-mail: djouadi{at}necker.fr.
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Abstract |
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Objective: We tested whether bezafibrate, a drug acting as a peroxisome proliferator-activated receptor (PPAR) agonist, could stimulate RC capacities.
Design: Fibroblasts or myoblasts from controls or patients deficient in complex I (CI), complex III (CIII), or complex IV (CIV) were cultured with or without bezafibrate.
Main Outcome Measures: Enzyme activities, mRNA and protein expression, and respiration rates were measured.
Results: In control cells, bezafibrate increased the CI, CIII, and CIV enzyme activities (+42 to +52%), as well as RC mRNAs (+40 to +120%) and RC protein levels (+50 to +150%). Nine of 14 patient cell lines tested exhibited a significant increase in the activity of the deficient RC complex after bezafibrate treatment (+46 to +133%), and full pharmacological correction could be achieved in seven cell lines. Similar effects were obtained using a PPAR
agonist. These changes were related to a drug-induced increase in the mutated mRNAs and RC protein levels. Finally, the molecular mechanisms by which the PPAR pathway could induce the expression of genes encoding structural subunits or ancillary proteins of the RC apparatus, leading to stimulate the activity and protein levels of RC complex, likely involved the PPAR
coactivator-1
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Conclusions: This study suggests a rationale for a possible correction of moderate RC disorders due to mutations in nuclear genes, using existing drugs, and brings new insights into the role of PPAR in the regulation of the mitochondrial RC in human cells.
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Introduction |
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TopAbstractIntroductionPatients and MethodsResultsDiscussionReferences |
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One possible strategy to increase residual enzyme activity is to act at the level of gene expression to stimulate the transcription. Regulation of RC genes involves several specific transcription factors, including the nuclear respiratory factors (NRFs) 1 and 2, which bind and activate the promoters of various nuclear genes encoding RC subunits or RC-associated proteins, and also regulate gene expression of the mitochondrial transcription factor A (Tfam) (5, 6). Among recently characterized regulatory factors of RC genes is the peroxisome proliferator activated receptor (PPAR) coactivator-1 (PGC-1). PGC-1 does not bind directly to DNA but rather docks on and coactivates the transcription factors already bound to the promoter region of target genes, like NRF1 and NRF2 (5, 6). Accordingly, PGC-1 potentially up-regulates not only the expression of nuclear genes encoding RC components, but also the expression of Tfam, and, therefore, is now considered to be pivotal in the coordinate regulation of nuclear and mitochondrial genomes. This is supported by gain-of-function and loss-of-function experiments, which underscore the importance of PGC-1 in OXPHOS regulation in the mouse skeletal muscle and heart (7).
PPARs are ligand-activated nuclear receptors involved in the regulation of energy metabolism by controlling the expression levels of numerous genes, in particular in the mitochondrial fatty acid β-oxidation pathway (8, 9). Accordingly, the PPARs have focused much attention as potential therapeutic targets for treatment of diabetes, obesity, or cardiovascular diseases. Interestingly, recent data clearly suggest that PPAR-signaling pathway could also impact the RC. Thus, targeted overexpression of PPAR in the mouse muscle results in a marked increase in the proportion of fibers with high mitochondrial content, as revealed by succinate dehydrogenase (10) and cytochrome c oxidase (COX) expression studies (11). More importantly, it has recently been shown that PPAR agonists such as fibrates, drugs that have long been known to ameliorate dyslipidemia, stimulate the expression of PGC-1 gene in mice skeletal muscle, both in vitro and in vivo (12). Altogether, these data provide a rationale by which PPAR agonists might regulate the RC.
In the present study, we therefore tested whether bezafibrate or high-affinity PPAR agonists might up-regulate the expression of the mitochondrial RC complexes, and if this could lead to stimulate the expression of deficient RC enzyme activity or associated proteins in fibroblasts from patients with mitochondrial disorders due to CI, CIII, or CIV deficiencies.
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Patients and Methods |
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Patients 1–8, reported previously, harbored nuclear mutations in RC enzyme subunits or assembly factors (Table 1
) (13, 14, 15, 16, 17, 18, 19). Six other patients (nos. 9–14) exhibited a CIV deficiency expressed in cell culture. As for the majority of COX deficient patients, the disease-causing mutations in these six patients are unknown (17, 20, 21) despite extensive efforts in sequence analysis of candidate genes.
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Skin fibroblast cell lines were established from patients 1–10, and 12 and 13. For patients 11 and 14, myoblasts derived from muscle biopsy were studied. Fibroblasts and myoblasts were grown under standard conditions (22). Bezafibrate (400 µM for fibroblasts or 200 µM for myoblasts) or vehicle (dimethylsulfoxide), or PPAR-selective compounds were added to the culture medium for the indicated time. To determine optimal conditions for induction of RC enzyme by bezafibrate, COX activity was measured in dose-response and kinetics experiments (Fig. 1). The PPAR-selective agonists used were GW 7647, GW 0742, and GW 7845 (obtained from Dr. Huet, GlaxoSmithKline, Les Ulis, France). The concentrations used, 1 µM for GW 7647, 10 nM for GW 0742, and 1 µM for GW 7845, are selective for PPARs , , and , respectively (22, 23).
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Fibroblasts pellets were homogenized, and enzyme activities were measured by spectrophotometric methods described in Ref. 24 . CI was assayed after fibroblast treatment with digitonin and Percoll as described in Ref. 25 .
COX histochemistry
Myoblasts were grown in 24-well plates, and the medium was replaced by 200 µl COX assay buffer [0.01% digitonin, 0.5 mg/ml catalase, 1 mg/ml cytochrome c, and 0.5 mg/ml 3,3'-diaminobenzidine tetrahydrochloride, in 0.1 M 3-(N-morpholino)propanesulfonic acid (pH 7.3). Myoblasts were incubated for 3 h at 37 C before photography.
Real-time quantitative PCR (RT-QPCR)
The RT-QPCRs were performed as previously described in Ref. 22 using primers listed in supplemental Table 1, which is published as supplemental data on The Endocrine Society’s Journals Online web site at http://jcem.endojournals.org. All primers spanned an intron/exon boundary. The results were normalized for comparison by measuring β-actin mRNA levels in each sample.
Western blot analysis
Cell protein extracts (20–25 µg) were resolved by 12% SDS-PAGE and transferred to polyvinylidene fluoride membranes. Membranes were blocked with 10% nonfat dry milk/0.1% Tween 20 and incubated with specific antibodies (Molecular Probes, Inc., Eugene, OR, or MitoSciences, Eugene, OR). Secondary antibody was used; the signals were revealed by chemiluminescence (enhanced chemiluminescence; Amersham Biosciences Inc., Piscataway, NJ) and quantified by computerized analysis.
Polarographic studies
Respiration of digitonin-permeabilized fibroblasts (digitonin 0.01% for 1 min) was measured using a Clark oxygen electrode at 37 C in 0.27 ml incubation medium containing: 10 mM KH2PO4, 300 mM mannitol, 10 mM KCl, 5 mM MgCl2, 1 mM ADP, 1 mg/ml BSA (pH 7.4), and 8 mM pyruvate + 1 mM malate or 10 mM succinate as mitochondrial substrates (26). The respiratory control ratio, i.e. maximal to basal respiration rates ratio, was 4.5–5.5, indicating good mitochondrial integrity.
Potassium cyanide (KCN) titration of COX activity in myoblasts
Respiration of digitonin-permeabilized myoblasts was measured polarographically at 37 C in 0.27 ml incubation medium containing 2 mM ADP, 6 µM rotenone, and 10 mM succinate or 400 µM N,N,N',N'-tetramethyl-p-phenylenediamine (TMPD), 1 mM ascorbate, 2 mM ADP, and 0.6 µM antimycin (27, 28). Auto-oxidation of TMPD plus ascorbate was negligible compared with the cell O2 consumption rates. Small amounts of 3 mM KCN solution were sequentially introduced at appropriate intervals to allow accurate measurements of constant slopes. The KCN-inhibited respiration measured after each KCN addition was expressed as percentage of the uninhibited respiration (27, 28).
Statistical analysis
Differences between vehicle and treated cells were analyzed by one-way ANOVA and the Fisher test, or by the unpaired t test. P < 0.05 was considered significant.
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Results |
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Cultured fibroblasts or myoblasts from 14 patients with mitochondrial disorders due to CI (n = 5), CIII (n = 1), or CIV (n = 8) deficiency were exposed to bezafibrate for 72 h, and the activities of the deficient RC complex were determined (Fig. 2). As shown in Fig. 2A, three out of five CI-deficient cell lines (patients 2, 4, and 5) exhibited a marked increase in CI activity [reduced nicotinamide adenine dinucleotide (NADH)-decylubiquinone oxidoreductase (NQR)] in response to bezafibrate (x1.4 to x2), and a stimulatory effect of bezafibrate (x1.4) on NQR activity was also observed in control fibroblasts. Furthermore, pharmacological treatment resulted in the restoration of normal NQR enzyme levels in patient 4 and 5 fibroblasts, which initially exhibited a 40% enzyme defect compared with control. CIII enzyme determinations [decylubiquinol cytochrome c reductase (QCCR)] revealed a potent induction of this enzyme activity by bezafibrate in control (x1.5), and in CIII-deficient fibroblasts as well (Fig. 2B). Indeed, more than a 2-fold increase in the QCCR level was observed in patient 6 fibroblasts treated by bezafibrate, leading to a complete correction of the initial enzyme defect (–68% relative to control). Exposure to bezafibrate also led to a marked (x1.5) increase in CIV enzyme activity (COX) in control cells, whereas various pharmacological profiles were found in the eight COX-deficient cell lines, in response to the drug (Fig. 2C). Thus, bezafibrate triggered a 1.4- to 2-fold increase in COX activity in patient 8 and 11–14 cells, and this led to restore normal enzyme activity levels in four of these cell lines (patients 11–14). In contrast, fibroblasts from patients 7, 9, and 10 showed no significant changes in COX activity in response to bezafibrate. COX histochemistry performed in control and patient 14 myoblasts (Fig. 2D) revealed a weak COX staining in untreated patient myoblasts, which was markedly increased after exposure to bezafibrate.
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-specific agonist restores RC enzyme activities in CI and CIV-deficient human cells
To determine which PPAR isoforms mediates the response to bezafibrate, cells were treated for 48 h by agonists specific for PPAR , or or . These experiments were performed in control, and in patients 5 and 14 that exhibited the highest increases in CI and CIV enzyme activities, when treated by bezafibrate (Fig. 3). Exposure to GW 7647 or GW 7845 induced no significant changes in RC enzyme activities. In contrast, treatment by GW 0742 triggered a 1.3-fold increase in CI and COX enzyme activities in controls. In patients 5 and 14, GW 0742 stimulated CI and COX enzyme activities by 1.7- and 1.6-fold, respectively, leading to the correction of the initial enzyme defects in both cell lines.
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Quantitative RT-PCR studies were performed to determine the effects of bezafibrate on mRNA levels of the disease-causing genes responsible for CI, CIII, or CIV deficiency (Fig. 4). Treatment by bezafibrate increased all mRNA levels in control fibroblasts (from x1.4 to x2.1 compared with vehicle-treated cells). In the patient cells, the mutated mRNAs were similarly increased by bezafibrate, except in patients 3 and 1, consistent with the lack of increase in CI enzyme activity in these two patients.
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Studies were performed using the antibody specific of the mutated gene when available (NDUFS3, SURF1) or antibodies representative of the mutated complex, i.e. core 2 for CIII, or COX2 (mitochondrial-encoded subunit) and COX4 (nuclear-encoded subunit) for CIV. Equal loading was checked with an antibody against mitochondrial porin (Fig. 5).
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). The basal levels of core 2 protein were slightly but not significantly reduced in untreated patient 6 cells (Fig. 5B). Treatment by bezafibrate markedly increased (>2-fold) the amount of core 2 protein in patient 6 and control fibroblasts. A severe decrease in SURF1 protein was found in untreated patient 7 cells (Fig. 5C), down to less than 10% of normal level, and no significant change occurred in response to bezafibrate. In control cells, in contrast, the SURF1 protein levels were strongly inducible by bezafibrate. Altogether, the lack of changes in SURF1 mutant protein in patient 7 cells treated with bezafibrate could account for the lack of increase in COX activity (Fig. 2C), despite a drug-induced increase in SURF1 mRNA (Fig. 4). Finally, the levels of COX2 and COX4 subunits (Fig. 5D) increased about 2-fold in control cells exposed to bezafibrate. In untreated patient 8 cells (COX10 deficient), both COX2 and COX4 proteins were markedly reduced (–80 and –50% relative to control, respectively). Treatment of patient 8 cells with bezafibrate induced a robust increase in COX2 and COX4 proteins (x4.7 and x2.3, respectively), resulting in the restoration of normal levels of both COX subunits. Effects of bezafibrate on respiratory rates in control and COX-deficient cells
To determine whether drug-induced changes in enzyme activity of the various RC complexes impacted on mitochondrial respiration, we performed polarographic assays in control and in two COX-deficient cell lines that exhibited quite different response to bezafibrate, i.e. patients 7 (SURF1) and 8 (COX10).
In control fibroblasts, drug-induced increases in respiration rates were observed (Table 2). Thus, pyruvate plus malate and succinate oxidation rates were significantly stimulated (+32 and +45%; P < 0.05) after treatment by bezafibrate. In patient 7, both oxidation rates were deficient, and no significant increases occurred after treatment by bezafibrate. In patient 8, in contrast, the initial defects in substrate oxidation (–25 to –30% relative to control) were fully corrected by bezafibrate, which induced a x1.3 to x1.5 increase in respiration.
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). These experiments were performed with succinate as a substrate, to titrate COX as RC integrated step, or with TMPD plus ascorbate, which directly feed the RC at the level of CIV, allowing to titrate COX as an isolated step (28, 29). Under these conditions it has been shown that titration curves from COX-deficient cells are steeper than those from healthy control, due to the diminished COX reserve in the deficient cells (30, 31). Accordingly, the slope of the titration curves in patient myoblasts (–1.98, Fig. 6B) was steeper (–20%) compared with control (–1.61, Fig. 6A). Bezafibrate treatment increased the slopes with: y = –1.36x + 99.2, r = 0.995 and y = –1.34x + 99.4, and r = 0.997 in patient and control cells, respectively. This correction was even more obvious when the experiments were performed in the presence of TMPD plus ascorbate (Fig. 6, C and D). Indeed, the steeper slope in untreated patient cells (–2.76, –26%) compared with control (–2.05) pointed to a rate limitation of respiration in the COX-deficient myoblasts. However, exposure to bezafibrate resulted in a marked increase in the slope (–1.86 vs. –2.76 in untreated cells), which then reached the control range, reflecting an increase in COX reserve within the mitochondria.
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Table 3 shows that PGC-1 mRNA levels were strongly increased (by 2- to 3-fold) by bezafibrate treatment in control and patients cells. Interestingly, bezafibrate generally induced an increase in gene expression of NRF1, NRF2, and Tfam, key transcription factors of mitochondrial proteins.
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Discussion |
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The results obtained from control fibroblasts clearly indicate a stimulatory effect of bezafibrate on RC, as shown by the drug-induced increases in NQR, QCCR, and COX activities, and by the parallel changes in the levels of proteins representative of CI, CIII, and CIV. In addition, bezafibrate significantly increased the transcript levels of a number of genes encoding RC subunits or ancillary proteins. As an initial step in testing bezafibrate in RC deficiencies, we studied a panel of patient cells carrying known disease-causing mutations in nuclear genes of CI, CIII, and CIV, as well as COX-deficient cells with unknown pathogenic mutations.
Consistent with the observations made in control cells, bezafibrate was found to potentially stimulate the residual enzyme activity of CI, CIII, and CIV in deficient cells. Indeed, nine out of the 14 patient cell lines tested (64%) exhibited a significant increase in the activity of the deficient RC complex after treatment by bezafibrate, and full pharmacological correction of the enzyme defect was achieved in seven cell lines initially presenting a CI, CIII, or CIV deficiency. These increases in enzyme activities were associated with parallel increases in mRNA levels of the mutated genes, and in some instances we investigated the changes in protein levels induced by bezafibrate. Altogether, our results showed that the mutation T145I/R199W of NDUFS3 gene (patient 2) resulted in a clear reduction in NDUFS3 protein level, which appeared consistent with the decreased CI activity. Furthermore, in this patient’s cells, the drug-induced increase in CI enzyme activity was clearly related to the parallel increases in NDUFS3 mRNAs and protein levels. In patient 6, the homozygous P99L mutation of BCS1 gene led to a slight decrease in core 2 protein level, in keeping with the initial description of this BCS1-deficient patient, which reported a reduced expression of two other CIII proteins (Rieske and core 1 subunits) (16). In patient 6, bezafibrate stimulated the expression of the BSC1 gene and increased core 2 protein level, which might account for the stimulation of CIII activity. The patient 8 cells, which harbored a homozygous N204K mutation of COX10 assembly factor, exhibited markedly low levels of COX2 and COX 4 proteins, as previously described (18, 33). Treatment by bezafibrate stimulated the expression of the mutated COX 10 gene, restored COX2 and COX 4 proteins to normal levels, and resulted in a marked stimulation of COX activity. Interestingly, the partial correction of COX deficiency by bezafibrate appeared sufficient to restore normal respiratory rates in treated cells.
Three cell lines carrying identified gene mutations (patients 1, 3, and 7) did not respond to bezafibrate. Our data suggest that this could be related to the lack of induction of mutated mRNA in patient 1 and 3 cells in response to bezafibrate. In patient 7 cells, the homozygous SURF1 gene mutation P183fsX189, predicting a truncated protein, was associated with extremely low levels of SURF1 protein, and consistent with a key role of SURF1 in CIV assembly, the residual COX activity was markedly reduced. Exposure to bezafibrate increased SURF1 mRNAs similarly in control and patient cells, and strongly up-regulated SURF1 protein in control fibroblasts. However, the defective SURF1 protein was not inducible by bezafibrate in patient 7 cells, which likely explains the lack of changes in COX activity in the treated cells.
Together, our data show that, in control human cells, bezafibrate can up-regulate the expression of genes encoding RC enzyme subunits or assembly factors, resulting in increases of the corresponding RC enzyme activities. This drug effect might account for the pharmacological stimulation or correction of the deficient-RC complex, which was achieved in some cells from patients with mitochondrial disorders, when the disease-causing mutations did not severely affect the production of mutated mRNA or protein.
This, together with recent data in animal models (10, 11), clearly suggests that PPARs could take part in OXPHOS regulation. However, the molecular mechanisms by which PPARs might control RC genes are not fully elucidated. The simplest explanation would be the presence of a functional PPAR response element (PPRE) on the 5'-flanking region of target genes. However, PPREs have never been found in the promoter regions of OXPHOS genes, despite extensive studies (5), suggesting that PPAR-mediated regulation of RC is likely indirect. In support of this, the present study showed that PPAR activation triggered a coordinate up-regulation of several key OXPHOS transcription regulators, and in particular of PGC-1. This marked induction of PGC-1 is consistent with the recent identification of a PPRE on the human PGC-1 gene promoter (34). Furthermore, forced expression studies in mammalian cells demonstrated that increasing PGC-1 level induced the expression of NRF1 and NRF2 (35). Therefore, induction of PGC-1 by bezafibrate might account for the parallel up-regulation of NRF1 and NRF2 in the treated cells. Altogether, enhanced expression of the transcription factors NRF1 and NRF2, and of their coactivator PGC-1, in response to bezafibrate, likely account for the stimulation of RC nuclear genes.
CI, CIII, and CIV are multisubunit enzymes encoded by the nuclear and the mitochondrial genomes, and parallel increases in the mRNA of COX2 (mitochondrial gene) and COX4 (nuclear gene) were found in bezafibrate-treated cells (data not shown), consistent with the Western blot data. Increases in COX2 and possibly other mitochondrial mRNAs can be explained by the induction of the mitochondrial transcription factor Tfam (6), in bezafibrate-treated cells. Interestingly, stimulation of Tfam gene expression might result from multiple events because both NRF1 and NRF2 regulate the human Tfam gene (5), and because overexpression of PGC-1 has also been reported to increase Tfam expression (35). These observations led us to propose that upon activation by bezafibrate, PPAR directly stimulates the expression level of PGC-1, which in turn regulates NRF1, NRF2, and Tfam expression. This could ensure a coordinate stimulation of nuclear and mitochondrial RC gene expression leading to subsequent increases in RC proteins and enzyme activities.
Our experiments were performed using bezafibrate, a widely used hypolipidemic drug considered as a pan (, , and ) PPAR agonist (36). Experiments using specific PPAR agonists indicate that bezafibrate effects on RC gene expression are likely mediated by the PPAR isoform, in agreement with recent literature data. Indeed, PPAR and not PPAR was shown to activate PGC-1 gene expression in muscle (12). In addition, targeted overexpression of PPAR in the mouse muscle results in a marked increase in complex II enzyme activity (10) and in COX mRNA (11). Interestingly, chronic mice treatment with a PPAR agonist also resulted in the stimulation of OXPHOS gene expression in skeletal muscle, together with a global increase in oxidative metabolism (10, 11, 37). Finally, recent data obtained in mice in which PPAR has been selectively ablated in skeletal muscle indicate that PGC-1 mRNAs were decreased and that transcript levels of several nuclear genes encoding components of the RC complexes were significantly lower compared with wild-type animals (34).
Overall, our study brings new insights into the role of PPAR in the control of energy metabolism by demonstrating a functional link between activation of the PPAR pathway and the regulation of mitochondrial RC in human cells. Exposure to bezafibrate can correct RC deficiency in patient cells, indicating the relevance of OXPHOS control by PPAR in a physiopathological context. This might have implications in the field of RC disorders but also in the context of common metabolic diseases like type 2 diabetes, for which OXPHOS regulation recently emerged as an important issue (38, 39), and PPARs are considered as major therapeutic targets. Finally, our approach suggests a new rationale to tackle the difficult question of inborn RC disorder treatment, which still remains largely unsolved.
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Footnotes |
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Disclosure Statement: The authors have nothing to declare.
First Published Online January 22, 2008
Abbreviations: CI, Complex I; CIII, complex III; CIV, complex IV; COX, cytochrome c oxidase; KCN, potassium cyanide; NADH, reduced nicotinamide adenine dinucleotide; NQR, NADH-decylubiquinone oxidoreductase; NRF, nuclear respiratory factor; OXPHOS, oxidative phosphorylation; PGC-1, peroxisome proliferator activated receptor coactivator-1; PPAR, peroxisome proliferator-activated receptor; PPRE, PPAR response element; QCCR, decylubiquinol cytochrome c reductase; RC, respiratory chain; RT-QPCR, real-time quantitative PCR; Tfam, mitochondrial transcription factor A; TMPD, N,N,N',N'-tetramethyl-p-phenylenediamine.
Received July 31, 2007.
Accepted January 10, 2008.
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References |
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controls muscle development and oxidative capability. FASEB J 17:2299–2301. PLoS Biol [Erratum (2005) 3:e61] 2:e294, but not PPAR, activates PGC-1 gene transcription in muscle. Biochem Biophys Res Commun 354:1021–1027[CrossRef][Medline] (PPAR) agonist but not PPAR corrects carnitine palmitoyl transferase 2 deficiency in human muscle cells. J Clin Endocrinol Metab 90:1791–1797 knock-out mice. Evidence for compensatory regulation by PPAR. J Biol Chem 277:26089–26097 expression is controlled in skeletal muscles by PPARβ, whose ablation results in fiber-type switching, obesity, and type 2 diabetes. Cell Metab 4:407–414[CrossRef][Medline]: a dagger in the heart of the metabolic syndrome. J Clin Invest 116:590–597[CrossRef][Medline]-responsive genes involved in oxidative phosphorylation are coordinately downregulated in human diabetes. Nat Genet 34:267–273[CrossRef][Medline]This article has been cited by other articles:
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  S Rahman and M G Hanna Diagnosis and therapy in neuromuscular disorders: diagnosis and new treatments in mitochondrial diseases J. Neurol. Neurosurg. Psychiatry, September 1, 2009; 80(9): 943 - 953. [Abstract] [Full Text] [PDF]
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 S. Srivastava, F. Diaz, L. Iommarini, K. Aure, A. Lombes, and C. T. Moraes PGC-1{alpha}/{beta} induced expression partially compensates for respiratory chain defects in cells from patients with mitochondrial disorders Hum. Mol. Genet., May 15, 2009; 18(10): 1805 - 1812. [Abstract] [Full Text] [PDF]
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