A molecular approach to dominance in hypophosphatasia

Hypophosphatasia is an inherited disorder characterized by defective bone mineralization and a deficiency of tissue-nonspecific alkaline phosphatase (TNSALP) activity. The disease is highly variable in its clinical expression, because of various mutations in the TNSALP gene. In approximately 14% of the patients tested in our laboratory, only one TNSALP gene mutation was found, despite exhaustive sequencing of the gene, suggesting that missing mutations are harbored in intron or regulatory sequences or that the disease is dominantly transmitted. The distinction between these two situations is of importance, especially in terms of genetic counseling, but dominance is sometimes difficult to conclusively determine by using familial analysis since expression of the disease may be highly variable, with parents of even severely affected children showing no or extremely mild symptoms of the disease. We report here the study of eight point mutations (G46 V, A99T, S164L, R167 W, R206 W, G232 V, N461I, I473F) found in patients with no other detectable mutation. Three of these mutations, G46 V, S164L, and I473F, have not previously been described. Pedigree and/or serum alkaline phosphatase data suggested possible dominant transmission in families with A99T, R167 W, and G232 V. By means of site-directed mutagenesis, transfections in COS-1 cells, and three-dimensional (3D) modeling, we evaluated the possible dominant effect of these eight mutations. The results showed that four of these mutations (G46 V, A99T, R167 W, and N461I) exhibited a negative dominant effect by inhibiting the enzymatic activity of the heterodimer, whereas the four others did not show such inhibition. Strong inhibition resulted in severe hypophosphatasia, whereas partial inhibition resulted in milder forms of the disease. Analysis of the 3D model of the enzyme showed that mutations exhibiting a dominant effect were clustered in two regions, viz., the active site and an area probably interacting with a region having a particular biological function such as dimerization, tetramerization, or membrane anchoring.

Model systems: Studying molecular recognition using bacterial n-hybrid systems

Since the first description of the yeast two-hybrid system, related genetic assays for protein-protein interactions have become popular and powerful tools for structure-function analysis on the scale of individual proteins or whole proteomes. After a somewhat surprising lag, similar systems have recently been described for use in bacterial hosts. n-hybrid modifications of the original yeast system have been used to examine interactions with DNA, RNA and small molecules, and other modifications have improved throughput for genomic applications. Bacterial n-hybrid systems are being designed for a similar array of uses. Will the bacterial systems be as popular as the yeast n-hybrid systems? Only time will tell.

Pubovaginal sling using cadaveric fascia and bone anchors: disappointing early results

PURPOSE

Pubovaginal sling procedures offer highly effective treatment for patients with female stress urinary incontinence. A recent modification of this technique is the use of cadaveric fascia lata as a sling material supported with titanium anchors placed bilaterally in the pubic bone. We reviewed our experience with this procedure and assessed our outcome.

MATERIALS AND METHODS

A total of 154 consecutive patients underwent a bone anchored, cadaveric fascia pubovaginal sling procedure by a single surgeon from July 1998 to June 1999. All patients were evaluated preoperatively with a detailed history, pelvic examination and radiographic or multichannel video urodynamic studies to diagnose stress urinary incontinence. Our technique begins with the nonincision placement of titanium bone anchors transvaginally into the pubic bone bilaterally. A 2 cm. wide tunnel is created bluntly beneath the vaginal epithelium between the 2 puncture sites with a right angle clamp. A 2 x 7 cm. strip of cadaveric fascia is then passed through the tunnel, into the retropubic space and secured to 2-0 polypropylene sutures attached to the anchors. After securing the sling, the transvaginal puncture sites are closed with 2-0 polyglactin sutures. Patients were seen postoperatively at 6 weeks, and 3 and 6-month followup. Patient age averaged 60 years (range 38 to 85), with an overall average length of followup from surgery of 10.6 months (range 6 to 16). All patients were mailed a self-administered questionnaire and participated in a telephone interview with an office nurse to retrospectively assess outcome and evaluate for recurrent stress urinary incontinence. Recurrent stress urinary incontinence was graded as 0-none, 1-rare, 2-moderate and 3-severe. Repeat pubovaginal sling procedure that was performed in patients with grades 2 to 3 stress urinary incontinence was considered a failure for the purpose of our study.

RESULTS

Of all 154 patients 58 (37.6%) had recurrent moderate to severe (grades 2 to 3) stress urinary incontinence at followup. A total of 26 patients underwent a second pubovaginal sling procedure for a reoperation rate of 16.9%. Intraoperative findings at reoperation revealed the titanium anchors to be in position, the polypropylene sutures to be intact, and retropubic fibrosis and scarring of the urethropelvic ligament suggesting appropriate retropubic placement of the sling in all cases. Uniformly all allogenic cadaveric fascia used for sling material appeared to be fragmented, attenuated or simply absent. Average time to reoperation was 9 months (range 3 to 15).

CONCLUSIONS

Early results using a bone anchored cadaveric fascia pubovaginal sling procedure were discouraging. Based on findings at reoperation, we attribute this result to the failure of our sling material and have abandoned the use of cadaveric fascia allografts in all pubovaginal slings at our institution.

Cloning and characterization of the mouse and human enamelin genes

Enamelin is likely to be essential for proper dental enamel formation. It is secreted by ameloblasts throughout the secretory stage and can readily be isolated from the enamel matrix of developing teeth. The gene encoding human enamelin is located on the long arm of chromosome 4, in a region previously linked to an autosomal-dominant form of amelogenesis imperfecta (AI). To gain information on the structure of the enamelin gene and to facilitate the future assessment of the role of enamelin in normal and diseased enamel formation, we have cloned and characterized the mouse and human enamelin genes. Both genes are about 25 kilobases long. The enamelin gene has 10 exons interrupted by 9 introns. Translation initiates in exon 3 and terminates in exon 10. All of the intron/exon junctions within the mouse and human enamelin coding regions are between codons, so there are no partial codons in any exon, and deletion of one or more coding exons by alternative RNA splicing would not shift the downstream reading frame.

An engineered leucine zipper a position mutant with an unusual three-state unfolding pathway

The leucine zipper is a dimeric coiled-coil structural motif consisting of four to six heptad repeats, designated (abcdefg)(n). In the GCN4 leucine zipper, a position 16 in the third heptad is occupied by an Asn residue whereas the other a positions are Val residues. Recently, we have constructed variants of the GCN4 leucine zipper in which the a position Val residues were replaced by Ile. The folding and unfolding of the wild-type GCN4 leucine zipper and the Val to Ile variant both adhere to a simple two-state mechanism. In this study, another variant of the GCN4 leucine zipper was constructed by moving the single Asn residue from a position 16 to a position 9. This switch causes the thermal unfolding of the GCN4 leucine zipper to become three state. The unfolding pathway of this variant was determined by thermal denaturation, limited proteinase K digestion, and sedimentation equilibrium analysis. Our data are consistent with a model in which the variant first unfolds from its N terminus and changes the oligomerization specificity from a native dimer to a partially unfolded intermediate containing a mixture of dimers and trimers and then completely unfolds to unstructured monomers.

Resistance to methotrexate due to AcrAB-dependent export from Escherichia coli

Many laboratory strains of Escherichia coli are resistant to methotrexate (MTX), a folate analogue that binds dihydrofolate reductase (DHFR). Mutations that inactivate either tolC or acrA confer MTX sensitivity. Further, overexpression of a fusion protein with DHFR activity reverses this sensitivity by titrating out intracellular MTX. These results suggest that MTX accumulates in cells where mutations in acrA or tolC have inactivated the TolC-dependent AcrAB multidrug resistance efflux pump.

Two-hybrid system for characterization of protein-protein interactions in E. coli

The yeast two-hybrid system has been used to characterize many protein-protein interactions. A two-hybrid system for E. coli was constructed in which one hybrid protein bound to a specific DNA site recruits another to an adjacent DNA binding site. The first hybrid comprises a test protein, the bait, fused to a chimeric protein containing the 434 repressor DNA binding domain. In the second hybrid, a second test protein, the prey, is fused downstream of a chimeric protein with the DNA binding specificity of the lambda repressor. Reporters were designed to express cat and lacZ under the control of a low-affinity lambda operator. At low expression levels, lambda repressor hybrids weakly repress the reporter genes. A high-affinity operator recognized by 434 repressor was placed nearby, in a position that does not yield repression by 434 repressor alone. If the test proteins interact, the 434 hybrid bound to the 434 operator stabilizes the binding of the lambda repressor hybrid to the lambda operator, causing increased repression of the reporter genes. Reconstruction experiments with the fos and jun leucine zippers detected protein-protein interactions between either homodimeric or heterodimeric leucine zippers.

The contribution of buried polar groups to the conformational stability of the GCN4 coiled coil

The dimeric interface of the leucine zipper coiled coil from GCN4 has been used to probe the contributions of hydrophobic and hydrogen bonding interactions to protein stability. We have determined the energetics of placing Ile or Asn residues at four buried positions in a two-stranded coiled coil. As expected, Ile is favored over Asn at these buried positions, but not as much as predicted by considering only the hydrophobic effect. It appears that interstrand hydrogen bonds form between the side-chains of the buried Asn residues and these contribute to the conformational stability of the coiled-coil peptides. However, these contributions are highly dependent on the locations of the Asn pairs. The effect of an Ile to Asn mutation is greatest at the N terminus of the peptide and decreases almost twofold as we move the substitution from the N to C-terminal heptads.

Characterization of the mouse and human PRSS17 genes, their relationship to other serine proteases, and the expression of PRSS17 in developing mouse incisors

The human PRSS17 (serine protease 17) gene, which is located on chromosome 19q in a cluster of genes encoding serine proteases, has been variously designated enamel matrix serine proteinase 1 (EMSP1), prostase, KLK4, and KLK-L1. We have cloned and characterized the mouse and human PRSS17 genes. Both have six exons and five introns. The mouse PRSS17 gene sequence is 10134bp; the human sequence is 7115bp. Computer analysis of the mouse PRSS17 gene sequence upstream of the translation initiation codon identified two potential transcription initiation sites, at nucleotides 2878 and 2336. The first nucleotide of the reported mouse PRSS17 cDNA sequence corresponds to position 2352 on the gene, only 16 bases downstream from one of the putative transcription initiation sites. Repetitive DNA sequences from the MSR1 family are found in both the mouse and human PRSS17 genes. Additionally, the human PRSS17 gene contains Tigger2, MER8, and Alu repetitive sequences. Phylogenetic analyses of human and rodent proteases suggest that the PRSS17 protein is not a member of the kallikrein family of serine proteases but that the PRSS17 gene may have originated prior to the divergence of the kallikrein and trypsin families of proteases. To better characterize the timing of PRSS17 expression in developing teeth, we performed in-situ hybridization on postnatal day 3 developing mouse mandibular incisors. PRSS17 mRNA was not detected in secretory stage ameloblasts but could be detected in odontoblasts, while transition-stage and maturation-stage ameloblasts were strongly positive. This pattern supports a role for the PRSS17 protein in the degradation of enamel proteins.

Characterization of a family with dominant hypophosphatasia

A kindred with dominant hypophosphatasia resulting from an alanine to threonine substitution at position 99 of the alkaline phosphatase protein is described. The clinical findings of individual members of the kindred were assessed by oral and physical examinations, or from the descriptions of multiple family members. The proband displayed enamel hypoplasia and premature loss of fully rooted primary anterior teeth, which were shown by histological examination to lack cementum. Serum alkaline phosphatase (ALP) and a vitamin B6 panel, and urine phosphoethanolamine (PEA) were measured on 21 family members. Based upon the clinical and laboratory tests, affected and unaffected status was assigned. Parametric linkage analysis of the kindred using different dominant models and frequency distributions for the disease allele and the mutation gave lodscores > 4.2 and confirmed the strong linkage between the disease and the mutation. Assuming the defined mutation causes the disease, the reliability of clinical and laboratory tests is assessed.

Chemically induced dimerization of dihydrofolate reductase by a homobifunctional dimer of methotrexate

BACKGROUND

Chemically induced dimerization (CID) can be used to manipulate cellular regulatory pathways from signal transduction to transcription, and to create model systems for study of the specific interactions between proteins and small-molecule chemical ligands. However, few CID systems are currently available. The properties of, and interactions between, Escherichia coli dihydrofolate reductase (DHFR) and the ligand methotrexate (MTX) meet many of the desired criteria for the development of a new CID system.

RESULTS

BisMTX, a homobifunctional version of MTX, was synthesized and tested for its ability to induce dimerization of DHFR. Gel-filtration analysis of purified DHFR confirmed that, in vitro, the protein was a monomer in the absence of dimerizer drug; in the presence of bisMTX, a complex of twice the monomeric molecular weight was observed. Furthermore, the off-rate was found to be 0.0002 s(-1), approximately 100 times slower than that reported for DHFR-MTX. Interestingly, the addition of excess bisMTX did not result in formation of the binary complex (1 protein:1 dimerizer) over the ternary complex (2 proteins:1 dimerizer), which suggests cooperative binding interactions (affinity modulation) between the two DHFR molecules in the bisMTX:DHFR(2) ternary complex.

CONCLUSIONS

The combination of DHFR and bisMTX provides a new CID system with properties that could be useful for applications in vivo. Formation of the bisMTX:DHFR(2) ternary complex in vitro is promoted over a wide range of dimerizer concentrations, consistent with the idea that formation of the ternary complex recruits energetically favorable interactions between the DHFR monomers in the complex.

Escherichia coli one- and two-hybrid systems for the analysis and identification of protein-protein interactions

Genetic methods based on fusion proteins allow the power of a genetic approach to be applied to the self-assembly of proteins or protein fragments, regardless of whether or not the normal function of the fused assembly domains is either known or amenable to selection or screening. The widespread adoption of variations of the yeast two-hybrid system originally described by S. Fields and O. Song (1989, Nature 340, 245-246) demonstrates the usefulness of these kinds of assays. This review describes some of the many systems used to select or screen for protein-protein interactions based on the regulation of reporter constructs by hybrid proteins expressed in bacteria, including recent implementations of generalizable two-hybrid systems for Escherichia coli.

Localization of EMSP1 expression during tooth formation and cloning of mouse cDNA

Enamel matrix serine proteinase 1 (EMSP1) is a proteolytic enzyme that has been isolated from the developing enamel of pig teeth. Its apparent function is to degrade the organic matrix in preparation for enamel maturation. The expression of EMSP1 has never been investigated in another organism besides the pig, and EMSP1 expression in the enamel organ has never been specifically demonstrated in ameloblasts. Here we report the expression of recombinant pig EMSP 1 (rpEMSP 1), the generation of rabbit polyclonal antibodies against rpEMSP1, the characterization of the antibodies and EMSP1 expression by Western blot and immunohistochemical analyses, the cloning and characterization of a full-length cDNA encoding mouse EMSP1, and the localization of EMSP1 expression in ameloblasts in mouse day 14 first and second molars by in situ hybridization. The full-length mouse EMSP1 cDNA clone has 1,237 nucleotides, excluding the poly(A+) tail, and encodes a preproprotein of 255 amino acids. Mouse EMSP1 shares 75% amino acid identity with pig EMSP1 and has three potential N-linked glycosylation sites, two of which are conserved in the pig homologue. Western blot analysis shows that the polyclonal antibodies are specific for EMSP1 and do not cross-react with trypsin. Immunohistochemistry of pig incisors shows discrete staining in the surface enamel at the earliest part of the maturation stage. In mouse molars, in situ hybridization gives a distinct and specific signal in maturation-stage ameloblasts, and in the junctional epithelium following tooth eruption. We conclude that EMSP1 is expressed by pig and mouse ameloblasts during the early maturation stage of amelogenesis.

Genetic selection of short peptides that support protein oligomerization in vivo

An important goal in protein engineering is to control associations between designed proteins. This is most often done by fusing known, naturally occurring oligomerization modules, such as leucine zippers [1] [2] [3], to the proteins of interest [4] [5] [6]. It is of considerable interest to design or discover new oligomerization domains that have novel binding specificities [7] [8] [9] [10] [11] in order to expand the 'toolbox' of the protein engineer and also to eliminate associations of the designed proteins with endogenous factors. We report here a simple genetic selection scheme through which to search libraries for peptides that are able to mediate homodimerization or higher-order self-oligomerization of a protein in vivo. We found several peptides that support oligomerization of the lambda repressor DNA-binding domain in Escherichia coli cells, some of them as efficiently as the endogenous dimerization domain or the GCN4 leucine zipper. Many are very small, comprising as few as six residues. This study strongly supports the notion that peptide sequence space is rich in small peptides, which might be useful in protein engineering and other applications.

The association of selected pathological features with prostate cancer in a single-needle biopsy accession

Isolated high-grade prostatic intraepithelial neoplasia (PIN) has been shown to be a positive predictor of prostate cancer (PCa) on follow-up biopsy. However, the incidence of isolated high-grade PIN in needle biopsy specimens has been reported with a highly variable frequency of 1% to 15%. The current study examined the relationship of various pathological features with PCa on a single biopsy accession. A study population of 388 community-based consecutive needle biopsy accessions was prospectively recorded by a single pathologist (T.M.W.). All of the individual biopsy specimens were coded for the presence of PCa, high-grade PIN, low-grade PIN, chronic inflammation (CI), intraluminal prostatic crystalloids (IPC) in benign glands, and mucinous metaplasia (MM). One hundred twenty-nine (33%) of the patients were diagnosed with PCa. The 8% incidence of isolated high-grade PIN was consistent with previous studies. The incidence of other pathological features were as follows: high-grade PIN, 14%; low-grade PIN, 13%; CI, 30%; IPC, 4%; and MM, 8%. Of the patients with high-grade PIN, 47% had PCa on a separate core biopsy, whereas 31% of patients without high-grade PIN were observed to have PCa (P=.021). Of the patients with CI, 21% were found to have PCa on a separate core, whereas 38% of patients without CI were found to have PCa (P=.0009). None of the other pathological features surveyed showed any significant association with PCa. High-grade PIN was a relatively common finding (14%) in this study and was positively associated with PCa on a separate core from the same accession biopsy. The negative association of CP with PCa within the same accession has not been reported previously and may be an artifact related to the clinical indications for a prostatic biopsy.

Effect of the gasoline additives on PAH emission

PAH emission from the powered engines fueled by a 95 leadfree gasoline (95-LFG), a 92 leadfree gasoline (92-LFG) and a Premium leaded gasoline (PLG) with two gasoline additives (SA and SB) were collected using a PAH sampling system with a particulate interception device. Twenty one PAHs were analyzed primarily by an GC/MS, while eight metal elements were determined mainly by an ICP-AES. This investigation showed that the gasoline additives contain more amounts of carcinogenic PAHs than gasolines do. Blending these additives do raise the PAH content in the gasolines, simultaneously, will emit more amount of PAHs from the tailpipe of engine exhaust. It is suggested that before a gasoline additive is commercialized, an assessment on its PAH emission should be evaluated to make sure that the additive will not emit more PAHs and cause adverse effect on public health.

Oligomerization properties of GCN4 leucine zipper e and g position mutants

Putative intersubunit electrostatic interactions between charged amino acids on the surfaces of the dimer interfaces of leucine zippers (g-e' ion pairs) have been implicated as determinants of dimerization specificity. To evaluate the importance of these ionic interactions in determining the specificity of dimer formation, we constructed a pool of > 65,000 GCN4 leucine zipper mutants in which all the e and g positions are occupied by different combinations of alanine, glutamic acid, lysine, or threonine. The oligomerization properties of these mutants were evaluated based on the phenotypes of cells expressing lambda repressor-leucine zipper fusion proteins. About 90% of the mutants do not form stable homooligomers. Surprisingly, approximately 8% of the mutant sequences have phenotypes consistent with the formation of higher-order (> dimer) oligomers, which can be classified into three types based on sequence features. The oligomerization states of mutants from two of these types were determined by characterizing purified fusion proteins. The Type I mutant behaved as a tetramer under all tested conditions, whereas the Type III mutant formed a variety of higher-order oligomers, depending on the solution conditions. Stable homodimers comprise less than 3% of the pool; several g-e' positions in these mutants could form attractive ion pairs. Putative repulsive ion pairs are not found among the homodimeric mutants. However, patterns of charged residues at the e and g positions do not seem to be sufficient to predict either homodimer or heterodimer formation among the mutants.

Gene expression from plasmids containing the araBAD promoter at subsaturating inducer concentrations represents mixed populations

Gene expression from plasmids containing the araBAD promoter can be regulated by the concentration of arabinose in the growth medium. Guzman et al. [Guzman, L.-M., Belin, D., Carson, M. J. & Beckwith, J. (1995) J. Bacteriol. 177, 4121-4130] showed that expression of a cloned gene could be modulated over several orders of magnitude in cultures grown in the presence of subsaturating concentrations of arabinose. We constructed plasmids expressing a fast-folding mutant Aequorea victoria green fluorescent protein from the araBAD promoter to examine the distribution of expressed gene products in individual cells at intermediate induction levels. Microscopic examination of cells grown at low arabinose concentrations shows mixtures of brightly fluorescent and dark cells, suggesting that intermediate expression levels in cultures reflect a population average of induced and uninduced cells. The kinetics of green fluorescent protein induction suggest that this reflects an "autocatalytic" induction mechanism due to accumulation of the inducer by active transport. This mechanism, which is analogous to the induction of the lac operon at subsaturating inducer concentrations in lacY+ cells, was described 40 years ago by Novick and Weiner [Novick, A. & Weiner, M. (1957) Proc. Natl. Acad. Sci. USA 43, 553-566].

Oriented DNA binding by one-armed lambda repressor heterodimers and contacts between repressor and RNA polymerase at P(RM)

Bacteriophage lambda repressor activates transcription from P(RM) by contacting the sigma subunit of E. coli RNA polymerase. Although mutations in repressors that are defective in activation affect exposed residues in the repressor-operator co-crystal, the subunit in repressor dimers that is responsible for activation has not been determined experimentally. Here, we describe an oriented heterodimer approach using one-armed repressor-leucine zipper fusion proteins to resolve this question. Protection against methylation of operator DNA in vitro by one-armed and two-armed repressor-leucine zipper heterodimers shows that the subunit containing the arm in a one-armed repressor heterodimer binds preferentially in the consensus half-site of lambda operators. The patterns of activation of the P(RM) promoter in vivo by one-armed heterodimers containing the pc2 mutation in only one subunit show that RNA polymerase at P(RM) contacts the N-terminal domain of the promoter-proximal subunit of the repressor dimer.

Buried asparagines determine the dimerization specificities of leucine zipper mutants

Regulation of gene expression by many transcription factors is controlled by specific combinations of homo- and heterodimers through a short alpha-helical coiled-coil known as a leucine zipper. The dimer interface of a leucine zipper involves side chains of the residues at the a, d, e, and g positions of the (abcdefg)n heptad repeat. To understand the basis for the specificity of dimer formation, we characterized GCN4 leucine zipper mutants with all 16 possible permutations and combinations of isoleucines and asparagines at four a positions in the dimer interface, using a genetic test for the specificity of dimer formation by lambda repressor-leucine zipper fusions. Heterodimers were detected by loss of repressor activity in the presence of a fusion to a dominant-negative mutant form of the DNA-binding domain of repressor. Reconstruction experiments using leucine zippers from GCN4, Jun, Fos, and C/EBP showed that this assay distinguishes pairs that form heterodimers from those that do not. We found that the mutants have novel dimerization specificities determined by the positioning of buried asparagine residues at the a positions. The pattern of buried polar residues could also explain the dimerization specificities of some naturally occurring leucine zippers. The altered specificity mutants described here should be useful for the construction of artificial regulatory circuitry.