Characterisation of the GRAF gene promoter and its methylation in patients with acute myeloid leukaemia and myelodysplastic syndrome

RHO subfamily of small GTPases in the development and function of hematopoietic cells

RHOA, RHOB, and RHOC comprise a subfamily of RHO GTPase proteins famed for controlling cytoskeletal dynamics. RHO proteins operate downstream of multiple signals emerging from the microenvironment, leading to diverse cell responses, such as proliferation, adhesion, and migration. Therefore, RHO signaling has been centrally placed in the regulation of blood cells. Despite their high homology, unique roles of RHOA, RHOB, and RHOC have been described in hematopoietic cells. In this article, we overview the contribution of RHO proteins in the development and function of each blood cell lineage. Additionally, we highlight the aberrations of the RHO signaling pathways found in hematological malignancies, providing clues for the identification of new therapeutic targets.

Vitamin D and K Supplementation Is Associated with Changes in the Methylation Profile of U266-Multiple Myeloma Cells, Influencing the Proliferative Potential and Resistance to Bortezomib

Multiple myeloma (MM) is a plasma cell malignancy that, despite recent advances in therapy, continues to pose a major challenge to hematologists. Currently, different classes of drugs are applied to treat MM, among others, proteasome inhibitors, immunomodulatory drugs, and monoclonal antibodies. Most of them participate in an interplay with the immune system, hijacking its effector functions and redirecting them to anti-MM activity. Therefore, adjuvant therapies boosting the immune system may be potentially beneficial in MM therapy. Vitamin D (VD) and vitamin K (VK) have multiple so called "non-classical" actions. They exhibit various anti-inflammatory and anti-cancer properties. In this paper, we investigated the influence of VD and VK on epigenetic alterations associated with the proliferative potential of MM cells and the development of BTZ resistance. Our results showed that the development of BTZ resistance is associated with a global decrease in DNA methylation. On the contrary, both control MM cells and BTZ-resistant MM cells exposed to VD alone and to the combination of VD and VK exhibit a global increase in methylation. In conclusion, VD and VK in vitro have the potential to induce epigenetic changes that reduce the proliferative potential of plasma cells and may at least partially prevent the development of resistance to BTZ. However, further ex vivo and in vivo studies are needed to confirm the results and introduce new supplementation recommendations as part of adjuvant therapy.

Associations of ARHGAP26 Polymorphisms with Alzheimer's Disease and Cardiovascular Disease

The Rho GTPase activating protein 26 (ARHGAP26) gene has been reported to be associated with neuropsychiatric diseases and neurodegenerative diseases including Parkinson's disease. We examined whether the ARHGAP26 gene is associated with Alzheimer's disease (AD) and/or cardiovascular disease (CVD). Multivariable logistic regression model was used to examine the associations of 154 single nucleotide polymorphisms (SNPs) within the ARHGAP26 gene with AD and CVD using the Alzheimer's Disease Neuroimaging Initiative 1 (ADNI-1) cohort. Fourteen SNPs were associated with AD (top SNP rs3776362 with p = 3.43 × 10^-3), while 37 SNPs revealed associations with CVD (top SNP rs415235 with p = 2.06 × 10^-4). Interestingly, 13 SNPs were associated with both AD and CVD. SNP rs3776362 was associated with CVD, Functional Activities Questionnaire (FAQ), and Clinical Dementia Rating Sum of Boxes (CDR-SB). A replication study using a Caribbean Hispanics sample showed that 17 SNPs revealed associations with AD, and 12 SNPs were associated with CVD. The third sample using a family-based study design showed that 9 SNPs were associated with AD, and 3 SNPs were associated with CVD. SNP rs6836509 within the ARHGAP10 gene (an important paralogon of ARHGAP26) was associated with AD and cerebrospinal fluid total tau (t-tau) level in the ADNI sample. Several SNPs were functionally important using the RegulomeDB, while a number of SNPs were associated with significant expression quantitative trait loci (eQTLs) using Genotype-Tissue Expression (GTEx) databases. In conclusion, genetic variants within ARHGAP26 were associated with AD and CVD. These findings add important new insights into the potentially shared pathogenesis of AD and CVD.

The role of GTPase-activating protein ARHGAP26 in human cancers

Rho GTPases are molecular switches that play an important role in regulating the behavior of a variety of tumor cells. RhoA GTPase-activating protein 26 (ARHGAP26) is a GTPase-activating protein and inhibits the activity of Rho GTPases by promoting the hydrolytic ability of Rho GTPases. It also affects tumorigenesis and progression of various tumors through several methods, including formation of abnormal fusion genes and circular RNA. This review summarizes the biological functions and molecular mechanisms of ARHGAP26 in different tumors, proposes the potential clinical value of ARHGAP26 in cancer treatment, and discusses current issues that need to be addressed.

Hypoxia-induced ARHGAP26 deficiency inhibits the proliferation and migration of human ductus arteriosus smooth muscle cell through activating RhoA-ROCK-PTEN pathway

The Rho family plays crucial roles in O₂ -induced vasoconstriction, cell proliferation, and migration. Rho GTPase-activating protein 26 (ARHGAP26) is a GTPase-activating protein for the small GTPases of the Rho family. Our previous studies have demonstrated that ARHGAP26 expression was significantly downregulated in patent human ductus arteriosus (DA) tissue. However, its role underlying the maintenance of DA patency is unclear. In this study, patent (fetal) and constricted (newborn) mouse DA tissues were harvested to confirm the differences in the levels of expression of ARHGAP26. Human DA smooth muscle cells (DASMCs) were isolated and cultured in vitro and used to test the function of ARHGAP26. The expression of ARHGAP26 was significantly lower in patent (fetal) than constricted (newborn) mouse DA. ARHGAP26-knocked-down human DASMCs showed reduced proliferation and migration, which are both crucial to anatomic closure of DA. Moreover, after culturing under hypoxic conditions, the expression of ARHGAP26 in human DASMCs was significantly lower and hypoxia-induced ARHGAP26 deficiency activated the phosphorylation level of phosphatase and tensin homolog (PTEN) in DASMCs by mediating the activity of RhoA and RhoA-associated kinase 1 (ROCK1). Use of Y27632, an inhibitor of ROCK which further reduces the phospholipid activity of PTEN can reverse the inhibitory effect of PTEN on the proliferation and migration of human DASMCs. This provides insight into the molecular regulation of the RhoA-ROCK-PTEN pathway in DA smooth muscle cells, which may be a suitable therapeutic target or diagnostic biomarker for perinatal DA tone management.

The 'endocytic matrix reloaded' and its impact on the plasticity of migratory strategies

An explosive growth in knowledge, in the last two decades, has conferred a new dimension to the process of endocytosis. Endocytic circuitries have come into focus as a pervasive system that controls virtual all aspects of cell biology. A few years ago, we proposed the term 'endocytic matrix' to define a cellular network of signalling wiring that is at the core of the cellular blueprint. A primary role of the endocytic matrix is the delivery of space-resolved and time-resolved signals to the cell in an interpretable format, and, as such, it has profound consequences on polarized cellular and supra-cellular functions, first and foremost, cell motility. Here, we describe a set of recent results that expand this notion and illuminate how endocytic matrix dynamically controls the plasticity of migratory strategies. We further highlight the impact of inter-organelle contact sites on motility and the role of organelle positioning in this process. Finally, we illustrate how global perturbation of the endocytic circuitry influences cellular and supra-cellular mechanics, ultimately controlling a solid-to-liquid-like transition in the mode of motility with potential consequences on cancer dissemination.

Graf regulates hematopoiesis through GEEC endocytosis of EGFR

GTPase regulator associated with focal adhesion kinase 1 (GRAF1) is an essential component of the GPI-enriched endocytic compartment (GEEC) endocytosis pathway. Mutations in the human GRAF1 gene are associated with acute myeloid leukemia, but its normal role in myeloid cell development remains unclear. We show that Graf, the Drosophila ortholog of GRAF1, is expressed and specifically localizes to GEEC endocytic membranes in macrophage-like plasmatocytes. We also find that loss of Graf impairs GEEC endocytosis, enhances EGFR signaling and induces a plasmatocyte overproliferation phenotype that requires the EGFR signaling cascade. Mechanistically, Graf-dependent GEEC endocytosis serves as a major route for EGFR internalization at high, but not low, doses of the predominant Drosophila EGFR ligand Spitz (Spi), and is indispensable for efficient EGFR degradation and signal attenuation. Finally, Graf interacts directly with EGFR in a receptor ubiquitylation-dependent manner, suggesting a mechanism by which Graf promotes GEEC endocytosis of EGFR at high Spi. Based on our findings, we propose a model in which Graf functions to downregulate EGFR signaling by facilitating Spi-induced receptor internalization through GEEC endocytosis, thereby restraining plasmatocyte proliferation.

The oncocytic subtype is genetically distinct from other pancreatic intraductal papillary mucinous neoplasm subtypes

In 2010, the World Health Organization reclassified the entity originally described as intraductal oncocytic papillary neoplasm as the 'oncocytic subtype' of intraductal papillary mucinous neoplasm. Although several key molecular alterations of other intraductal papillary mucinous neoplasm subtypes have been discovered, including common mutations in KRAS, GNAS, and RNF3, those of oncocytic subtype have not been well characterized. We analyzed 11 pancreatic 'oncocytic subtype' of intraductal papillary mucinous neoplasms. Nine pancreatic 'oncocytic subtype' of intraductal papillary mucinous neoplasms uniformly exhibited typical entity-defining morphology of arborizing papillae lined by layers of cells with oncocytic cytoplasm, prominent, nucleoli, and intraepithelial lumina. The remaining two were atypical. One lacked the arborizing papilla and had flat oncocytic epithelium only; the other one had focal oncocytic epithelium in a background of predominantly intestinal subtype intraductal papillary mucinous neoplasm. Different components of this case were analyzed separately. Formalin-fixed, paraffin-embedded specimens of all cases were microdissected and subjected to high-depth-targeted next-generation sequencing for a panel of 300 key cancer-associated genes in a platform that enabled the identification of sequence mutations, copy number alterations, and select structural rearrangements involving all targeted genes. Fresh frozen specimens of two cases were also subjected to whole-genome sequencing. For the nine typical pancreatic 'oncocytic subtype' of intraductal papillary mucinous neoplasms, the number of mutations per case, identified by next-generation sequencing, ranged from 1 to 10 (median=4). None of these cases had KRAS or GNAS mutations and only one had both RNF43 and PIK3R1 mutations. ARHGAP26, ASXL1, EPHA8, and ERBB4 genes were somatically altered in more than one of these typical 'oncocytic subtype' of intraductal papillary mucinous neoplasms but not in the other two atypical ones. In the neoplasm with flat oncocytic epithelium, the only mutated gene was KRAS. All components of the intestinal subtype intraductal papillary mucinous neoplasms with focal oncocytic epithelium manifested TP53, GNAS, and RNF43 mutations. In conclusion, this study elucidates that 'oncocytic subtype' of intraductal papillary mucinous neoplasm is not only morphologically distinct but also genetically distinct from other intraductal papillary mucinous neoplasm subtypes. Considering that now its biologic behavior is also being found to be different than other intraductal papillary mucinous neoplasm subtypes, 'oncocytic subtype' of intraductal papillary mucinous neoplasm warrants being recognized separately.

GRAF1 deficiency blunts sarcolemmal injury repair and exacerbates cardiac and skeletal muscle pathology in dystrophin-deficient mice

Background

The plasma membranes of striated muscle cells are particularly susceptible to rupture as they endure significant mechanical stress and strain during muscle contraction, and studies have shown that defects in membrane repair can contribute to the progression of muscular dystrophy. The synaptotagmin-related protein, dysferlin, has been implicated in mediating rapid membrane repair through its ability to direct intracellular vesicles to sites of membrane injury. However, further work is required to identify the precise molecular mechanisms that govern dysferlin targeting and membrane repair. We previously showed that the bin–amphiphysin–Rvs (BAR)–pleckstrin homology (PH) domain containing Rho-GAP GTPase regulator associated with focal adhesion kinase-1 (GRAF1) was dynamically recruited to the tips of fusing myoblasts wherein it promoted membrane merging by facilitating ferlin-dependent capturing of intracellular vesicles. Because acute membrane repair responses involve similar vesicle trafficking complexes/events and because our prior studies in GRAF1-deficient tadpoles revealed a putative role for GRAF1 in maintaining muscle membrane integrity, we postulated that GRAF1 might also play an important role in facilitating dysferlin-dependent plasma membrane repair.

Methods

We used an in vitro laser-injury model to test whether GRAF1 was necessary for efficient muscle membrane repair. We also generated dystrophin/GRAF1 doubledeficient mice by breeding mdx mice with GRAF1 hypomorphic mice. Evans blue dye uptake and extensive morphometric analyses were used to assess sarcolemmal integrity and related pathologies in cardiac and skeletal muscles isolated from these mice.

Results

Herein, we show that GRAF1 is dynamically recruited to damaged skeletal and cardiac muscle plasma membranes and that GRAF1-depleted muscle cells have reduced membrane healing abilities. Moreover, we show that dystrophin depletion exacerbated muscle damage in GRAF1-deficient mice and that mice with dystrophin/GRAF1 double deficiency phenocopied the severe muscle pathologies observed in dystrophin/dysferlin-double null mice. Consistent with a model that GRAF1 facilitates dysferlin-dependent membrane patching, we found that GRAF1 associates with and regulates plasma membrane deposition of dysferlin.

Conclusions

Overall, our work indicates that GRAF1 facilitates dysferlin-dependent membrane repair following acute muscle injury. These findings indicate that GRAF1 might play a role in the phenotypic variation and pathological progression of cardiac and skeletal muscle degeneration in muscular dystrophy patients.

Electronic supplementary material

The online version of this article (doi:10.1186/s13395-015-0054-6) contains supplementary material, which is available to authorized users.

GRAF1a is a brain-specific protein that promotes lipid droplet clustering and growth, and is enriched at lipid droplet junctions

Lipid droplets are found in all cell types. Normally present at low levels in the brain, they accumulate in tumours and are associated with neurodegenerative diseases. However, little is known about the mechanisms controlling their homeostasis in the brain. We found that GRAF1a, the longest GRAF1 isoform (GRAF1 is also known as ARHGAP26), was enriched in the brains of neonates. Endogenous GRAF1a was found on lipid droplets in oleic-acid-fed primary glial cells. Exclusive localization required a GRAF1a-specific hydrophobic segment and two membrane-binding regions, a BAR and a PH domain. Overexpression of GRAF1a promoted lipid droplet clustering, inhibited droplet mobility and severely perturbed lipolysis following the chase of cells overloaded with fatty acids. Under these conditions, GRAF1a concentrated at the interface between lipid droplets. Although GRAF1-knockout mice did not show any gross abnormal phenotype, the total lipid droplet volume that accumulated in GRAF1(-/-) primary glia upon incubation with fatty acids was reduced compared to GRAF1(+/+) cells. These results provide additional insights into the mechanisms contributing to lipid droplet growth in non-adipocyte cells, and suggest that proteins with membrane sculpting BAR domains play a role in droplet homeostasis.

High expression of GTPase regulator associated with the focal adhesion kinase (GRAF) is a favorable prognostic factor in acute myeloid leukemia

BACKGROUND

GRAF is a recognized tumor suppressor gene that was found inactivated in AML. However, the prognostic role of a GRAF transcript has not been studied in patients with AML.

METHODS

In this study, we investigated the expression of the GRAF transcript by real time quantitative PCR in 60 AML patients and 30 healthy age and sex matched controls.

RESULTS

GRAF expression was significantly lower in patients with AML when compared to controls (P=0.008). There were no significant differences in clinical features, FAB subtypes and cytogenetic risk subgroups between patients with high and low GRAF expression levels. Kaplan-Meier analysis showed that patients with high GRAF expression had longer overall survival (OS). Multivariate analysis revealed that, besides WBC count, GRAF expression was also an independent prognostic factor for AML.

CONCLUSION

We provide evidence that high GRAF expression is a favorable prognostic marker in patients with AML.

Polymorphisms in microRNA target sites modulate risk of lymphoblastic and myeloid leukemias and affect microRNA binding

BACKGROUND

MicroRNA dysregulation is a common event in leukemia. Polymorphisms in microRNA-binding sites (miRSNPs) in target genes may alter the strength of microRNA interaction with target transcripts thereby affecting protein levels. In this study we aimed at identifying miRSNPs associated with leukemia risk and assessing impact of these miRSNPs on miRNA binding to target transcripts.

METHODS

We analyzed with specialized algorithms the 3' untranslated regions of 137 leukemia-associated genes and identified 111 putative miRSNPs, of which 10 were chosen for further investigation. We genotyped patients with acute myeloid leukemia (AML, n = 87), chronic myeloid leukemia (CML, n = 140), childhood acute lymphoblastic leukemia (ALL, n = 101) and healthy controls (n = 471). Association between SNPs and leukemia risk was calculated by estimating odds ratios in the multivariate logistic regression analysis. For miRSNPs that were associated with leukemia risk we performed luciferase reporter assays to examine whether they influence miRNA binding.

RESULTS

Here we show that variant alleles of TLX1_rs2742038 and ETV6_rs1573613 were associated with increased risk of childhood ALL (OR (95% CI) = 3.97 (1.43-11.02) and 1.9 (1.16-3.11), respectively), while PML_rs9479 was associated with decreased ALL risk (OR = 0.55 (0.36-0.86). In adult myeloid leukemias we found significant associations between the variant allele of PML_rs9479 and decreased AML risk (OR = 0.61 (0.38-0.97), and between variant alleles of IRF8_ rs10514611 and ARHGAP26_rs187729 and increased CML risk (OR = 2.4 (1.12-5.15) and 1.63 (1.07-2.47), respectively). Moreover, we observed a significant trend for an increasing ALL and CML risk with the growing number of risk genotypes with OR = 13.91 (4.38-44.11) for carriers of ≥3 risk genotypes in ALL and OR = 4.9 (1.27-18.85) for carriers of 2 risk genotypes in CML. Luciferase reporter assays revealed that the C allele of ARHGAP26_rs187729 creates an illegitimate binding site for miR-18a-3p, while the A allele of PML_rs9479 enhances binding of miR-510-5p and the C allele of ETV6_rs1573613 weakens binding of miR-34c-5p and miR-449b-5p.

CONCLUSIONS

Our study implicates that microRNA-binding site polymorphisms modulate leukemia risk by interfering with the miRNA-mediated regulation. Our findings underscore the significance of variability in 3' untranslated regions in leukemia.

ADAR1 regulates ARHGAP26 gene expression through RNA editing by disrupting miR-30b-3p and miR-573 binding

Rho GTPase activating protein 26 (ARHGAP26) is a negative regulator of the Rho family that converts the small G proteins RhoA and Cdc42 to their inactive GDP-bound forms. It is essential for the CLIC/GEEC endocytic pathway, cell spreading, and muscle development. The present study shows that ARHGAP26 mRNA undergoes extensive A-to-I RNA editing in the 3' UTR that is specifically catalyzed by ADAR1. Furthermore, the mRNA and protein levels of ARHGAP26 were decreased in cells in which ADAR1 was knocked down. Conversely, ADAR1 overexpression increased the abundance of ARHGAP26 mRNA and protein. In addition, we found that both miR-30b-3p and miR-573 target the ARHGAP26 gene and that RNA editing of ARHGAP26 mediated by ADAR1 abolished the repression of its expression by miR-30b-3p or miR-573. When ADAR1 was overexpressed, the reduced abundance of ARHGAP26 protein mediated by miR-30b-3p or miR-573 was rescued. Importantly, we also found that knocking down ADAR1 elevated RhoA activity, which was consistent with the reduced level of ARHGAP26. Conversely, when ADAR1 was overexpressed, the amount of RhoA-GTP decreased. The similar expression patterns of ARHGAP26 and ADAR1 in human tissue samples further confirmed our findings. Taken together, our results suggest that ADAR1 regulates the expression of ARHGAP26 through A-to-I RNA editing by disrupting the binding of miR-30b-3p and miR-573 within the 3' UTR of ARHGAP26. This study provides a novel insight into the mechanism by which ADAR1 and its RNA editing function regulate microRNA-mediated modulation of target genes.

Active RHOA favors retention of human hematopoietic stem/progenitor cells in their niche

BACKGROUND

Hematopoietic stem/progenitor cells (HSPCs) maintain the hematopoietic system by balancing their self-renewal and differentiation events. Hematopoietic stem cells also migrate to various sites and interact with their specific microenvironment to maintain the integrity of the system. Rho GTPases have been found to control the migration of hematopoietic cells and other cell types. Although the role of RAC1, RAC2 and CDC42 has been studied, the role of RHOA in human hematopoietic stem cells is unclear.

RESULTS

By utilizing constitutively active and dominant negative RHOA, we show that RHOA negatively regulates both in vitro and in vivo migration and dominant negative RHOA significantly increased the migration potential of human HSC/HPCs. Active RHOA expression favors the retention of hematopoietic stem/progenitor cells in the niche rather than migration and was found to lock the cells in the G0 cell cycle phase thereby affecting their long-term self-renewal potential.

CONCLUSION

The current study demonstrates that down-regulation of RHOA might be used to facilitate the migration and homing of hematopoietic stem cells without affecting their long-term repopulating ability. This might be of interest especially for increasing the homing of ex vivo expanded HSPC.

Abnormal methylation of GRAF promoter Chinese patients with acute myeloid leukemia

The epigenetic disturbances are recognized as an alternative mechanism contributing to the pathogenesis of acute myeloid leukemia (AML). GTPase regulator associated with focal adhesion kinase (GRAF), a putative tumor suppressor gene, was revealed with mutations and promoter methylation in AML and myelodysplastic syndrome. In this study, we investigated the methylation status of GRAF promoter in Chinese AML patients. Aberrant methylation of GRAF promoter was detected in 66.7% (88/132) of the cases analyzed. The methylation of GRAF gene could be detected in all FAB subtypes and in all cytogenetic risk groups. There were no significant differences in clinical features, FAB subtypes and cytogenetic risk groups between patients with and without GRAF methylation. GRAF transcript was significantly lower in AML group compared to controls (3.30 vs 56.06, P<0.001). Both patients with methylated GRAF gene and those without methylated GRAF gene had significantly lower GRAF transcript than controls (P<0.001). Furthermore, GRAF transcript was significantly lower in patients with methylated GRAF than those without methylated GRAF (1.64 vs 6.42, P=0.005). These findings suggest that the hypermethylation of GRAF promoter might be one of early events in the development of AML.

GTPase regulator associated with the focal adhesion kinase (GRAF) transcript was down-regulated in patients with myeloid malignancies

BACKGROUND

GTPase regulator associated with the focal adhesion kinase (GRAF), a putative tumor suppressor gene, is found inactivated in hematopoietic malignancies by either genetic or epigenetic abnormalities. However, the expression level of GRAF gene has not yet been studied in leukemia. The aim of this study was to investigate the expression level of GRAF gene in those patients with myeloid malignancies including acute myeloid leukemia (AML), myelodysplastic syndrome (MDS) and chronic myeloid leukemia (CML).

METHODS

The expression levels of GRAF transcript were determined in 94 patients using real-time quantitative PCR (RQ-PCR). Clinical and laboratory data of these patients were collected and analyzed.

RESULTS

The significantly decreased level of GRAF transcript was observed in three myeloid malignancies compared to controls. Within AML, there was no difference in the level of GRAF transcript among different FAB subtypes (P > 0.05). Difference was not observed in the amount of GRAF mRNA between CML at chronic phase and controls. As CML progressed, GRAF transcript significantly decreased. In MDS, three cases with 5q deletion had lower GRAF transcript than four without 5q deletion (median 0.76 vs 2.99) (P > 0.05).

CONCLUSION

our results demonstrate that the GRAF transcript is decreased in myeloid malignancies.

Decreased expression of GRAF1/OPHN-1-L in the X-linked alpha thalassemia mental retardation syndrome

Background

ATRX is a severe X-linked disorder characterized by mental retardation, facial dysmorphism, urogenital abnormalities and alpha-thalassemia. The disease is caused by mutations in ATRX gene, which encodes a protein belonging to the SWI/SNF DNA helicase family, a group of proteins involved in the regulation of gene transcription at the chromatin level. In order to identify specific genes involved in the pathogenesis of the disease, we compared, by cDNA microarray, the expression levels of approximately 8500 transcripts between ATRX and normal males of comparable age.

Methods

cDNA microarray was performed using total RNA from peripheral blood mononuclear cells of ATRX and normal males. Microarray results were validated by quantitative real-time polymerase chain reaction.

Results

cDNA microarray analysis showed that 35 genes had a lower expression (30-35% of controls) while 25 transcripts had a two-fold higher expression in comparison to controls. In the microarray results the probe for oligophrenin-1, a gene known for its involvement in mental retardation, showed a decreased hybridization signal. However, such gene was poorly expressed in blood mononuclear cells and its decrease was not confirmed in the quantitative real-time RT-PCR assay. On the other hand, the expression of an homologous gene, the GTPase regulator associated with the focal adhesion kinase 1/Oligophrenin-1-like (GRAF1/OPHN-1-L), was relatively high in blood mononuclear cells and significantly decreased in ATRX patients. The analysis of the expression pattern of the GRAF1/OPHN-1-L gene in human tissues and organs revealed the predominant brain expression of a novel splicing isoform, called variant-3.

Conclusions

Our data support the hypothesis of a primary role for altered gene expression in ATRX syndrome and suggest that the GRAF1/OPHN-1-L gene might be involved in the pathogenesis of the mental retardation. Moreover a novel alternative splicing transcript of such gene, predominantly expressed in brain tissues, was identified.

GRAF1-dependent endocytosis

The role of endocytosis in controlling a multitude of cell biological events is well established. Molecular and mechanistic characterization of endocytosis has predominantly focused on CME (clathrin-mediated endocytosis), although many other endocytic pathways have been described. It was recently shown that the BAR (Bin/amphiphysin/Rvs) and Rho GAP (GTPase-activating protein) domain-containing protein GRAF1 (GTPase regulator associated with focal adhesion kinase-1) is found on prevalent, pleiomorphic endocytic membranes, and is essential for the major, clathrin-independent endocytic pathway that these membranes mediate. This pathway is characterized by its ability to internalize GPI (glycosylphosphatidylinositol)-anchored proteins, bacterial toxins and large amounts of extracellular fluid. These membrane carriers are highly dynamic and associated with the activity of the small G-protein Cdc42 (cell division cycle 42). In the present paper, we review the role of GRAF1 in this CLIC (clathrin-independent carrier)/GEEC (GPI-anchored protein-enriched early endocytic compartment) endocytic pathway and discuss the current understanding regarding how this multidomain protein functions at the interface between membrane sculpting, small G-protein signalling and endocytosis.

Rho GTPases in hematopoiesis and hemopathies

Rho family GTPases are intracellular signaling proteins regulating multiple pathways involved in cell actomyosin organization, adhesion, and proliferation. Our knowledge of their cellular functions comes mostly from previous biochemical studies that used mutant overexpression approaches in various clonal cell lines. Recent progress in understanding Rho GTPase functions in blood cell development and regulation by gene targeting of individual Rho GTPases in mice has allowed a genetic understanding of their physiologic roles in hematopoietic progenitors and mature lineages. In particular, mouse gene-targeting studies have provided convincing evidence that individual members of the Rho GTPase family are essential regulators of cell type-specific functions and stimuli-specific pathways in regulating hematopoietic stem cell interaction with bone marrow niche, erythropoiesis, and red blood cell actin dynamics, phagocyte migration and killing, and T- and B-cell maturation. In addition, deregulation of Rho GTPase family members has been associated with multiple human hematologic diseases such as neutrophil dysfunction, leukemia, and Fanconi anemia, raising the possibility that Rho GTPases and downstream signaling pathways are of therapeutic value. In this review we discuss recent genetic studies of Rho GTPases in hematopoiesis and several blood lineages and the implications of Rho GTPase signaling in hematologic malignancies, immune pathology. and anemia.