Characterization of alternatively spliced products and tissue-specific isoforms of USP28 and USP25.

BACKGROUND: The ubiquitin-dependent protein degradation pathway is essential for the proteolysis of intracellular proteins and peptides. Deubiquitinating enzymes constitute a complex protein family involved in a multitude of cellular processes. The ubiquitin-specific proteases (UBP) are a group of enzymes whose predicted function is to reverse the ubiquitinating reaction by removing ubiquitin from a large variety of substrates. We have lately reported the characterization of human USP25, a specific-ubiquitin protease gene at 21q11.2, with a specific pattern of expression in murine fetal brains and adult testis. RESULTS: Database homology searches at the DNA and protein levels and cDNA library screenings led to the identification of a new UBP member in the human genome, named USP28, at 11q23. This novel gene showed preferential expression in heart and muscle. Moreover, cDNA, expressed sequence tag and RT-PCR analyses provided evidence for alternatively spliced products and tissue-specific isoforms. Concerning function, USP25 overexpression in Down syndrome fetal brains was shown by real-time PCR. CONCLUSIONS: On the basis of the genomic and protein sequence as well as the functional data, USP28 and USP25 establish a new subfamily of deubiquitinating enzymes. Both genes have alternatively spliced exons that could generate protein isoforms with distinct tissue-specific activity. The overexpression of USP25 in Down syndrome fetal brains supports the gene-dosage effects suggested for other UBP members related to aneuploidy syndromes.

The first non-LTR retrotransposon characterised in the cephalochordate amphioxus, BfCR1, shows similarities to CR1-like elements.

BfCR1 is the first non-long terminal repeat retrotransposon to be characterised in the amphioxus genome. Sequence alignment of the predicted translation product reveals that BfCR1 belongs to the CR1-like retroposon class, a family widely distributed in vertebrate and invertebrate lineages. Structural analysis shows conservation of the specific motifs of the ORF2-CR1 elements: the N-terminal endonuclease, the reverse transcriptase and the C-terminal domains. The BfCR1 element possesses an atypical 3' terminus consisting of the tandem repeat (AAG)6. We gathered evidence supporting the mobility of this element and report an estimated 15 copies of BfCR1, mostly truncated, per haploid genome, a remarkably low number when compared to that of vertebrates. Phylogenetic analysis, including the amphioxus element, seems to indicate that (i) CR1-like retroposons cluster in a monophyletic group and (ii) the CR1-like family was already present in the chordate ancestor. Our data provide further support for the horizontal transmission of CR1-like elements during early vertebrate evolution.

USP25, a novel gene encoding a deubiquitinating enzyme, is located in the gene-poor region 21q11.2.

We have identified a new gene, USP25, spanning over 150 kb at 21q11. 2, one of the lowest gene-density regions of the human genome. USP25 is made up of 25 exons and encodes a 1087-aa protein. Database comparisons reveal high homology with members of the ubiquitin protease family (UBP). Basal expression was observed in all human tissues tested, and two main transcripts were identified. The homologous murine gene has also been characterized. In situ hybridization in mouse embryonic brains showed a clear correlation of expression with proliferative neuroepithelial cells and postmitotic neurons. Moreover, high expression was observed in adult mouse testis. UBPs belong to a complex family of deubiquitinating enzymes that specifically cleave ubiquitin conjugates on a great variety of substrates. These enzymes have an essential role in protein degradation via the 26S proteasome and thus regulate many cellular pathways. An increase in USP25 gene dosage in Down syndrome patients could seriously disturb the balance between ubiquitinated and deubiquitinated substrates.