Isabelle Broutin

INF2 Mutations in Charcot–Marie–Tooth Disease with Glomerulopathy

BACKGROUND Charcot-Marie-Tooth neuropathy has been reported to be associated with renal diseases, mostly focal segmental glomerulosclerosis (FSGS). However, the common mechanisms underlying the neuropathy and FSGS remain unknown. Mutations in INF2 were recently identified in patients with autosomal dominant FSGS. INF2 encodes a formin protein that interacts with the Rho-GTPase CDC42 and myelin and lymphocyte protein (MAL) that are implicated in essential steps of myelination and myelin maintenance. We therefore hypothesized that INF2 may be responsible for cases of Charcot-Marie-Tooth neuropathy associated with FSGS. METHODS We performed direct genotyping of INF2 in 16 index patients with Charcot-Marie-Tooth neuropathy and FSGS who did not have a mutation in PMP22 or MPZ, encoding peripheral myelin protein 22 and myelin protein zero, respectively. Histologic and functional studies were also conducted. RESULTS We identified nine new heterozygous mutations in 12 of the 16 index patients (75%), all located in exons 2 and 3, encoding the diaphanous-inhibitory domain of INF2. Patients presented with an intermediate form of Charcot-Marie-Tooth neuropathy as well as a glomerulopathy with FSGS on kidney biopsy. Immunohistochemical analysis revealed strong INF2 expression in Schwann-cell cytoplasm and podocytes. Moreover, we demonstrated that INF2 colocalizes and interacts with MAL in Schwann cells. The INF2 mutants perturbed the INF2-MAL-CDC42 pathway, resulting in cytoskeleton disorganization, enhanced INF2 binding to CDC42 and mislocalization of INF2, MAL, and CDC42. CONCLUSIONS INF2 mutations appear to cause many cases of FSGS-associated Charcot-Marie-Tooth neuropathy, showing that INF2 is involved in a disease affecting both the kidney glomerulus and the peripheral nervous system. These findings provide new insights into the pathophysiological mechanisms linking formin proteins to podocyte and Schwann-cell function. (Funded by the Agence Nationale de la Recherche and others.).

Mutations in INF2 Are a Major Cause of Autosomal Dominant Focal Segmental Glomerulosclerosis

The recent identification of mutations in the INF2 gene, which encodes a member of the formin family of actin-regulating proteins, in cases of familial FSGS supports the importance of an intact actin cytoskeleton in podocyte function. To determine better the prevalence of INF2 mutations in autosomal dominant FSGS, we screened 54 families (78 patients) and detected mutations in 17% of them. All mutations were missense variants localized to the N-terminal diaphanous inhibitory domain of the protein, a region that interacts with the C-terminal diaphanous autoregulatory domain, thereby competing for actin monomer binding and inhibiting depolymerization. Six of the seven distinct altered residues localized to an INF2 region that corresponded to a subdomain of the mDia1 diaphanous inhibitory domain reported to co-immunoprecipitate with IQ motif-containing GTPase-activating protein 1 (IQGAP1). In addition, we evaluated 84 sporadic cases but detected a mutation in only one patient. In conclusion, mutations in INF2 are a major cause of autosomal dominant FSGS. Because IQGAP1 interacts with crucial podocyte proteins such as nephrin and PLCε1, the identification of mutations that may alter the putative INF2-IQGAP1 interaction provides additional insight into the pathophysiologic mechanisms linking formin proteins to podocyte dysfunction and FSGS.

NKX2-1 Mutations Leading to Surfactant Protein Promoter Dysregulation Cause Interstitial Lung Disease in "Brain-Lung-Thyroid Syndrome''

NKX2‐1 (NK2 homeobox 1) is a critical regulator of transcription for the surfactant protein (SP)‐B and ‐C genes (SFTPB and SFTPC, respectively). We identified and functionally characterized two new de novo NKX2‐1 mutations c.493C>T (p.R165W) and c.786_787del2 (p.L263fs) in infants with closely similar severe interstitial lung disease (ILD), hypotonia, and congenital hypothyroidism. Functional analyses using A549 and HeLa cells revealed that NKX2‐1‐p.L263fs induced neither SFTPB nor SFTPC promoter activation and had a dominant negative effect on wild‐type (WT) NKX2‐1. In contrast,NKX2‐1‐p.R165W activated SFTPC, to a significantly greater extent than did WTNKX2‐1,whileSFTPB activation was only significantly reduced in HeLa cells. In accordance with our in vitro data, we found decreased amounts of SP‐B and SP‐C by western blot in bronchoalveolar lavage fluid (patient with p.L263fs) and features of altered surfactant protein metabolism on lung histology (patient with NKX2‐1‐p.R165W). In conclusion, ILD in patients with NKX2‐1 mutations was associated with altered surfactant protein metabolism, and both gain and loss of function of the mutated NKX2‐1 genes on surfactant protein promoters were associated with ILD in “Brain‐Lung‐Thyroid syndrome”.

Novel NOBOX loss‐of‐function mutations account for 6.2% of cases in a large primary ovarian insufficiency cohort

Primary ovarian insufficiency (POI) is a disorder associated with female infertility, which affects approximately 1% of women under 40 years of age. A genetic component has been suggested as one possible cause of the majority of cases of nonsyndromic forms. Newborn Ovary Homeobox (NOBOX) is an ovary‐specific gene, playing a critical role in ovary in mice, as its absence leads to sterility mimicking a POI. In this study, we sequenced NOBOX in a cohort of 178 women with idiopathic POI. Among 19 identified variations, we described one nonsense (c.907C>T/p.R303X) and four missense (c.271G>T/p.G91W, c.349C>T/p.R117W, c.1025G>C/p.S342T, and c.1048G>T/p.V350L) NOBOX heterozygous mutations in 12 patients. We reproduced each of the five mutations and tested their effects on the signaling activity in transfected cells. We demonstrated that these mutations compromised the ability of the proteins to bind to and transactivate the well‐known growth differentiation factor 9 (GDF9) promoter. The pattern of our findings suggests that the genetic mechanism in humans responsible for POI in women involves haploinsufficiency rather than dominant negative gene action. The identification, characterization, and the very high 6.2% prevalence of these new mutations in POI patients suggest considering NOBOX as the first autosomal candidate gene involved in this syndrome. Hum Mutat 32:1108–1113, 2011. ©2011 Wiley‐Liss, Inc.

Structural effects of monovalent anions on polymorphic lysozyme crystals.

Understanding direct salt effects on protein crystal polymorphism is addressed by comparing different crystal forms (triclinic, monoclinic, tetragonal and orthorhombic) for hen, turkey, bob white quail and human lysozymes. Four new structures of hen egg-white lysozyme are reported: crystals grown in the presence of NapTS diffracted to 1.85 A, of NaI to 1.6 A, of NaNO(3) to 1.45 A and of KSCN to 1.63 A. These new structures are compared with previously published structures in order to draw a mapping of the surface of different lysozymes interacting with monovalent anions, such as nitrate, chloride, iodide, bromide and thiocyanate. An analysis of the structural sites of these anions in the various lysozyme structures is presented. This study shows common anion sites whatever the crystal form and the chemical nature of anions, while others seem specific to a given geometry and a particular charge environment induced by the crystal packing.

Structural and Thermodynamic Bases for the Design of Pure Prolactin Receptor Antagonists X-RAY STRUCTURE OF Del1-9-G129R-hPRL

Competitive antagonists of the human prolactin (hPRL) receptor are a novel class of molecules of potential therapeutic interest in the context of cancer. We recently developed the pure antagonist Del1-9-G129R-hPRL by deleting the nine N-terminal residues of G129R-hPRL, a first generation partial antagonist. We determined the crystallographic structure of Del1-9-G129R-hPRL, which revealed no major change compared with wild type hPRL, indicating that its pure antagonistic properties are intrinsically due to the mutations. To decipher the molecular bases of pure antagonism, we compared the biological, physicochemical, and structural properties of numerous hPRL variants harboring N-terminal or Gly129 mutations, alone or combined. The pure versus partial antagonistic properties of the multiple hPRL variants could not be correlated to differences in their affinities toward the hPRL receptor, especially at site 2 as determined by surface plasmon resonance. On the contrary, residual agonism of the hPRL variants was found to be inversely correlated to their thermodynamic stability, which was altered by all the Gly129 mutations but not by those involving the N terminus. We therefore propose that residual agonism can be abolished either by further disrupting hormone site 2-receptor contacts by N-terminal deletion, as in Del1-9-G129R-hPRL, or by stabilizing hPRL and constraining its intrinsic flexibility, as in G129V-hPRL.

Tripartite assembly of RND multidrug efflux pumps.

Tripartite multidrug efflux systems of Gram-negative bacteria are composed of an inner membrane transporter, an outer membrane channel and a periplasmic adaptor protein. They are assumed to form ducts inside the periplasm facilitating drug exit across the outer membrane. Here we present the reconstitution of native Pseudomonas aeruginosa MexAB–OprM and Escherichia coli AcrAB–TolC tripartite Resistance Nodulation and cell Division (RND) efflux systems in a lipid nanodisc system. Single-particle analysis by electron microscopy reveals the inner and outer membrane protein components linked together via the periplasmic adaptor protein. This intrinsic ability of the native components to self-assemble also leads to the formation of a stable interspecies AcrA–MexB–TolC complex suggesting a common mechanism of tripartite assembly. Projection structures of all three complexes emphasize the role of the periplasmic adaptor protein as part of the exit duct with no physical interaction between the inner and outer membrane components.

Enzyme structural plasticity and the emergence of broad-spectrum antibiotic resistance.

The emergence of multi‐resistant pathogenic bacteria is a worldwide health issue. Recently, clinical variants of a single antibiotic‐modifying acetyltransferase, AAC(6′)‐Ib—a variant of aminoglycoside 6′‐N‐acetyltransferase—have been identified that confer extended resistance to most aminoglycosides and, more surprisingly, to structurally unrelated fluoroquinolones. The corresponding gene is carried by mobile genetic elements and is present in most multi‐resistant pathogenic strains, hence making it a serious threat to current therapies. Here, we report the crystal structures of both narrow‐ and broad‐spectrum resistance variants of this enzyme, which reveal the structural basis for the emergence of extended resistance. The active site shows an important plasticity and has adapted to new substrates by a large‐scale gaping process. We have also obtained co‐crystals with both substrates, and with a simple transition state analogue, which provides new clues for the design of inhibitors of this resistance mechanism.

Multiple mutations lead to MexXY/OprM-dependent aminoglycoside resistance in clinical strains of Pseudomonas aeruginosa

ABSTRACT Constitutive overproduction of the pump MexXY-OprM is recognized as a major cause of resistance to aminoglycosides, fluoroquinolones, and zwitterionic cephalosporins in Pseudomonas aeruginosa. In this study, 57 clonally unrelated strains recovered from non-cystic fibrosis patients were analyzed to characterize the mutations resulting in upregulation of the mexXY operon. Forty-four (77.2%) of the strains, classified as agrZ mutants were found to harbor mutations inactivating the local repressor gene (mexZ) of the mexXY operon (n = 33; 57.9%) or introducing amino acid substitutions in its product, MexZ (n = 11; 19.3%). These sequence variations, which mapped in the dimerization domain, the DNA binding domain, or the rest of the MexZ structure, mostly affected amino acid positions conserved in TetR-like regulators. The 13 remaining MexXY-OprM strains (22.8%) contained intact mexZ genes encoding wild-type MexZ proteins. Eight (14.0%) of these isolates, classified as agrW1 mutants, overexpressed the gene PA5471, which codes for the MexZ antirepressor AmrZ, with 5 strains exhibiting growth defects at 37°C and 44°C, consistent with mutations impairing ribosome activity. Interestingly, one agrW1 mutant appeared to harbor a 7-bp deletion in the coding sequence of the leader peptide, PA5471.1, involved in ribosome-dependent, translational attenuation of PA5471 expression. Finally, DNA sequencing and complementation experiments revealed that 5 (8.8%) strains, classified as agrW2 mutants, harbored single amino acid variations in the sensor histidine kinase of ParRS, a two-component system known to positively control mexXY expression. Collectively, these results demonstrate that clinical strains of P. aeruginosa exploit different regulatory circuitries to mutationally overproduce the MexXY-OprM pump and become multidrug resistant, which accounts for the high prevalence of MexXY-OprM mutants in the clinical setting.

The catalytic site of serine proteinases as a specific binding cavity for xenon

BACKGROUND Under moderate pressure, xenon can bind to proteins and form weak but specific interactions. Such protein-xenon complexes can be used as isomorphous derivatives for phase determination in X-ray crystallography. RESULTS Investigation of the serine proteinase class of enzymes shows that the catalytic triad, the common hydrolytic motif of these enzymes, is a specific binding site for one xenon atom and shows high occupancy at pressures below 12 bar. Complexes of xenon with two different serine proteinases, elastase and collagenase, were analyzed and refined to 2.2 A and 2.5 A resolution, respectively. In both cases, a single xenon atom with a low temperature factor is located in the active site at identical positions. Weak interactions exist with several side chains of conserved amino acids at the active site. Xenon binding does not induce any major changes in the protein structure and, as a consequence, crystals of the xenon complexes are highly isomorphous with the native protein structures. Xenon is also found to bind to the active site of subtilisin Carlsberg, a bacterial serine proteinase, that also has a catalytic triad motif. CONCLUSIONS As the region around the active site shows conserved structural homology in all serine proteinases, it is anticipated that xenon binding will prove to be a general feature of this class of proteins.