METHODS FOR INDUCING FULL ABLATION OF HEMATOPOIESIS

Abstract

The inventors have identified an autosomal dominant (AD) missense mutation in the RAC2 gene (coding for Ras-related botulinum toxin substrate 2 (RAC2)) in three Severe combined immunodeficiencies (SCID) patients whose clinical presentation overlaps with the RD SCID form but who lack AK2 mutations and deafness. Using biochemical and in vitro differentiation assays, the inventors demonstrated that the RAC2 mutation was closely related to an impairment in cell differentiation capacity and defects in cellular and mitochondrial networks. Taken as a whole, the data demonstrate that a dominant gain-of-function (GOF) mutation in the RAC2 protein's GDP/GTP binding site inhibits HSPC differentiation and leads to a severe AD form of SCID with a clinical presentation of RD. Accordingly, the results prompt to consider that introduction of the identified RAC2 mutein in the hematopoietic lineage would be suitable for inducing full ablation of hematopoiesis.

C

Claims

1. A method of full ablating hematopoiesis and/or treating a hematopoietic cell malignancy in a patient in need thereof comprising administering to the patient a therapeutically effective amount of i) a polypeptide comprising the amino acid sequence as set forth in SEQ ID NO:1 wherein the amino residue (G) at position 12 is mutated, or ii) a polynucleotide encoding for a polypeptide comprising the amino acid sequence as set forth in SEQ ID NO:1 wherein the amino residue (G) at position 12 is mutated.

2. A method for inhibiting proliferation and differentiation of a population of hematopoietic stem cells comprising contacting said population with an effective amount of i) a polypeptide comprising the amino acid sequence as set forth in SEQ ID NO:1 wherein the amino residue (G) at position 12 is mutated, or ii) a polynucleotide encoding for a polypeptide comprising the amino acid sequence as set forth in SEQ ID NO:1 wherein the amino residue (G) at position 12 is mutated.

3. A method of inducing cell death of a population of hematopoietic cells comprising contacting said population with an effective amount of i) a polypeptide comprising the amino acid sequence as set forth in SEQ ID NO:1 wherein the amino residue (G) at position 12 is mutated, or ii) a polynucleotide encoding for a polypeptide comprising the amino acid sequence as set forth in SEQ ID NO:1 wherein the amino residue (G) at position 12 is mutated.

4. The method of claim 3, wherein the population of hematopoietic cells is a population of malignant hematopoietic cells.

5. (canceled)

6. The method of claim 1 for preparing the patient for bone marrow transplantation.

7. The method of claim 6 wherein the bone marrow transplantation is hematopoietic stem cell transplantation.

8. The method of claim 1 wherein the hematopoietic cell malignancy is selected from the group consisting of leukemias, lymphomas and multiple myelomas.

9. The method of claim 1 wherein the hematopoietic cell malignancy is selected from the group consisting of myelodysplastic syndrome (MDS); myeloproliferative diseases such as polycythemia vera, essential thrombocytosis (ET), myelofibrosis; and diseases with features of both myelodysplastic syndromes and myeloproliferative diseases such as chronic myelomonocytic leukemia (CMML), juvenile myelomonocytic leukemia (JMML), atypical chronic myeloid leukemia (aCML), myelodysplastic/myeloproliferative disease and acute myelogenous leukemia (AML).

10. The method according to claim 1 wherein the amino residue (G) at position 12 is substituted.

11. The method of claim 10 wherein the amino residue (G) at position 12 is substituted by an amino acid residue (R).

12. The method according to claims 1 wherein the polynucleotide is a messenger RNA (mRNA).

13. The method according to claim 1 wherein the polynucleotide is inserted in a vector.

14. The method according to claim 1 wherein the polypeptide or the polynucleotide is conjugated to at least one other molecule selected from the group consisting of polynucleotides, polypeptides, lipids, lectins, carbohydrates, vitamins, cofactors, and drugs.

15. The method of claim 14 wherein the polypeptide or the polynucleotide is conjugated to a molecule having a specific affinity for hematopoietic cells.

16. The method according to claim 1 wherein the the polypeptide or polynucleotide is formulated with lipidoids.

17. The method according to wherein the polypeptide or the polynucleotide is formulated using one or more liposomes, lipoplexes, or lipid nanoparticles.

18. The method of claim 15 wherein the molecule having a specific affinity for hematopoietic cells, is an antibody or peptide having a binding affinity for a protein expressed at the surface of a hematopoietic stem cell.

Description

FIGURES

[0037] FIG. 1: Identification and effects of the p.G12R RAC2 mutation on RAC2's GTPase activity. A. Pedigrees of the three patients from two unrelated kindreds. Black boxes and black circles represent the affected male (P1) and females (P2, P3), respectively. White boxes and circles represent unaffected individuals. Arrows represent the probands, and a double horizontal bar represent consanguinity. B. A representative electropherogram of RAC2 DNA sequencing for control and patient cells, showing the c.34G>A mutation. C. A representative immunoblot of RAC2 protein expression in lysates from control fibroblasts (C1, C2) and fibroblasts derived from the affected individuals (P1, P2 and P3). The loading control corresponds to GADPH expression. D. HEK293T cells were either not transduced (NT, control) or were transduced with a lentiviral empty vector (WPI) containing the wild type (WT) form of RAC2 cDNA, the mutated form described here (G12R) or (as a positive control) the constitutively activated RAC2 GTP form (G12V). Two days after transduction, cells were recovered for analysis using the G-LISA assay (15 μg of total protein per well). The results are quoted as the mean±standard error of the mean (SEM) of three independent experiments. ***p<0.001; ****p<0.0001.

[0038] FIG. 2: G12R Mutation Inhibits HSPC Proliferation and Differentiation

[0039] A. Proliferation of CD34-positive cells in a 7-day culture. Flow cytometry was used to analyse the proportion (in %) of GFP+-expressing cells in the live cell (7-AAD-negative) gate. The proportions of ROS-low, Di1C1(5)-low and annexin V-positive GFP+ live cells were determined on day 4. The results are quoted as the mean±SEM of four independent experiments. B. Neutrophil differentiation in a 7-day culture. Flow cytometry was used to analyse the number of granulocytes (CD11b+CD15+) among the GFP+ live cells. The proportions of ROS low, Di1C1(5) low and annexin V+ cells among the GFP+ live cells were analysed on day 4. The results are quoted as the mean±SEM of three independent experiments. C. T cell differentiation in a 7-day culture (n=3). Flow cytometry was used to analyse the proportion of GFP+ live cells. The number of T cell progenitors (CD7+) was evaluated on day 7 in the GFP+ live cells. The results are quoted as the mean±SEM of three independent experiments. For all experiments: *p<0.05; **p<0.01 and ***p<0.001.

[0040] FIG. 3. Test of Molm-13 cells from acute myelogenous leukemia (AML, translocation MLL-AF9). A. Molm-13 cells were transduced in the presence of a lentiviral construct containing an empty vector (WPI) or a vector with the wild-type form of RAC2 (WT) or a vector containing the mutated form of RAC2 (G12R). The transduced cells all express a GFP reporter gene making it possible to detect by flow cytometry the % of GFP+ cells and the number of GFP+ transduced cells during a 7-day culture. B. At day 4, flow cytometry analysis was performed to evaluate the proportion of GFP+ transduced cells which enters apoptosis (% annexin V+), the mitochondrial membrane polarization (% Δ μm) and the ROS production. The results are representative of 4 independent experiments. **p<0.01 and ***p<0.001.

[0041] FIG. 4. Test of MV4-11 cells from AML (translocation MLL-AF4). MV4-11 cells were transduced in the presence of a lentiviral construct containing an empty vector (WPI) or a vector with the wild-type form of RAC2 (WT) or a vector containing the mutated form of RAC2 (G12R). The transduced cells all express a GFP reporter gene making it possible to detect by flow cytometry the % of GFP+ cells and the number of GFP+transduced cells during a 7 day culture. The results are representative of 2 independent experiments.

[0042] FIG. 5. Test of HL60 cells from AML. HL-60 cells were transduced in the presence of a lentiviral construct containing an empty vector (WPI) or a vector with the wild-type form of RAC2 (WT) or a vector containing the mutated form of RAC2 (G12R). Following the transduction step, cells were cultured in the presence of retinoic acid in order to induce their differentiation toward the granulocytic lineage. A-B. The transduced cells all express a GFP reporter gene making it possible to detect by flow cytometry the number of GFP+transduced cells 2 days (J2) and 4 days (J4) after the beginning of the culture and the number of granulocytes (CD11b+CD15+). The results are representative of 3 independent experiments *p<0.05 *** p<0.001. C-D. At day 4, flow cytometry analysis was performed to evaluate the proportion of cell with a low mitochondrial membrane polarisation level and a low ROS production level. The percentage of cell entering apoptosis was 2% and 10% for the WT and G12R condition, respectively. The histogramms are representative of 3 independent experiments.

[0043] FIG. 6. A. Mononuclear cells from peripheral blood from healthy donors were cultured in the presence of Phytohemagglutinin A and transduced with a lentiviral construct containing an empty vector (WPI) or a vector containing the mutated form of RAC2 (G12R). The transduced cells all express a GFP reporter gene making it possible to follow by flow cytometry the % of GFP +cells, the number of GFP+ transduced cells and the proportion of GFP+ cells which enters apoptosis (% annexin V+).The results are representative of 2 independent experiments. B. B-EBV lines from healthy donors were transduced in the presence of a lentiviral construct containing an empty vector (WPI) or a vector containing the mutated form of RAC2 (G12R). The transduced cells all express a GFP reporter gene making it possible to follow by flow cytometry the% of GFP+ cells. The results are representative of 3 independent experiments ***p<0.001.

EXAMPLE

[0044] Material & Methods

[0045] Patients and Human Cord Blood Samples

[0046] The study was conducted in accordance with the French legislation and the principles of the Declaration of Helsinki. Informed consent was obtained from the patients' parents or legal guardians, and the study protocol was approved by the regional independent ethics committee and the French Ministry of Research (DC 2014-2272/2015/ DC-2008-329.) Primary fibroblast cell lines were obtained from skin biopsies.

[0047] Human cord blood (CB) samples eligible for research purposes were obtained from the Cord Blood Bank at St Louis Hospital (Paris, France; authorization 2014/09/23). Mononuclear cells were isolated by density separation on Lymphoprep (Abcys). CD34-positive HSPCs were sorted magnetically using the autoMACSpro separator (Miltenyi Biotec), and the cells' purity was checked with a MACSQuant analyzer (Miltenyi Biotec).

[0048] Genetic, Sequencing and Gene Expression Analysis

[0049] Genomic DNA was isolated by phenol/chloroform extraction from fibroblasts (P1, P2 and P3) or peripheral blood mononuclear cells (PBMCs) (P3 father). Whole-exome sequencing was performed with an Illumina TruSeq exome enrichment kit (Illumina), using 100 bp paired-end reads. Eighty-five percent of target regions were observed with a coverage >20×. Variant calling was not based on a particular genetic model. The G12R RAC2 variant was not found in a number of in-house and public sequence databases. Sanger sequencing confirmed the mutation in P1, P2 and P3 (ABI Prism 3700 sequencer, Life Technologies).

[0050] For Western blot analyses, primary fibroblasts from control or patients were lysed in a Tris buffer (20 mM Tris, pH 7.9; 300 mM NaCl; 1% Nonidet P-40) supplemented with protease and phosphatase inhibitors. Cell extracts were separated by SDS-PAGE, blotted, and stained with anti-RAC2 (ab154711, Abcam) or anti-GAPDH (SC-32233, Santa Cruz) antibodies. After staining with an HRP-conjugated secondary antibody, the immunoblot was developed using an ECL+ kit (Amersham). Protein levels were quantified with Fiji software.

[0051] Construction and production of the lentiviral vectors. The backbone of the replication-defective, self-inactivating pWPI lentiviral vector was provided by Addgene (https://www.addgene.org/). All the constructs (G12R, G12V, D57N and E62K) were generated by GenScript (https://www.genscript.com). Lentiviral supernatants were produced by the vector facility at SFR BioSciences Gerland-Lyon Sud (Lyon, France). All procedures with genetically modified cells were approved by the French National Biotechnology Council and the French Ministry of Research (reference: 4983/3).

[0052] RAC2 Activation Assays and Immunoblotting on the HEK293T Cell Line

[0053] The HEK293T cells were cultured overnight prior to transduction with the appropriate lentiviral supernatant at a multiplicity of infection of 20. After two days of culture, the cell pellets were frozen in liquid nitrogen. Levels of activated RAC2 were determined using the G-LISA® RAC Activation Assay Biochem Kit™ (#BK125, Cytoskeleton Inc.). The G-LISA® kit was performed according to the manufacturer's instructions, except that 10 μg/ml RAC2 specific antibody (AT2G11, sc-517424, Santa Cruz Biotechnology, Inc.) and 1/2000 HRP-conjugated anti-mouse antibody (#1721011, Bio-Rad) were used to detect the amount of captured active RAC2. The reaction was visualized by the addition of 100 μl of chromogenic substrate (1-step UltraTMB, 34028, Thermo Scientific) for 3 min, and stopped with 50 μl H2SO4 1N. Absorbance at 450 nm was measured using FLUOstar OPTIMA microplate reader. Whole cell extracts were obtained by lysing cells in G-LISA lysis buffer supplemented with a protease inhibitor cocktail. Proteins were separated by SDS-PAGE, and immunoblotted with anti-RAC2 (AT2G11, sc-517424, Santa Cruz Biotechnology, Inc.) or anti GAPDH (14C10, Cell Signaling Technology) antibodies. Immunoblots were revealed by chemiluminescence using the ChemiDoc MP System (Bio-Rad). Protein levels were quantified with Image Lab software.

[0054] Holotomography

[0055] To measure the refractive index within the cell (i.e. label-free visualization of cellular organelles), the cell sample was seeded into a 35-mm glass bottom-dish (Ibidi-Dishes™ #81156; Ibidi, GmbH) and placed into the viewing area of the 3D Cell Explorer microscope (Nanolive SA). Cell images were processed using STEVE software (Nanolive).

[0056] In Vitro Culture of Human Cord Blood CD34+ Cells

[0057] Cord blood CD34+ HSPCs were cultured overnight (as previously described.sup.5) and then transduced with the appropriated lentiviral supernatant at a multiplicity of infection of 80. After two days of culture, the GFP+ cells were measured by flow cytometry prior to in vitro culture. The CD34+ cells' ability to differentiate along the granulocyte or T cell lineage was measured as described elsewhere .sup.6,29.

[0058] Flow Cytometry, Mitotracker DIIC1(5) and CellRox Staining

[0059] Monoclonal antibodies against CD7(MT-701), CD11b (D12), CD34 (8G12), and mouse IgGlk, IgG2a and IgG2b isotype controls, and the reagents annexin V and 7-aminoactinomycin D (7AAD) were obtained from BD Biosciences (San Jose, Calif.). CD15 (80H5) and mouse IgM control antibodies were purchased from Beckman Coulter (San Diego). The mitochondrial membrane potential was measured using a MitoProbe™ DiIC1(5) assay kit (M34151), and the ROS level was measured using a CellRox® assay kit (C10492), according to the manufacturer's instructions (Life Technologies). After staining, cells were analyzed on a Gallios flow cytometer, and the data were processed using Kaluza software (all from Beckman Coulter).

[0060] Homology Modelling

[0061] Three-dimensional homology models were built for the G12R mutant of human RAC2 (Uniprot P15153) using MODELLER software v20 (Webb and Sali, 2016, 2017). The crystal structure of WT human RAC2 (PDB code: 1DS6) was used as a template.

[0062] Statistical Analysis

[0063] For all analyses, three or more independent experiments were performed. Data are reported as the mean±standard error of the mean (SEM). Two-tailed, unpaired t-test was performed using Prism 4 software (GraphPad). The threshold for statistical significance was set to p<0.05.

[0064] Results

[0065] A Missense Mutation (G12R) in the RAC2 GTPase Protein Leads to a SCID Phenotype

[0066] In our cohort of patients with a SCID phenotype, a few individuals do not have yet a molecular diagnosis. Three patients from two unrelated kindreds (P1 from one kindred, and P2 and P3 as the mother and daughter from another kindred, respectively) presented with all the clinical features of RD other than deafness (FIG. 1A). As observed in patients with RD, the complete absence of neutrophils was responsible for the occurrence of severe infections earlier than is usually observed in other forms of SCID (Table 1) picard.sup.2. The analysis of other blood cell lineages highlighted the absence of T and B lymphocytes and circulating monocytes. Conversely, the platelet counts were normal, and haemoglobin level was slightly lower than normal in only one patient (Table 1). Overall, the peripheral cell count profile corresponded to the severe hypoplastic morphology of the patients' bone marrow (Table 1, and data not shown). Haematopoietic stem cell transplantation (HSCT) performed soon after birth was successful in P1 (previously described as P4.sup.12) and P2—demonstrating that the inherited defect was intrinsic and not extrinsic (i.e. not micro-environmental) (Table 1).

[0067] By performing whole-genome sequencing (WES) of the patients' fibroblasts, we identified a heterozygous missense mutation (c.34G>A, p.G12R) in the RAC2 gene as the only possible genetic cause. The mutation was confirmed by Sanger sequencing in the three patients but was absent in P3's father—the only relative from whom we could obtain a DNA sample (FIG. 1B). The presence of the same phenotype and the same RAC2 G12R missense mutation in P2 and her daughter P3 highlighted the AD inheritance pattern.

[0068] The p.G12R missense mutation is located in the G1 box—a highly conserved guanine nucleotide binding region.sup.13—and was not annotated in our in-house database (n=14154 in the cohort) or in the human Genome Aggregation Database (gnomAD). The mutation was predicted to be deleterious by four different in silico prediction software, including Combined Annotation Dependent Depletion. It is noteworthy that this gene mutation differed from the loss-of-function (LOF) or gain-of-function (GOF) mutations previously reported to be responsible for mild neutrophil defects and/or lymphopenia.sup.14-18. It is noteworthy that the G12R missense mutation was associated with normal levels of RAC2 protein expression in the patients' fibroblasts (FIG. 1C).

[0069] The G12R Mutation Located in the GDP/GTP-Binding Domain Disrupts Cell Homeostasis

[0070] In order to understand the functional impact of this G12R mutation, we generated a three-dimensional homology model using wild-type (WT) RAC2's X-ray structure.sup.19 as a template (Data not shown). A bulky, flexible arginine is predicted to block the entrance to the GDP/GTP binding pocket in the G1 box (Data not shown). Accordingly, the arginine's charged guanidinium group might disrupt the positively charged pocket and thus influence the GTP hydrolysis rate—as previously demonstrated for other small GTPases.sup.20.

[0071] To test this model biochemically, we quantified the active RAC2 GTP-bound form in extracts of naturally RAC2-negative HEK293T cells. The cells were transduced with an empty lentiviral backbone(WPI) with green fluorescent protein (GFP) as a tracker or the vector containing either the WT form of RAC2 cDNA, the mutated form described here (G12R) or as positive control, the constitutively activated RAC2 GTP form (G12V).sup.21. A high level of the active GTP-bound RAC2 form was observed with both G12V and G12R (FIG. 1D)—demonstrating that substitution by arginine at position 12 leads to sustained activation of the RAC2 signalling pathway. The G12R mutation is therefore a GOF mutation.

[0072] To determine how the expression of a constitutively active form of RAC2 impacts cell division and survival, we performed a holotomographic analysis of primary fibroblasts from P3. In agreement with the low observed cell proliferation rate in vitro (data not shown), the patient's fibroblasts were characterized by very slow cellular dynamics and fragmented nuclei that were suggestive of cytokinesis failure (Data not shown). Moreover, and in contrast to control fibroblasts, the cell shape and mitochondrial network were disrupted in P3's fibroblasts (Data not shown). These two observations highlighted a link between the G12R mutation, defective mitochondrial activity, and dramatic changes in cellular mitosis that reflect a defect in the RAC signalling pathways driving cytokinesis.sup.22.

[0073] The G12R Mutation Disrupts Mitochondrial Activity and Blocks HSPC Differentiation

[0074] To understand the impact of constitutive RAC2 activation on haematopoiesis, CD34-positive human cord blood HSPCs (given the absence of patient bone marrow samples) were transduced with the WPI, WT, G12R or G12V RAC2 cDNAs (with GFP as tracker) and then cultured with cytokines for 7 days (FIG. 2b). The expression of constitutively activated RAC2 forms (G12R and G12V) led to the disappearance of GFP+transduced cells within 4 days—a time point at which reactive oxygen species (ROS) production (quantified as the proportion of “ROS low” cells) was blocked significantly, and mitochondrial membrane depolarization (the proportion of “DilC1(5) low” cells) and apoptosis (the proportion of annexin V-positive cells) had increased significantly (FIG. 2A). It is noteworthy that these changes were not observed in the GFP-negative subset. Taken as a whole, these findings underlined the specific correlation between mutations at position 12 in the RAC2 protein and mitochondrial dysfunction. Our results are in line with previous reports indicating that (i) mitochondrial activity is required for HSPC function.sup.23,24, and (ii) disruption of the RAC2 signalling pathway affect mitochondrial dynamics.sup.25 and ROS production.sup.26-28. (FIG. 2A).

[0075] As RAC2 is highly expressed in human hematopoietic bone marrow subsets and during human thymopoiesis, we transduced HSPCs with WPI, WT, G12R or G12V RAC2 cDNAs and stimulated them to differentiate along the granulocyte, monocyte and T-lymphoid lineages, all absent in the three patients. After 7 days of culture with granulocyte-colony-stimulating factor, the GFP+CD15+CD11b+ neutrophil counts were significantly lower in the G12R and G12V conditions than in the WPI or WT conditions (FIG. 2B). The mitochondrial membrane potential and ROS production were also found to be disrupted, and were associated with elevated levels of apoptosis (FIG. 2B). Differentiation towards the monocyte lineage and differentiation in colony-forming unit assays gave similar results (data not shown). Upon exposure to the Notch ligand delta-like-4 (DL4) culture system (which enables the differentiation of HSPCs into CD7+ T-cell progenitors in 7 days.sup.29), the GFP+ subset count in the G12R and G12V conditions was very low (relative to the WT and WPI conditions), and GFP+CD7+ T cell progenitors were almost completely absent (FIG. 2C). These results are in line with a previous report in which expression of the G12V constitutive active form of RAC2 prevented mouse thymocytes differentiation and leads to apoptosis.sup.30. Taken as a whole, our results underline the key function of the GDP/GTP-bound RAC2 balance in the survival and differentiation of the lymphomyeloid compartment and emphasize the RAC2 signalling pathway's involvement in HSPC function. We next compared the impact of G12R with that of previously described LOF and GOF missense mutations in RAC2 (p.D57N and p.E62K, respectively).sup.14,15,18 on the proliferation of human cord blood HSPCs. During the 8-day culture, the number of GFP+-transduced HSPCs was low in the E62K and G12R groups but was significantly higher in the E62K condition than in the G12R condition. Unlike the G12R mutation, the D57N and E62K mutations had no impact on mitochondrial activity.

[0076] Taken as a whole, our observations suggest that the G12R missense mutation in RAC2 is correlated with the level of the active GTP-bound form of RAC2 and with HSPC survival and function. These findings fit with the clinical and immunological phenotype of the SCID observed in our three patients and with the less severe phenotype displayed by patients carrying E62K or D57 mutations.

[0077] The Three Types of LAM Tested (Molm13, MV4-11 and HL60)

[0078] Molm-13 cells from acute myelogenous leukemia (AML, translocation MLL-AF9) were transduced in the presence of a lentiviral construct containing an empty vector (WPI), a vector containing the wild form of RAC2 (WT) or a vector containing the mutated form of RAC2 (G12R). The transduced cells all express a reporter gene GFP (green fluorescent protein) making it possible to follow by flow cytometry the percentage of GFP+ cells and the number of transduced GFP+ cells. The expression of constitutively activated RAC2 form (G12R) led to the disappearance of GFP+ transduced cells within 7 days. (FIG. 3A). This observation is correlated with an increased proportion of GFP+ apoptotic cells (% annexin V+) having defective mitochondrial membrane polarization and defective reactive oxygen species (ROS) production. (FIG. 3B).

[0079] MV4-11 cells from AML (translocation MLL-AF4) were also transduced in the presence of a lentiviral construct containing an empty vector (WPI), a vector containing the wild form of RAC2 (WT) or a vector containing the form mutated from RAC2 (G12R). The transduced cells expressing a GFP reporter gene were followed by flow cytometry and we observed a decreased percentage and number of GFP+ cells in the G12R condition (FIG. 4). The cells were also stained with May-Grunwald Giemsa and ragmentation of the nucleus was observed only in the cells transduced with the G12R construct (data not shown).

[0080] HL60 cells from AML were transduced in the presence of a lentiviral construct containing the wild form of RAC2 (WT) or a vector containing the mutated form of RAC2 (G12R). After transduction, the cells were cultured for 4 days in the presence of retinoic acid to induce their differentiation into granulocytes (CD11b+CD15+). The number of GFP+ cells was measured at day 2 (J2) and day 4 (J4) and the number of granulocytes at day 4. As observed on FIGS. 5A and 5B, the number of GFP+ and CD11b+CD15+ cells was drastically reduced in the G12R condition as compared to the WT condition. This observation was correlated with a higher mitochondrial membrane depolarisation (quantified as the proportion of DILC low cells; FIG. 5C) and a higher proportion of cells unable to produce ROS (quantified as the % of ROS low; FIG. 5D). Of note, the proportion of GFP+ cells entering apoptosis was 2% and 10% for the WT and G12R condition, respectively. The cells were also stained with May-Grünwald Giemsa and fragmented nuclei were only observed in the cells transduced with the G12R construct.

[0081] We also studied the effect of the RAC2 G12R missense mutation on primary cells. Briefly,mononuclear cells from healthy donors were transduced in the presence of a lentiviral construct containing an empty vector (WPI) or a vector containing the mutated form of RAC2 (G12R). The GFP+ transduced were followed by flow cytometry during a 8-day culture. As observed on FIG. 6A, the % and number of GFP+ cells were drastically reduced in the G12R condition as compared with the control condition. This observation is associated with a higher % of apoptotic cells (FIG. 6A). B-EBV lines from healthy donors were also transduced in the presence of a lentiviral construct containing an empty vector (WPI) or a vector containing the mutated form of RAC2 (G12R). In the time-course kinetic represented on FIG. 6B, the % of GFP+ cells is significantly decreased in the G12R condition

[0082] Discussion:

[0083] In three patients with SCID, we identified a heterozygous, dominant missense mutation (p.G12R) in the highly conserved G1 box domain of the RAC2 protein. The mutation affected the innate and adaptive immune systems. Given the severity of the clinical presentation, the patients had to undergo HSCT in the first few weeks of life. The observation of AD inheritance broadens the clinical spectrum of RD. We now distinguish between two forms of RD: a recessive syndromic form associated with deafness and AK2 mutations (type I), and a non-syndromic form (without deafness) specifically associated with the AD G12R mutation in RAC2 (type II). The absence of sensorineural hearing loss in the type II AD-SCID form might be due to the specific expression of RAC2 in the hematopoietic lineage and the fact that RAC1 and RAC3 (but not RAC2) are the RAC GTPases involved in the development of the mouse inner ear.sup.31.

[0084] The G12R-RAC2 mutation identified here had a drastic effect on cell proliferation and survival—especially in HSPCs. Furthermore, the mutation disrupted mitochondrial activity in HSPCs and impaired cell differentiation toward lymphoid and myeloid lineages. These features might explain the absence of circulating lymphocytes and neutrophils in our three patients, and highlight the non-redundant regulatory role of RAC2 at different haematopoietic checkpoints.

[0085] Surprisingly, in our experiments, the D57N LOF variant has no impact on the HSPC pool, and the E62K GOF variant only has a moderate effect (relative to G12R). Taken as a whole, our findings emphasize that the various RAC2 mutations differ in their effects on HSPCs, which probably accounts for the broad spectrum of clinical phenotypes observed in patients with RAC2 defects (ranging from neutrophil defects and/or leukopenia—without any reported alteration of haematopoiesis—to the SCID form with bone-marrow hypoplasia we described here). This type of heterogeneity has already been reported for RAG1 mutations, where the phenotype ranges from autoimmune disease to SCID.sup.32.

[0086] The location of the amino-acid substitution (inside the GDP/GTP binding pocket (G12R) or in the switch II domain (E62K)) might explain the difference between these two GOF mutations. This hypothesis is in line with (i) the high level of the active (GTP-bound) form of RAC2 observed in the G12R condition, relative to its absence in the E62K condition, and (ii) the low level of RAC2 protein expression observed in the E62K condition - suggesting that glutamate substitution at position 62 may influence the protein's stability. Consequently, we suggest that G12R mutation is the only one able to activate downstream targets in a constitutive, sustained manner. Our results for the E62K variant differ those described by Hsu et al .sup.18 as they evaluated the GTP binding capacity of purified E62K protein; here, we measured RAC2 GTP activity in a cell lysate. In light of these results, the two GOF mutations do not drive the same level of RAC2 GTP activity and the G-LISA assay appears to be an appropriate method for measuring the level of RAC2 activation in live cells.

[0087] In summary, the p.G12R RAC2 mutation has a drastic impact on the maintenance and differentiation of the HSPC compartment, and might thus explain the severity of the patients' clinical and immunological phenotype. To the best of our knowledge, the present study is the first to report an AD form of SCID and physicians should consider RAC2 gene sequencing for patients with SCID and RD clinical presentation. We also observed a drastic impact of the G12R missense mutation in various cell lines (B-EBV, AML) or mature primary cells suggesting that targeting the GDP/GTP binding domain of RAC2 protein could represent a novel therapeutic strategy to induce leukemic cells death.

TABLE-US-00002 TABLE 1 Haematological characteristics and outcomes for the three patients Age Patients matched (age at P1 P2 P3 control presentation) (3 days) (10 days) (9 days) value Infection at birth Sepsis coloured Sepsis/ amniotic fluid meningitis sepsis/pneumonia brain abscesses White blood cells 0.6 0.3 0.5  7-18 (×10−.sup.9/l) Lymphocytes 0.4 0.1 0.5 3.4-7.6 (×10−.sup.9/l) B lymphocytes 0.09 0.004 0 0.3-2.sup.  (×10−.sup.9/l) T lymphocytes 0.24 0.07 0 2.5-5.5 (×10−.sup.9/l) Monocytes 0.01 NE 0 0.1-1.1 (×10−.sup.9/l) Neutrophils 0.2 NE 0 1.5-8.5 (×10−.sup.9/l) Platelets 248 220 429 175-500 (×10−.sup.9/l) Haemoglobin 18 13 10 12.5-16.6 (g/dl) Bone marrow Hypoplasia Hypoplasia Hypoplasia aspirate HSCT 1.sup.st T-depleted HSCT (2 M) 1.sup.st T-depleted (age at HSCT (3 M) Busulfan 8 mg/kg HSCT (2 M) transplantation, Busulfan 8 mg/kg Endoxan 50 mg/kg Busulfan 3.6 mg/kg months) Endoxan 200 mg/kg Fludarabine Conditioning SAL 25 mg/kg 160 mg/m.sup.2 regimen 2.sup.nd T-depleted SAL 5 mg/kg HSCT (6 M) 2.sup.nd HSCT (3 M) Busulfan 16 mg/kg Fludarabine Cyclophosphamide 120 mg/m.sup.2 200 mg/kg SAL 5 mg/kg SAL 25 mg/kg Donor cells 1.sup.st HSCT: MMFD/f MMFD/f 1.sup.st HSCT: MMFD/f 2.sup.nd HSCT: MMFD/f 2.sup.nd HSCT: MMFD/f Outcome A/W GVHD resolved at Graft failure; D120; A/W death (5.5 M post-HSCT) BM = bone-marrow; HSCT = haematopoietic stem cell transplantation; M = months; ALS: anti-lymphocyte serum; MMFD/f: mismatched family donor/father; GVHD: graft versus host disease; A/W: alive and we

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