Skeletal muscle vulnerability in a child with Pitt-Hopkins syndrome

BACKGROUND

TCF4 acts as a transcription factor that binds to the immunoglobulin enhancer Mu-E5/KE5 motif. Dominant variants in TCF4 are associated with the manifestation of Pitt-Hopkins syndrome, a rare disease characterized by severe mental retardation, certain features of facial dysmorphism and, in many cases, with abnormalities in respiratory rhythm (episodes of paroxysmal tachypnea and hyperventilation, followed by apnea and cyanosis). Frequently, patients also develop epilepsy, microcephaly, and postnatal short stature. Although TCF4 is expressed in skeletal muscle and TCF4 seems to play a role in myogenesis as demonstrated in mice, potential myopathological findings taking place upon the presence of dominant TCF4 variants are thus far not described in human skeletal muscle.

METHOD

To address the pathological effect of a novel deletion affecting exons 15 and 16 of TCF4 on skeletal muscle, histological and immunofluorescence studies were carried out on a quadriceps biopsy in addition to targeted transcript studies and global proteomic profiling.

RESULTS

We report on muscle biopsy findings from a Pitt-Hopkins patient with a novel heterozygous deletion spanning exon 15 and 16 presenting with neuromuscular symptoms. Microscopic characterization of the muscle biopsy revealed moderate fiber type I predominance, imbalance in the proportion of fibroblasts co-expressing Vimentin and CD90, and indicate activation of the complement cascade in TCF4-mutant muscle. Protein dysregulations were unraveled by proteomic profiling. Transcript studies confirmed a mitochondrial vulnerability in muscle and confirmed reduced TCF4 expression.

CONCLUSION

Our combined findings, for the first time, unveil myopathological changes as phenotypical association of Pitt-Hopkins syndrome and thus expand the current clinical knowledge of the disease as well as support data obtained on skeletal muscle of a mouse model.

HEB in the spotlight: Transcriptional regulation of T-cell specification, commitment, and developmental plasticity

The development of T cells from multipotent progenitors in the thymus occurs by cascades of interactions between signaling molecules and transcription factors, resulting in the loss of alternative lineage potential and the acquisition of the T-cell functional identity. These processes require Notch signaling and the activity of GATA3, TCF1, Bcl11b, and the E-proteins HEB and E2A. We have shown that HEB factors are required to inhibit the thymic NK cell fate and that HEBAlt allows the passage of T-cell precursors from the DN to DP stage but is insufficient for suppression of the NK cell lineage choice. HEB factors are also required to enforce the death of cells that have not rearranged their TCR genes. The synergistic interactions between Notch1, HEBAlt, HEBCan, GATA3, and TCF1 are presented in a gene network model, and the influence of thymic stromal architecture on lineage choice in the thymus is discussed.

Colorectal tumors are effectively eradicated by combined inhibition of {beta}-catenin, KRAS, and the oncogenic transcription factor ITF2

Colorectal carcinomas (CRC) harbor well-defined genetic abnormalities, including aberrant activation of β-catenin (β-cat) and KRAS, but independent targeting of these molecules seems to have limited therapeutic effect. In this study, we report therapeutic effects of combined targeting of different oncogenes in CRC. Inducible short hairpin RNA (shRNA)-mediated silencing of β-cat, ITF2, or KRAS decreased proliferation by 88%, 72%, and 45%, respectively, with no significant apoptosis in any case. In contrast, combined blockade of β-cat and ITF2 inhibited proliferation by 99% with massive apoptosis. Similar effects occurred after combined shRNA against β-cat and KRAS. In vivo, single oncogene blockade inhibited the growth of established tumors by up to 30%, whereas dual β-cat and ITF2 targeting caused 93% inhibition. Similar tumor growth suppression was achieved by double β-cat/KRAS shRNA in vivo. Our findings illustrate an effective therapeutic principle in CRC based on a combination targeting strategy that includes the ITF2 oncogene, which represents a novel therapeutic target.

ID2-VEGF-related pathways in the pathogenesis of Kaposi's sarcoma: a link disrupted by rapamycin

The Id-proteins are a family of four related proteins implicated in the control of differentiation and cell-cycle progression. Down-regulation of Id-gene expression is essential for the differentiation of several cell types. In addition, deregulated Id2 activity inhibits the Rb tumor suppressor pathway and promotes the expression of vascular endothelial growth factor (VEGF). Several members of VEGF family could be involved in Kaposi's sarcoma (KS) development and progression. Lymphatic vascular endothelial hyaluronan receptor-1 (LYVE-1) is the first marker of lymphatic endothelial competence during development in the mature vasculature, and is also expressed on KS spindle cells. Rapamycin (RAPA), an immunosuppressive drug, has been shown to reverse KS growth and to reduce tumor angiogenesis. We evaluate, in transplantation-associated KS and in cultured KS-cells the RAPA effect on Id2 and on de novo lymphangiogenesis. Markers of lymphatic-endothelial-cells (VEGFR-3, LYVE-1) and Id2, expressed at low levels within the normal skin, were up-regulated in KS and returned to normal levels after RAPA introduction. The association between Id2 and lymphangiogenesis is suggested by co-localization of Id2, VEGFR-3 and LYVE-1. RAPA inhibition on Id2 expression was confirmed in vitro in KS-cells, both in basal conditions and upon stimulation with VEGF. In conclusion, our data would suggest a novel molecular mechanism for the antineoplastic effects of RAPA in posttransplant KS.

Epithelial mesenchymal transition traits in human breast cancer cell lines

Epithelial mesenchymal transition (EMT) has long been associated with breast cancer cell invasiveness and evidence of EMT processes in clinical samples is growing rapidly. Genome-wide transcriptional profiling of increasingly larger numbers of human breast cancer (HBC) cell lines have confirmed the existence of a subgroup of cell lines (termed Basal B/Mesenchymal) with enhanced invasive properties and a predominantly mesenchymal gene expression signature, distinct from subgroups with predominantly luminal (termed Luminal) or mixed basal/luminal (termed Basal A) features (Neve et al Cancer Cell 2006). Studies providing molecular and cellular analyses of EMT features in these cell lines are summarised, and the expression levels of EMT-associated factors in these cell lines are analysed. Recent clinical studies supporting the presence of EMT-like changes in vivo are summarised. Human breast cancer cell lines with mesenchymal properties continue to hold out the promise of directing us towards key mechanisms at play in the metastatic dissemination of breast cancer.

Role of the basic helix-loop-helix protein ITF2 in the hormonal regulation of Sertoli cell differentiation

Sertoli cells are a post-mitotic terminally differentiated cell population that forms the seminiferous tubules in the adult testis and provides the microenvironment and structural support for developing germ cells. During pubertal development, Sertoli cells are responsive to follicle-stimulating hormone (FSH) to promote the expression of differentiated gene products. The basic helix-loop-helix (bHLH) and inhibitors of differentiation (Id) transcription factors are involved in the differentiation of a variety of cell lineages during development. Both bHLH and Id transcription factors have been identified in Sertoli cells. A yeast two-hybrid screen was conducted using a rat Sertoli cell cDNA library to identify bHLH dimerization partners for the Id1 transcription factor. The ubiquitous bHLH protein ITF2 (i.e., E2-2) was identified as one of the interacting partners. The current study investigates the expression and function of ITF2 in Sertoli cells. ITF2 was found to be ubiquitously expressed in all testicular cell types including germ cells, peritubular myoid cells, and Sertoli cells. Stimulation of cultured Sertoli cells with FSH or dibutryl cAMP resulted in a transient decrease in expression of ITF2 mRNA levels followed by a rise in expression with FSH treatment. ITF2 expression was at its highest in mid-pubertal 20-day-old rat Sertoli cells. ITF2 was found to directly bind to negative acting Id HLH proteins and positive acting bHLH proteins such as scleraxis. Transient overexpression of ITF2 protein in cultured Sertoli cells stimulated transferrin promoter activity, which is a marker of Sertoli cell differentiation. Co-transfections of ITF2 and Id proteins sequestered the inhibitory effects of the Id family of proteins. Observations suggest ITF2 can enhance FSH actions through suppressing the inhibitory actions of the Id family of proteins and increasing the actions of stimulatory bHLH proteins (i.e., scleraxis) in Sertoli cells.

Molecular mechanisms regulating expression and function of transcription regulator inhibitor of differentiation 3

The transcription factor antagonist inhibitor of differentiation 3 (Id3) has been implicated in many diverse developmental, physiological and pathophysiological processes. Its expression and function is subjected to many levels of complex regulation. This review summarizes the current understanding of these mechanisms and describes how they might be related to the diverse functions that have been attributed to the Id3 protein. Detailed understanding of these mechanisms should provide insights towards the development of therapeutic approaches to various diseases, including cancer and atherogenesis.

A negative regulatory element-dependent inhibitory role of ITF2B on IL-2 receptor alpha gene

Despite the fact that the negative regulatory element (NRE) within the upstream regulatory region of human IL-2 receptor alpha (IL-2Ralpha) gene has been identified two decades ago, mechanisms of the NRE function on the gene are hitherto unknown. In this paper, we report for the first time that the immunoglobulin transcription factor 2B (ITF2B) encoded by transcription factor 4 (TCF4) gene is a NRE binding protein. The full-length TCF4 cDNA clone was obtained from a HTLV-1 transformed human peripheral T cell MACHERMAKER cDNA library with NRE as the bait in yeast one-hybrid system. The NRE binding ability of ITF2B was further confirmed in chromatin-immunoprecipitation assay. Competitive RT-PCR-based promoter activity assay showed that over-expression of ITF2B protein inhibited the expression of IL-2Ralpha gene in Jurkat cells in an NRE-dependent manner. The function of ITF2B on the inhibition of both the IL-2Ralpha and the 5'LTR activity of HIV-1 shed light on the essence of NRE binding protein as a potential target for immune therapy and treatment in AIDS patients.

Identification of novel MyoD gene targets in proliferating myogenic stem cells

A major control point for skeletal myogenesis revolves around the muscle basic helix-loop-helix gene family that includes MyoD, Myf-5, myogenin, and MRF4. Myogenin and MRF4 are thought to be essential to terminal differentiation events, whereas MyoD and Myf-5 are critical to establishing the myogenic cell lineage and producing committed, undifferentiated myogenic stem cells (myoblasts). Although mouse genetic studies have revealed the importance of MyoD and Myf-5 for myoblast development, the genetic targets of MyoD and Myf-5 activity in undifferentiated myoblasts remain unknown. In this study, we investigated the function of MyoD as a transcriptional activator in undifferentiated myoblasts. By using conditional expression of MyoD, in conjunction with suppression subtractive hybridizations, we show that the Id3 and NP1 (neuronal pentraxin 1) genes become transcriptionally active following MyoD induction in undifferentiated myoblasts. Activation of Id3 and NP1 represents a stable, heritable event that does not rely on continued MyoD activity and is not subject to negative regulation by an activated H-Ras G12V protein. These results are the first to demonstrate that MyoD functions as a transcriptional activator in myogenic stem cells and that this key myogenic regulatory factor exhibits different gene target specificities, depending upon the cellular environment.

Gene expression profile in fibroblast growth factor 2-transformed endothelial cells

Fibroblast growth factor-2 (FGF2) exerts paracrine and autocrine functions on endothelial cells. FGF2-overexpressing murine aortic endothelial cells (FGF2-T-MAE cells) induce opportunistic hemangioendothelioma-like tumors when inoculated in immunodeficient mice. To evaluate the impact of FGF2-mediated activation on gene expression profile in transformed endothelial cells, we performed subtractive suppression hybridization analysis between FGF2-T-MAE cells and parental MAE cells. The two cell populations were compared for differential gene expression also by gene macroarray hybridization with 32P-labeled cDNAs. The two approaches allowed the identification of 27 transcripts whose expression was upregulated by FGF2 in endothelial cells. With the exception of one unknown gene, the differentially expressed transcripts encoded for proteins involved in the modulation of cell cycle, differentiation, and cell adhesion. Among them, the stress-inducible genes A170, GADD45 and GADD153 are upregulated by FGF2 transfection or recombinant growth factor treatment. Their expression was also induced in vascular tumors originated by parental or FGF2-transfected MAE cells in nude mice. This study extends the number of genes involved in tumor angiogenesis and/or endothelial cell transformation, a finding with possible implications for the discovery of novel targets for angiostatic therapy.

The Id proteins and angiogenesis

Since the identification of the Id proteins over a decade ago, a great many cell cycle and cell fate decisions have been shown to be under the control of these proteins as described in other sections of this review issue. Perhaps the most unsuspected activity of this class of proteins has been their essential role in angiogenesis, both in the forebrain during development and during the growth and metastasis of tumors in adults. This section of the review issue will focus on the key observations which have led to these conclusions, speculations about potential mechanisms and the outlook for potential therapeutic interventions.

Involvement of ITF2 in the transcriptional regulation of melanogenic genes

In response to agouti signal protein, melanocytes switch from producing eumelanin to pheomelanin concomitant with the down-regulation of melanogenic gene transcription. We previously reported that a ubiquitous basic helix-loop-helix transcription factor, known as ITF2, is up-regulated during this switch, and we now report that treatment of melanocytes with melanocyte-stimulating hormone down-regulates expression of ITF2. To more fully characterize the involvement of ITF2 in regulating melanogenic gene transcription, ITF2 sense or antisense constructs were introduced into melan-a melanocytes. Gene and protein expression analyses and luciferase reporter assays using promoters from melanogenic genes showed that up-regulation of ITF2 suppressed melanogenic gene expression as well as the expression of Mitf, a melanocyte-specific transcription factor. In addition, stable ITF2 sense transfectants had significant reductions in pigmentation and a less dendritic phenotype compared with mock transfectants. In contrast, ITF2 antisense-transfected melanocytes were more pigmented and more dendritic. These results demonstrate that up-regulation of ITF2 during the pheomelanin switch is functionally significant and reveal that differential expression of a ubiquitous basic helix-loop-helix transcription factor can modulate expression of melanogenic genes and the differentiation of melanocytes.

Genomic organization and promoter analysis of the murine Id3 gene

Overexpression of the helix-loop-helix motif-containing transcription inhibitor Id3 has been shown to repress muscle-specific gene expression. Consistent with its putative negative regulatory role in the myogenic process, Id3 is highly expressed in proliferating myoblasts but down regulated when myoblasts are induced to differentiate. To investigate how Id3 expression may be transcriptionally regulated, we isolated a mouse Id3 genomic DNA fragment and characterized its organization and promoter activity. Comparison of the Id3 gene from human and mouse demonstrated a conserved exon-intron organization in which the first intron interrupts the C-terminal protein coding region and the second intron interrupts the 3' untranslated region at analogous positions in the two species. Sequence analysis of the 5'-flanking region revealed an unexpected mouse strain-specific genetic polymorphism due to a single base substitution. Deletion analysis revealed that as little as 180 base pairs of the mouse Id3 promoter upstream of the transcription start site is sufficient for a high level of gene expression in proliferating C2C12 myoblasts. In particular, the region between the nucleotide position -180 and -34 appeared to be crucial for maximal reporter gene activity and interacted specifically with C2C12 nuclear proteins. Finally, we showed that, despite the creation of a putative transcription factor-binding site, the genetic polymorphism observed did not affect Id3 promoter activity in proliferating C2C12 cells.

Analysis of the inhibition of MyoD activity by ITF-2B and full-length E12/E47

MyoD heterodimerizes with E type factors (E12/E47 and ITF-2A/ITF-2B) and binds E box sequences within promoters of muscle-specific genes. In transient transfection assays, MyoD activates transcription in the presence of ITF-2A but not ITF-2B, which contains a 182-amino acid N-terminal extension. The first 83 amino acids of the inhibitory N terminus of ITF-2B show high sequence homology to the N terminus of full-length E12/E47. Previous studies that showed activation of MyoD by E12 used an artificially N-terminally truncated form. Here we show that the full-length form of E12 inhibits MyoD function. A conserved alpha-helix motif, capable of interacting with the transcriptional machinery, was not essential for inhibition. Furthermore, the fusion of N-terminal ITF-2B sequences or non-inhibiting ITF-2A sequences to truncated E12 was sufficient in converting the activator into an inhibitor. Overexpression of ITF-2B did not inhibit C2C12 myogenesis or affect levels of endogenous muscle gene expression, consistent with the finding that inhibitory E type proteins are present in muscle. Furthermore, we found that MyoD co-transfected with either ITF-2B or ITF-2A converted fibroblasts into myoblasts with the same frequency. Our findings suggest that the ability of E type proteins to inhibit MyoD activity is dependent on the context of the E box.

HASH-1 and E2-2 are expressed in human neuroblastoma cells and form a functional complex

The basic helix-loop-helix (bHLH) transcription factor mammalian achaete-scute homolog-1 (MASH-1 in mouse and HASH-1 in human) is essential for proper development of olfactory and most peripheral autonomic neurons, and for the formation of distinct neuronal circuits within the central nervous system. We have previously shown that HASH-1 is expressed in neuroblastoma tumors and cell lines, and in this study we have used the yeast two-hybrid system to isolate HASH-1 interacting proteins from a human neuroblastoma cDNA library. Two of the isolated clones contained cDNA from the E2-2 gene (also known as ITF2/SEF2-1). We show that E2-2 interacts with HASH-1 in both yeast and mammalian cells. The HASH-1/E2-2 complex binds an E-box (CACCTG) in vitro, and transactivates an E-box containing reporter construct in vivo. Furthermore, E2-2 seems to be one of the major HASH-1 interacting proteins in extracts from neuroblastoma cells. In conclusion, E2-2 forms a functional complex with HASH-1, and might therefore be involved in the development of specific parts of the central and peripheral nervous systems.

Involvement of p27(kip1) and cyclin D3 in the regulation of cdk2 activity during skeletal muscle differentiation

Terminal myogenic differentiation involves an irreversible transition from a proliferative state to a post-mitotic quiescent state. We showed here that in addition to the previously reported down regulation of G(1)-related cyclin-associated kinase activities, this transition was also accompanied by an extensive reorganization of the cyclin-cdk complexes, including a dramatic shift of cdk2 from cyclin A to cyclin D3. Moreover, the inhibition of cdk activity also correlated with an increase in the expression of the p27(kip1) cdk inhibitor and in its association with the cyclin-cdk2 complexes. Since depletion of p27 substantially reduced the cdk inhibitor activity present in differentiated muscle cells, we believe that the increase in p27 expression along with the reorganization of the cyclin-cdk2 complexes may play an important role in the inhibition of cdk2 activity during the differentiation process.

Thymocyte selection is regulated by the helix-loop-helix inhibitor protein, Id3

E2A, HEB, E2-2, and daughterless are basic helix-loop-helix (bHLH) proteins that play key roles in multiple developmental pathways. The DNA binding activity of E2A, HEB, and E2-2 is regulated by a distinct class of inhibitor HLH proteins, the Id gene products. Here, we show that Id3 is required for major histocompatability (MHC) class I- and class II-restricted thymocyte positive selection. Additionally, H-Y TCR-mediated negative selection is severely perturbed in Id3 null mutant mice. Finally, we show that E2A and Id3 interact genetically to regulate thymocyte development. These observations identify the HLH inhibitory protein Id3 as an essential component required for proper thymocyte maturation.

Forced expression of Id-1 in the adult mouse small intestinal epithelium is associated with development of adenomas

Ids are dominant-negative helix-loop-helix (HLH) proteins that play overlapping yet distinct roles in antagonizing basic HLH transcription factors. Although Ids affect myogenesis, neurogenesis, and B-cell development, little is known about their in vivo functions in epithelia. We have examined the effects of forced expression of Id-1 in the small intestinal epithelium of adult chimeric mice. 129/Sv embryonic stem cells, transfected with DNA containing Id-1 under the control of transcriptional regulatory elements that function in all intestinal epithelial cell lineages, were introduced into C57Bl/6 (B6) blastocysts heterozygous for the ROSA26 marker. The B6 ROSA26/+ intestinal epithelium of the resulting adult chimeras produces Escherichia coli beta-galactosidase, allowing identification of this internal control cell population. Chimeras produced from nontransfected embryonic stem cells served as additional controls. Immunohistochemical studies of the control chimeras indicated that the small intestinal epithelium supports a complex pattern of endogenous Id expression. Id-1 is restricted to the cytoplasm; levels do not decrease as descendants of multipotent intestinal stem cells differentiate. Id-2 and Id-3 are only detectable in nuclei; levels increase markedly as epithelial cells differentiate. Forced expression of Id-1 in the 129/Sv epithelium results in a decline in Id-2 and Id-3 to below the limits of immunodetection. A subset of chimeric-transgenic mice lacked growth factor- and defensin-producing Paneth cells in their 129/Sv epithelium and also developed intestinal adenomas. These changes were not present in normal control chimeras. Adenomas were composed of proliferating beta-Gal-positive and -negative epithelial cells, suggesting that they arose through cooperative interactions between 129/Sv(Id-1) and B6 ROSA26/+ cells. These chimeras provide a model for studying how perturbations in Id expression affect tumorigenesis.