The small GTPase RhoH is an atypical regulator of haematopoietic cells

Abstract

Rho GTPases are a distinct subfamily of the superfamily of Ras GTPases. The best-characterised members are RhoA, Rac and Cdc42 that regulate many diverse actions such as actin cytoskeleton reorganisation, adhesion, motility as well as cell proliferation, differentiation and gene transcription. Among the 20 members of that family, only Rac2 and RhoH show an expression restricted to the haematopoietic lineage.RhoH was first discovered in 1995 as a fusion transcript with the transcriptional repressor LAZ3/BCL6. It was therefore initially named translation three four (TTF) but later on renamed RhoH due to its close relationship to the Ras/Rho family of GTPases. Since then, RhoH has been implicated in human cancer as the gene is subject to somatic hypermutation and by the detection of RHOH as a translocation partner for LAZ3/BCL6 or other genes in human lymphomas. Underexpression of RhoH is found in hairy cell leukaemia and acute myeloid leukaemia.Some of the amino acids that are crucial for GTPase activity are mutated in RhoH so that the protein is a GTPase-deficient, so-called atypical Rho GTPase. Therefore other mechanisms of regulating RhoH activity have been described. These include regulation at the mRNA level and tyrosine phosphorylation of the protein's unique ITAM-like motif. The C-terminal CaaX box of RhoH is mainly a target for farnesyl-transferase but can also be modified by geranylgeranyl-transferase. Isoprenylation of RhoH and changes in subcellular localisation may be an additional factor to fine-tune signalling.Little is currently known about its signalling, regulation or interaction partners. Recent studies have shown that RhoH negatively influences the proliferation and homing of murine haematopoietic progenitor cells, presumably by acting as an antagonist for Rac1. In leukocytes, RhoH is needed to keep the cells in a resting, non-adhesive state, but the exact mechanism has yet to be elucidated. RhoH has also been implicated as a regulatory molecule in the NFkappaB, PI3 kinase and Map kinase pathways. The recent generation of RhoH knockout mice showed a defect in thymocyte selection and TCR signalling of thymic and peripheral T-cells. However, RhoH-deficient mice did not develop lymphomas or showed obvious defects in haematopoiesis.

Figure 1

Alignment of the most prominent human Rho GTPases. RhoA, Rac1, Cdc42 and RhoH were aligned using the ClustalW alignment program [71]. Conserved residues are presented in red and RhoH specific amino acids are depicted in blue. The ITAM-like motif in RhoH comprising the sequence YxxA(X)₆YxxA is underlined. Gray-shaded boxes mark the phosphate-binding loop, and switch I and switch II regions, respectively. The Rho family specific insertion motif that is absent in the other members of the Ras superfamily, is shown in bold typing.

Figure 2

Rho GTPases can be isoprenylated at their C-terminal CaaX box. The CKIF motif in human and mouse RhoH is a target for the in vitro modification by A) farnesyl-transferase and B) geranylgeranyl-transferase. The isoprenylation reaction was performed as described elsewhere [23].

Figure 3

RhoH in Cancer. The two major mechanisms that link RhoH to cancer are aberrant somatic hypermutation and the formation of fusion transcripts with genes such as the transcriptional repressor LAZ3/BCL6. Both mechanisms have been found in a variety of human cancers and the figure summarises the current knowledge by sorting the described cancers corresponding to their WHO classification [36]. Cancers involving somatic mutation of the RHOH gene are highlighted in blue, while cancers, where a fusion of RhoH was detected, are presented in green. Cases of leukaemia characterised by underexpression of RhoH caused by an unknown mechanism are shown in white.

Figure 4

RhoH is indispensable for correct TCR activation. Illustration of the proposed mechanism that links RhoH to the active TCR complex. The ITAM-like motif of RhoH (see figure 1) allows binding of the Zap70 molecule and shuttling to the TCR CD3ζ chain. Zap70 binding to the CD3ζ chain eventually results in the activation of the linker for activation of T-cells (LAT) molecule and the so-called LAT signalosome. In the absence of RhoH, translocation of Zap70 to the immunological synapse and CD3ζ phosphorylation are impaired leading to reduced TCR-induced signalling and impaired thymocyte selection.