Research Article: Identification of an Xiap-Like Pseudogene on Mouse Chromosome 7

Date Published: November 30, 2009

Publisher: Public Library of Science

Author(s): Aneta Kotevski, Wendy D. Cook, David L. Vaux, Bernard A. Callus, Andreas Bergmann. http://doi.org/10.1371/journal.pone.0008078

Abstract: The most thoroughly characterized mammalian IAP is XIAP/BIRC4, which can inhibit caspases 9, 3 and 7, but may also regulate apoptosis through interactions with other proteins such as Smac/DIABLO, HtrA2/Omi, XAF1, TAK1, cIAP1, and cIAP2.High throughput sequencing of the mouse genome revealed the existence of a gene resembling Xiap/Birc4 on mouse chromosome 7. To confirm the existence of this gene, and to determine its functional significance, we performed Southern and Northern blot analysis. This showed the presence of the Xiap-like gene in both wild-type and Xiap gene knock-out mice, but the corresponding mRNA was not detected in any tissues examined by Northern blot. Analysis of the gene sequence in all three possible reading frames predicts that expression of this gene would not give rise to a full-length protein, but only non-functional truncated polypeptides. Because its nucleotide sequence is 92% identical to Xiap, but it has no introns corresponding to those of Xiap, we conclude that Xiap-ps1 is a pseudogene generated by retro-transposition of a spliced Xiap message to chromosome 7.

Partial Text: The inhibitor of apoptosis proteins (IAPs) are a family of proteins that bear one or more baculoviral IAP repeat (BIR) domains [1], [2]. The most thoroughly characterized mammalian IAP is XIAP/BIRC4, which can inhibit caspases 9, 3 and 7 [2], [3], but may also regulate apoptosis through interactions with other proteins such as Smac/DIABLO, HtrA2/Omi, XAF1, TAK1, cIAP1, and cIAP2 [4], [5].

From the Southern analyses (Fig. 2) we have confirmed that a novel Xiap-like gene exists on mouse chromosome 7. As we detected the presence of the Xiap-ps1 gene in genomic DNA samples from Xiap−/− tissues, the bands detected were not the result of the probe hybridising to the Xiap gene on the X chromosome. We failed to detect any evidence by Northern analysis that Xiap-ps1 is expressed in tissues in which Xiap is clearly expressed at the mRNA level. Analysis of the nucleotide sequence showed that Xiap-ps1 gene is devoid of intronic sequences found in Xiap and analysis of the various splicing variants showed that Xiap-ps1 shows similarity to the regions in common to all variants, starting 7 nucleotides upstream of the initiation codon in exon 2 of Xiap (Fig. 1b). This suggests that Xiap-ps1 has arisen from retrotransposition of a processed Xiap mRNA to chromosome 7. We hypothesise that a retroviral infection occurred in mouse germ cells that allowed insertion of reverse-transcribed spliced Xiap mRNA into chromosome 7. A similar event has been shown to have occurred during evolution of the great apes, leading in that case to a transcribed and translated product, BIRC8 (ILP-2), from a single-exon gene [10].

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http://doi.org/10.1371/journal.pone.0008078

 

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