Elsevier

Gene

Volume 343, Issue 2, 22 December 2004, Pages 231-238
Gene

A fragile X mental retardation-like gene in a cnidarian

https://doi.org/10.1016/j.gene.2004.10.007Get rights and content

Abstract

The fragile X mental retardation syndrome in humans is caused by a mutational loss of function of the fragile X mental retardation gene 1 (FMR1). FMR1 is an RNA-binding protein, involved in the development and function of the nervous system. Despite of its medical significance, the evolutionary origin of FMR1 has been unclear. Here, we report the molecular characterization of HyFMR1, an FMR1 orthologue, from the cnidarian hydroid Hydractinia echinata. Cnidarians are the most basal metazoans possessing neurons. HyFMR1 is expressed throughout the life cycle of Hydractinia. Its expression pattern correlates to the position of neurons and their precursor stem cells in the animal. Our data indicate that the origin of the fraxile X related (FXR) protein family dates back at least to the common ancestor of cnidarians and bilaterians. The lack of FXR proteins in other invertebrates may have been due to gene loss in particular lineages.

Introduction

The fragile X syndrome is the most common form of inherited mental retardation in humans. The disease is caused by a loss of function mutation in the fragile X mental retardation gene, FMR1, mostly due to the expansion of a CGG repeat in the 5′ untranslated region of the gene. Repeat expansion is followed by hypermethylation resulting in the transcriptional silencing of the FMR1 gene (Pieretti et al., 1991, Verkerk et al., 1991).

FMR1 (also termed fragile X mental retardation protein [FMRP]) is an RNA binding protein that contains at least three types of RNA binding motifs: a ribonucleoprotein K homology domain (KH domain; FMR1 has two such domains), an arginine and glycine rich domain (RGG box) and an N-terminal NDF domain shown to have RNA and protein binding properties (Siomi et al., 1994, Adinolfi et al., 2003, Ramos et al., 2003a). The so-called FMR1/FXR interaction domain located near the amino terminus of the protein is responsible for the dimerization and interaction with the two other members of the same protein family (Zhang et al., 1995, Siomi et al., 1996). A ribosomal interaction domain mediates interactions with the 60 S ribosomal subunit (Khandjian et al., 1996, Siomi et al., 1996, Tamanini et al., 1996, Feng et al., 1997). Due to the presence of a nuclear export signal sequence (NES), it has been proposed that FMR1 may shuttle between the nucleus and cytoplasm while carrying its target RNAs (Eberhart et al., 1996, Bardoni et al., 1997). Functional studies have revealed a role of FMR1 in dendritic and synapse development (Laggerbauer et al., 2001, Zhang et al., 2001, Morales et al., 2002, Sung et al., 2003). Recently, FMR1 has also been proposed to function in the RNAi machinery as it was shown to be present in the RISC, the RNA-induced silencing complex (Caudy et al., 2002, Jin et al., 2004).

Vertebrate genomes contain one copy of the FMR1 gene and two autosomal paralogues termed fraxile X related 1 (FXR1) and FXR2 (Zhang et al., 1995). The evolutionary history of the three genes is unclear. The first invertebrate orthologue of FMR1 was isolated from Drosophila (Wan et al., 2000) and was termed dFMR1. Its gene product exhibits a very high sequence similarity with the human and other vertebrate FXR proteins and is the only invertebrate FXR characterized so far. Database searches of available complete genome sequences of Caenorhabditis elegans and Saccharomyces cerevisiae have not revealed any FMR1/FXR homologue (Wan et al., 2000).

We have isolated and analyzed the cDNA of a member of the FXR gene family from the marine hydroid Hydractinia echinata (for a description of the animal, see Frank et al., 2001). This animal belongs to the most ancient extant eumetazoan phylum, the Cnidaria. The fact that cnidarians are the most basal, living metazoans possessing a nervous system is intriguing in this regard. It raises the possibility that the function of FMR1 has been conserved during the evolution of the Eumetazoa, and that it has been primarily related to the development of the nervous system.

Section snippets

Isolation of HyFMR1 full-length cDNA

A cDNA fragment of 600 bp with a predicted amino acid sequence showing high similarity to FXR proteins was isolated during a differential gene expression analysis from an arrayed, nonamplified cDNA library. The full-length cDNA was obtained by rapid amplification of cDNA ends (RACE) polymerase chain reaction (PCR) according to the SMART RACE protocol (Clontech) from polyp and larvae cDNA. The oligonucleotides 5′-ACAAGGGGCCAATATCCAAG-3′ and 5′-TCTGGCTCAGGGTCAATCTTTA-3′ were used for the 3′ and

Full-length cDNA and predicted amino acid sequence

Using RACE PCR, we have isolated a cDNA clone of 1992 bp. The cDNA contained an open reading frame of 1863 bp, encoding a putative protein of 621 amino acid residues with a predicted molecular mass of 70.6 kDa. The nucleotide sequence is deposited at the EMBL database under accession number AJ829441. The predicted amino acid sequence showed the highest similarity with the human FMR1 in the first 2/3 of the protein. Similarity decreased towards the C-terminus known to be less conserved among all

Discussion

The FXR protein family was originally described in vertebrates, where its members fulfill a role in neuronal development and synapse plasticity, mostly through translational repression of specific neuronal mRNAs (Laggerbauer et al., 2001, Zhang et al., 2001, Sung et al., 2003). The first report of an FMR1 homologue outside the Vertebrata came from a study on Drosophila. Functional studies in this animal have revealed that dFMR1 is involved in neurite extension, guidance and branching (Zhang et

Acknowledgements

We thank Günter Plickert (University of Cologne) for anti-RFamide antibodies. Werner A. Müller and Regina Teo commented upon an early version of the manuscript. Funds were provided by the DFG, the German Research Foundation (grant number FR 1346/5-1 to U.F.). J.G.F. was a doctoral fellow of the DFG's Graduiertenkolleg 484 and the Landes Graduiertenförderungsprogramm Baden-Württemberg.

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