Alternative titles; symbols
HGNC Approved Gene Symbol: RACGAP1
Cytogenetic location: 12q13.12 Genomic coordinates (GRCh38) : 12:49,989,162-50,033,440 (from NCBI)
| Location | Phenotype |
Phenotype MIM number |
Inheritance |
Phenotype mapping key |
|---|---|---|---|---|
| 12q13.12 | Anemia, congenital dyserythropoietic, type IIIb, autosomal recessive | 619789 | Autosomal recessive | 3 |
Rho GTPases control a variety of cellular processes. There are 3 subtypes of Rho GTPases in the Ras superfamily of small G proteins: RHO (see 165370), RAC (see RAC1; 602048), and CDC42 (116952). GTPase-activating proteins (GAPs) bind activated forms of Rho GTPases and stimulate GTP hydrolysis. Through this catalytic function, Rho GAPs negatively regulate Rho-mediated signals. GAPs may also serve as effector molecules and play a role in signaling downstream of Rho and other Ras-like GTPases.
By screening a Jurkat cDNA library using a yeast 2-hybrid system with an activated form of RAC as bait, followed by screening a placenta cDNA library, Toure et al. (1998) isolated a cDNA encoding RACGAP1, which they called MGCRACGAP. The predicted 527-amino acid RACGAP1 protein has a large N-terminal region containing a protein kinase C (see 176960)-like cysteine-rich motif. The authors determined that RACGAP1 shares highest homology with the Drosophila RnRacGAP and the chimerins of rat and human (see 602857). Functional analysis showed that the GAP domain of RACGAP1 exhibits strong GAP activity towards CDC42, RAC1, and RAC2 (602049). Northern blot analysis detected an approximately 3.2-kb RACGAP1 transcript that was most abundantly expressed in testis, with low expression in most other tissues. Western blot analysis detected a RACGAP1 protein of 58 kD in testis extracts. In situ hybridization showed that RACGAP1 expression is restricted to germ cells in mature testis.
By yeast 2-hybrid screen, Toure et al. (2001) found that the non-Rho-binding N-terminus of RACGAP1 interacted with the anion exchanger SLC26A8 (608480). By mutation analysis, they determined that the N-terminus of RACGAP1 interacted with the cytoplasmic C-terminal domain of SLC26A8. In situ hybridization of adult testis showed overlap of SLC26A8 and RACGAP1 expression in spermatocytes. The specific pattern of expression suggested to Toure et al. (2001) that SLC26A8 expression was shorter than that of RACGAP1.
Zhao and Fang (2005) identified RACGAP1 as a substrate of the anaphase-promoting complex/cyclosome, a ubiquitin ligase that controls mitotic progression, in HeLa cells. RACGAP1 was required for assembly of anillin (ANLN; 616027) and myosin into the contractile ring and to activate myosin through RhoA (165390)-mediated phosphorylation of the myosin regulatory light chain (MYL2; 160781) at the initiation of cytokinesis. RACGAP1 associated with ECT2 (600586), a guanine nucleotide exchange factor for RhoA, at anaphase and during cytokinesis, and RACGAP1 was required for localization of ECT2 to the central spindle and the contractile ring. Knockdown of ECT2 or RACGAP1 led to absence of a contractile ring and lack of furrow ingression. Zhao and Fang (2005) concluded that RACGAP1 is a master regulator for initiation of cytokinesis that controls assembly of the contractile ring and ingression of the cleavage furrow.
Canman et al. (2008) noted that, during cytokinesis, the GTPase RhoA orchestrates contractile ring assembly and constriction. RhoA signaling is controlled by the central spindle, a set of microtubule bundles that forms between the separating chromosomes. Centralspindlin is a protein complex consisting of ZEN4 (KIF23; 605064) and CYK4 and is required for central spindle assembly and cytokinesis in C. elegans. Canman et al. (2008) found that 2 separation-of-function mutations in the GAP domain of CYK4 lead to cytokinesis defects that mimic centralspindlin loss of function. These defects could be rescued by depletion of the GTPase RAC or its effectors, but not by depletion of RhoA. Canman et al. (2008) concluded that inactivation of RAC by CYK4 functions in parallel with RhoA activation to drive contractile ring constriction during cytokinesis.
The centralspindlin complex, a conserved component of the spindle midzone and midbody, is composed of 2 molecules of the kinesin protein MKLP1 (605064) and 2 molecules of the Rho family GTPase-activating protein RACGAP1 (summary by Lekomtsev et al., 2012). Lekomtsev et al. (2012) identified a plasma membrane tethering activity in the centralspindlin protein complex, a conserved component of the spindle midzone and midbody, and demonstrated that the C1 domain of RACGAP1 associates with the plasma membrane by interacting with polyanionic phosphoinositide lipids. Using x-ray crystallography, Lekomtsev et al. (2012) determined the structure of this atypical C1 domain. Mutations in the hydrophobic cap and in basic residues of the C1 domain of RACGAP1 prevent association of the protein with the plasma membrane, and abrogate cytokinesis in human and chicken cells. Artificial membrane tethering of centralspindlin restores cell division in the absence of the C1 domain of RACGAP1. Although C1 domain function is dispensable for the formation of the midzone and midbody, it promotes contractility and is required for the attachment of the plasma membrane to the midbody, a long-postulated function of this organelle. Lekomtsev et al. (2012) suggested that centralspindlin links the mitotic spindle to the plasma membrane to secure the final cut during cytokinesis in animal cells.
Gross (2022) mapped the RACGAP1 gene to chromosome 12q13.12 based on an alignment of the RACGAP1 sequence (GenBank BC032754) with the genomic sequence (GRCh38).
In a boy with congenital dyserythropoietic anemia type IIIb (CDAN3B; 619789), Wontakal et al. (2022) identified compound heterozygous missense mutations in the RACGAP1 gene: L396Q (604980.0001) and P432S (604980.0002). The mutations, which were found by whole-exome sequencing, were each inherited from an unaffected parent, consistent with autosomal recessive transmission. Both variants affected conserved residues in the GAP domain, and neither was present in the gnomAD database. Knockdown of RACGAP1 in erythroid cell lines in vitro caused a striking defect in enucleation and an increase in multinucleated erythroblasts that could not be rescued by expression of either mutation. Further in vitro studies showed that the mutations had variable effects on the ability to stimulate other GTPases. The L396Q variant showed a decrease in stimulation of CDC42 (116952) and RAC1 (602048) with no change in the ability to stimulate RHOA (165390) compared to wildtype. In contrast, P432S showed no effect on CDC42 or RAC1, but had a significant increase in the ability to stimulate RHOA compared to wildtype. RNAi-mediated knockdown of RACGAP1 in HeLa cells resulted in impaired cytokinesis that was partially rescued by L396Q, but not by P432S. The L396Q variant demonstrated similar cellular localization as wildtype during cytokinesis, but a portion of L396Q cells showed late furrow regression. The P432S variant localized abnormally with a ring-like pattern instead of accumulating at the spindle midzone; this variant resulted in a severe cytokinesis defect, which the authors postulated was due to increased RHOA activity that could interfere with contractile ring formation and cause mislocalization of the centralspindlin. The authors noted that mutation in the KIF23 gene (605064), which is a component of the centralspindlin, causes a similar autosomal dominant form of CDAN3 (CDAN3A; 105600), suggesting a common pathogenetic mechanism.
In 3 unrelated patients with CDAN3B, Hernandez et al. (2023) identified homozygous mutations in the RACGAP1 gene: the previously identified P432S mutation and a novel T220A mutation (604980.0003). An in vitro erythroid differentiation analysis demonstrated that both mutations resulted in impaired cell growth. Analysis of GTPase balance in patient lymphoblastoid cells demonstrated that both the P432S and T220A mutations had reduced activation levels of RHOA and CDC42 and increased activation of RAC1, leading to inhibition of cytokinesis and increased multinucleation.
In a 3-year-old boy with congenital dyserythropoietic anemia type IIIb (CDAN3B; 619789), Wontakal et al. (2022) identified compound heterozygous missense mutations in the RACGAP1 gene: a c.1187T-A transversion, resulting in a leu396-to-gln (L396Q) substitution, and a c.1294C-T transition, resulting in a pro432-to-ser (P432S; 604980.0002) substitution. The mutations, which were found by whole-exome sequencing, were each inherited from an unaffected parent, consistent with autosomal recessive transmission. Both variants affected conserved residues in the GAP domain, and neither was present in the gnomAD database. Detailed in vitro studies showed that the mutations were unable to rescue multinucleated erythrocytes or cytokinesis defects in RACGAP1-null cells. Moreover, the L396Q variant resulted in a loss-of-function effect on target GTPases, whereas P432S showed increased stimulation of target RHOA (165390).
For discussion of the c.1294C-T transition in the RACGAP1 gene, resulting in a pro432-to-ser (P432S) substitution, that was found in compound heterozygous state in a patient with congenital dyserythropoietic anemia type IIIb (CDAN3B; 619789) by Wontakal et al. (2022), see 604980.0001.
In a patient (patient A.II.2) with CDAN3B, Hernandez et al. (2023) identified homozygosity for the P432S mutation. The mutation, which was identified by whole-exome sequencing, was present in heterozygous state in the parents and an unaffected sib. The mutation had a minor allele frequency of 0.000008 in the gnomAD database. An in vitro erythroid differentiation analysis demonstrated that the P432S mutation resulted in impaired cell growth and reduced erythrocytes.
In 2 unrelated patients (B.II.1 and C.II.3), the latter of whom was born to consanguineous parents, with congenital dyserythropoietic anemia type IIIb (CDAN3B; 619789), Hernandez et al. (2023) identified homozygosity for a c.658A-G transition in the RACGAP1 gene, resulting in a thr220-to-ala (T220A) substitution at a conserved residue. The mutation was identified by whole-exome sequencing, and the parents of patient B.II.1 were shown to be mutation carriers. The mutation had a minor allele frequency of 0.000004 in the gnomAD database. An in vitro erythroid differentiation analysis demonstrated that the T220A mutation resulted in impaired cell growth.
Canman, J. C., Lewellyn, L., Laband, K., Smerdon, S. J., Desai, A., Bowerman, B., Oegema, K. Inhibition of Rac by the GAP activity of centralspindlin is essential for cytokinesis. Science 322: 1543-1546, 2008. [PubMed: 19056985] [Full Text: https://doi.org/10.1126/science.1163086]
Gross, M. B. Personal Communication. Baltimore, Md. 3/18/2022.
Hernandez, G., Romero-Cortadellas, L., Ferrer-Cortes, X., Venturi, V., Dessy-Rodriguez, M., Olivella, M., Husami, A., De Soto, C. P., Morales-Camacho, R. M., Villegas, A., Gonzalez-Fernandez, F. A., Morado, M., Kalfa, T. A., Quintana-Bustamante, O., Perez-Montero, S., Tornador, C., Segovia, J. C., Sanchez, M. Mutations in the RACGAP1 gene cause autosomal recessive congenital dyserythropoietic anemia type III. Haematologica 108: 581-587, 2023. [PubMed: 36200420] [Full Text: https://doi.org/10.3324/haematol.2022.281277]
Lekomtsev, S., Su, K.-C., Pye, V. E., Blight, K., Sundaramoorthy, S., Takaki, T., Collinson, L. M., Cherepanov, P., Divecha, N., Petronczki, M. Centralspindlin links the mitotic spindle to the plasma membrane during cytokinesis. Nature 492: 276-279, 2012. [PubMed: 23235882] [Full Text: https://doi.org/10.1038/nature11773]
Toure, A., Dorseuil, O., Morin, L., Timmons, P., Jegou, B., Reibel, L., Gacon, G. MgcRacGAP, a new human GTPase-activating protein for Rac and Cdc42 similar to Drosophila rotundRacGAP gene product, is expressed in male germ cells. J. Biol. Chem. 273: 6019-6023, 1998. [PubMed: 9497316] [Full Text: https://doi.org/10.1074/jbc.273.11.6019]
Toure, A., Morin, L., Pineau, C., Becq, F., Dorseuil, O., Gacon, G. Tat1, a novel sulfate transporter specifically expressed in human male germ cells and potentially linked to RhoGTPase signaling. J. Biol. Chem. 276: 20309-20315, 2001. [PubMed: 11278976] [Full Text: https://doi.org/10.1074/jbc.M011740200]
Wontakal, S. N., Britto, M., Zhang, H., Han, Y., Gao, C., Tannenbaum, S., Durham, B. H., Lee, M. T., An, X., Mishima, M. RACGAP1 variants in a sporadic case of CDA III implicate the dysfunction of centralspindlin as the basis of the disease. Blood 139: 1413-1418, 2022. [PubMed: 34818416] [Full Text: https://doi.org/10.1182/blood.2021012334]
Zhao, W., Fang, G. MgcRacGAP controls the assembly of the contractile ring and the initiation of cytokinesis. Proc. Nat. Acad. Sci. 102: 13158-13163, 2005. [PubMed: 16129829] [Full Text: https://doi.org/10.1073/pnas.0504145102]