Sry High Mobility Group Box Research Articles

Inherited Human XY Sex Reversal and Gonadal Neoplasia Due to Enhanced Formation of Non-Specific Enhanceosomes by an Architectural Transcription Factor

The development of organisms is regulated by a fine-tuned gene-regulatory network, which is driven by transcription factors (TFs). In the embryogenesis, these TFs control diverse cell fates and final body plan. This is precisely regulated by a specific DNA-binding process and enhanceosome formation. A model is provided by testis determination in mammals, which is initiated by a Y-encoded architectural transcription factor, SRY. Mutations in SRY cause gonadal dysgenesis leading to various developmental defects. Such mutations cluster in SRY’s high mobility group (HMG) box, a sequence-specific DNA-binding domain shared by a conserved family of TFs. Here, we have characterized several mutations at the same position in HMG box, which are compatible with either male or female phenotypes as observed in an XY father and XY daughter, respectively. These mutations, at a function-unknown motif in the SRY HMG box, markedly disturb the specific DNA affinity. On transient transfection of human and rodent cell lines, the SRY variants exhibit decreased specific DNA-binding activity (relative to wild type) are associated with mis-formed enhanceosomes. The variants’ gene regulatory activities were reduced by 2-fold relative to wild-type SRY at similar levels of mRNA expression. When engineered mutations that functions to increase the DNA-binding specificity were deployed to SRY variants, the transcriptional activity was in association with restored occupancy of sex-specific enhancer elements in principal downstream gene Sox9. Our findings define a novel mechanism of impaired organogenesis, disturbed specific DNA-binding activity of a master transcription factor, leading to a developmental decision poised at the edge of ambiguity.

Human Sex Determination at the Edge of Ambiguity: INHERITED XY SEX REVERSAL DUE TO ENHANCED UBIQUITINATION AND PROTEASOMAL DEGRADATION OF A MASTER TRANSCRIPTION FACTOR

A general problem is posed by analysis of transcriptional thresholds governing cell fate decisions in metazoan development. A model is provided by testis determination in therian mammals. Its key step, Sertoli cell differentiation in the embryonic gonadal ridge, is initiated by SRY, a Y-encoded architectural transcription factor. Mutations in human SRY cause gonadal dysgenesis leading to XY female development (Swyer syndrome). Here, we have characterized an inherited mutation compatible with either male or female somatic phenotypes as observed in an XY father and XY daughter, respectively. The mutation (a crevice-forming substitution at a conserved back surface of the SRY high mobility group box) markedly destabilizes the domain but preserves specific DNA affinity and induced DNA bend angle. On transient transfection of diverse human and rodent cell lines, the variant SRY exhibited accelerated proteasomal degradation (relative to wild type) associated with increased ubiquitination; in vitro susceptibility to ubiquitin-independent ("default") cleavage by the 20S core proteasome was unchanged. The variant's gene regulatory activity (as assessed in a cellular model of the rat embryonic XY gonadal ridge) was reduced by 2-fold relative to wild-type SRY at similar levels of mRNA expression. Chemical proteasome inhibition restored native-like SRY expression and transcriptional activity in association with restored occupancy of a sex-specific enhancer element in principal downstream gene Sox9, demonstrating that the variant SRY exhibits essentially native activity on a per molecule basis. Our findings define a novel mechanism of impaired organogenesis, accelerated ubiquitin-directed proteasomal degradation of a master transcription factor leading to a developmental decision poised at the edge of ambiguity.

Transcriptional activity of testis-determining factor SRY is modulated by the Wilms' tumor 1 gene product, WT1.

The Wilms' tumor 1 (WT1) and sex-determining region of the Y chromosome (SRY) genes are essential for development of the mammalian gonads and mutations in these genes are associated with gonadal dysgenesis in humans. The SRY gene encodes a transcription factor with one high-mobility group (HMG) box as a DNA-binding domain. WT1 encodes a transcription factor that contains four contiguous C2H2-type zinc-finger motifs as a DNA/RNA binding or protein-protein interaction domain. Here we report that WT1 binds to and acts synergistically with SRY to activate transcription from a promoter containing SRY-binding sites. This interaction is mediated by the WT1 zinc-finger domain and the SRY HMG box. WT1 mutants associated with Denys-Drash syndrome (DDS), which is characterized by Wilms' tumor, pseudohermaphroditism, and nephropathy, fail to interact with SRY. Wildtype WT1 is recruited to SRY-binding sites in an SRY-dependent manner, whereas DDS mutants are not recruited as efficiently. These results suggest that WT1 forms a complex with SRY to regulate transcription and that this WT1-SRY interaction is important in testis development.

SRY Interacts with and Negatively Regulates Androgen Receptor Transcriptional Activity

This study investigated interactions between SRY, the Y chromosome encoded male sex determining factor, and the androgen receptor (AR). Coexpression of AR and SRY caused marked repression of AR transcriptional activity on a series of androgen-responsive reporter genes. Mammalian one- and two-hybrid experiments demonstrated an AR-SRY interaction mediated by the AR DNA binding domain. Precipitations with glutathione S-transferase fusion proteins indicated that AR-SRY interactions were direct and mediated by the AR DNA binding domain and the SRY high mobility group box DNA binding domain. Transient expression of SRY in LNCaP prostate cancer cells repressed expression of an androgen-dependent prostate-specific antigen (PSA) reporter gene and stable SRY expression repressed the endogenous PSA gene. SRY protein expression was increased by proteosome inhibitors and by the androgen-liganded AR in transient and stable transfectants. AR transcriptional activity was also repressed by DAX1, and the effects of SRY and DAX1 on the AR were additive. These findings indicate that interactions between the AR, SRY, and DAX1 contribute to normal male development and function and suggest a general role for protein-protein interactions between high mobility group box proteins and steroid hormone receptors in regulating tissue-specific gene expression.

Muscle Differentiation Is Antagonized by SOX15, a New Member of the SOX Protein Family

SOX proteins belong to a multigenic family characterized by a unique DNA binding domain, known as the high mobility group box, that is related to that of the testis determining gene SRY. cDNA sequences for more than 30 SOX genes have been identified, and some are known to have diverse roles in vertebrate differentiation and development. Here, we report the isolation and characterization of mouse Sox15 that was uncovered during a screen for high mobility group box containing transcription factors that are expressed at different levels during skeletal muscle differentiation. Sox15 cDNAs were found at a much higher frequency in myoblasts prior to their differentiation into myotubes. Electrophoretic mobility shift assays indicated that recombinant SOX15 protein was capable of binding to a consensus DNA binding site for SOX proteins. When overexpressed in C2C12 myoblasts, wild type SOX15, but not a C-terminal truncated form or the related protein SOX11, specifically inhibited activation of muscle-specific genes and expression of the basic helix-loop-helix myogenic factors myogenin and MyoD, resulting in a failure of the cells to differentiate into myotubes. These results suggest a specific and repressive role for SOX15, requiring the C-terminal domain, during myogenesis.

Protein-directed DNA structure. I. Raman spectroscopy of a high-mobility-group box with application to human sex reversal.

Protein-directed reorganization of DNA underlies mechanisms of transcription, replication, and recombination. A molecular model for DNA reorganization in the regulation of gene expression is provided by the sequence-specific high-mobility-group (HMG) box. Structures of HMG-box complexes with DNA are characterized by expansion of the minor groove, sharp bending toward the major groove, and local unwinding of the double helix. The Raman vibrational signature of such DNA reorganization has been identified in a study of the SRY HMG box, encoded by the human male-determining region of the Y chromosome. We observe in the human SRY-HMG:DNA complex extraordinarily large perturbations to Raman bands associated with vibrational modes of the DNA backbone and accompanying large increases in intensities of Raman bands attributable to base unstacking. In contrast, DNA major-groove binding, as occurs for the bZIP protein GCN4 [Benevides, J. M., Li, T., Lu, X.-J., Srinivasan, A. R., Olson, W. K., Weiss, M. A., and Thomas, G. J., Jr. (2000) Biochemistry 39, 548-556], perturbs the Raman signature of DNA only marginally. Raman markers of minor-groove recognition in the human SRY-HMG:DNA complex are due primarily to perturbation of specific vibrational modes of deoxyribose moieties and presumably reflect desolvation at the nonpolar interface of protein and DNA. These Raman markers may be diagnostic of protein-induced DNA bending and are proposed as a baseline for comparative analysis of mutations in SRY that cause human sex reversal.

The SRY Cantilever Motif Discriminates Between Sequence- and Structure-Specific DNA Recognition: Alanine Mutagenesis of an HMG Box

Summary The high-mobility-group (HMG) box defines a DNA-bending motif conserved among architectural transcription factors. A “hydrophobic wedge” at the protein surface provides a mechanism of DNA bending: disruption of base stacking by insertion of a sidechain “cantilever.” First described in the mammalian testis-determining factor SRY, the cantilever motif consists of adjacent aromatic and nonpolar sidechains at the crux of the HMG box (residues 12 and 13). Here, the role of these side chains in DNA recognition is investigated by alanine mutagenesis. F12A and I13A substitutions in the SRY HMG box each permit native folding and thermal stability (as monitored by circular dichroism and 1H-NMR) but eliminate sequence-specific DNA-binding activity (as detected by gel-mobility shift). On binding to the sharp angles of a four-way DNA junction (4WJ), however, the substitutions each promote formation of a high-molecular-weight aggregate, presumably by DNA-dependent oligomerization. The substitutions have opposite effects on initial binding to the 4WJ: whereas such binding is attenuated ten-fold by F12A, it is enhanced by I13A. A foreshortened “alanine cantilever”, not observed among specific HMG boxes, occurs in a nonspecific domain (HMG-1A) and may enhance architecture-selective DNA recognition.

Imino proton exchange provides an 1H-NMR footprint of protein-DNA interactions: general strategy and application to the SRY HMG box.

A novel 1H nuclear magnetic resonance (NMR) strategy for "footprinting" specific protein-DNA target sites is demonstrated. Relative rates of site-specifc imino-proton exchange in the free and bound DNA duplex are determined by use of laminar-shifted shaped pulses in NOESY spectra. 2D exchange crosspeaks between imino (omega2 dimension) (omega1 dimension) resonances in principle provide site-specific probes of protein binding. Chemical exchange is distinguished from nuclear Overhauser enhancements(NOEs) to bound water by use of ROESY spectroscopy. This strategy is illustrated in 1H-NMR studies of the SRY high-mobility group (HMG) box, the Y-chromosome-encoded "master switch" for testis determination in man. In a specific complex between the protein and a 15-basepair DNA site, imino-proton exchange was observed to be damped selectively within the six basepair subsite 5'-ATTGTT, previously identified by random binding-site selection as an optimal SRY target sequence. The extent of damping correlates with sites of protein-DNA contacts in the minor groove but not with the magnitude of 1H-NMR complexation shifts. SRY binding has recently been shown to introduce significant distortions in DNA structure. The DNA is sharply bent and underwound; the minor groove is widened and major groove compressed. Our results demonstrate that despite such distortions base pairing is stably maintained. Protein binding in the DNA minor groove shields DNA imino protons from exchange with solvent.

Sry-negative XX sex reversal in the German shorthaired pointer dog.

Present hypotheses indicate that a testis differentiation cascade in mammals is induced by Sry, a gene encoding a DNA binding protein of the high mobility group (HMG) class. In XX sex reversal, individuals lacking a Y chromosome develop testicular tissue. Sry translocation from the Y to the X chromosome has been found in some, but not all, of these individuals. XX sex reversal in the German shorthaired pointer dog may be a model of Sry-negative XX sex reversal in humans. The purposes of this study were to report the familial occurrence of sex reversal and determine whether the conserved Sry HMG box, the region of the Sry protein essential for testis induction, is present in genomic DNA of affected dogs. Canine Sry HMG box sequences were used as primers in polymerase chain reactions. A 104 bp Sry HMG box product was generated from normal males, but not from females or XX sex reversed dogs. Parallel control reactions using hypoxanthine phosphoribosyl transferase primers generated a 177 bp product from all dogs. The pedigree of affected dogs and the absence of Sry HMG box sequences in their genomic DNA suggest that this disorder is due to a mutant autosomal gene in the testis differentiation cascade.

Sry‐negative XX sex reversal in the American cocker spaniel dog

The Sry gene product serves an important function in male sex determination through testis induction. However, testicular development has been reported in SRY-negative XX sex reversed humans. XX sex reversal of the American cocker spaniel, inherited as an autosomal recessive trait, may be a homolog of this disorder. The purpose of this study was to determine whether the Sry high mobility group (HMG) box is present in genomic DNA of affected dogs. Conserved Sry HMG box and hypoxanthine phosphoribosyltransferase (HPRT) sequences were used as primers in polymerase chain reactions. A 167 bp Y-specific canine Sry HMG box sequence was cloned from genomic DNA of normal male dogs. Internal primers generated a 104 bp Sry HMG box product from normal males, but not from females or XX sex reversed dogs. Parallel reactions generated an HPRT product from all dogs. Results indicate that the Sry HMG box is absent in genomic DNA of XX sex reversed dogs. We speculate that activation of the testis differentiation cascade in the absence of Sry in this model is due to a mutant autosomal gene.

The SRY high-mobility-group box recognizes DNA by partial intercalation in the minor groove: a topological mechanism of sequence specificity.

SRY, a putative transcription factor encoded by the sex-determining region of the human Y chromosome, regulates a genetic switch in male development. Impairment of this switch leads to intersex abnormalities of the newborn and is observed in association with mutations in the SRY DNA-binding domain [the high-mobility-group (HMG) box]. Here we show that the SRY HMG box exhibits a novel mechanism of DNA recognition: partial intercalation of a nonpolar side chain in the DNA minor groove. Base stacking (but not base pairing) is interrupted at the site of insertion. Sequence specificity reflects topological requirements of partial intercalation rather than direct readout of base-specific functional groups. Our results predict that the SRY HMG box inserts an alpha-helix into a widened minor groove at the center of a sharp DNA bend. A similar mechanism may underlie binding of SRY and homologous HMG proteins to four-way junctions (Holliday intermediates) and other noncanonical DNA structures.