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De novo insertion of an intron into the mammalian sex determining gene, SRY.

ABSTRACT: Two theories have been proposed to explain the evolution of introns within eukaryotic genes. The introns early theory, or "exon theory of genes," proposes that introns are ancient and that recombination within introns provided new exon structure, and thus new genes. The introns late theory, or "insertional theory of introns," proposes that ancient genes existed as uninterrupted exons and that introns have been introduced during the course of evolution. There is still controversy as to how intron-exon structure evolved and whether the majority of introns are ancient or novel. Although there is extensive evidence in support of the introns early theory, phylogenetic comparisons of several genes indicate recent gain and loss of introns within these genes. However, no example has been shown of a protein coding gene, intronless in its ancestral form, which has acquired an intron in a derived form. The mammalian sex determining gene, SRY, is intronless in all mammals studied to date, as is the gene from which it recently evolved. However, we report here comparisons of genomic and cDNA sequences that now provide evidence of a de novo insertion of an intron into the SRY gene of dasyurid marsupials. This recently (approximately 45 million years ago) inserted sequence is not homologous with known transposable elements. Our data demonstrate that introns may be inserted as spliced units within a developmentally crucial gene without disrupting its function.

SUBMITTER: O'Neill RJ

PROVIDER: S-EPMC19134 | biostudies-literature | 1998 Feb

REPOSITORIES: biostudies-literature

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De novo insertion of an intron into the mammalian sex determining gene, SRY.

O'Neill R J RJ Brennan F E FE Delbridge M L ML Crozier R H RH Graves J A JA

Proceedings of the National Academy of Sciences of the United States of America 19980201 4

Two theories have been proposed to explain the evolution of introns within eukaryotic genes. The introns early theory, or "exon theory of genes," proposes that introns are ancient and that recombination within introns provided new exon structure, and thus new genes. The introns late theory, or "insertional theory of introns," proposes that ancient genes existed as uninterrupted exons and that introns have been introduced during the course of evolution. There is still controversy as to how intron ...[more]

PMID: 9465071

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Project description:Long interspersed (L1) and Alu elements are actively amplified in the human genome through retrotransposition of their RNA intermediates by the -100 still retrotranspositionally fully competent L1 elements. Retrotransposition can cause inherited disease if such an element is inserted near or within a functional gene. Using direct cDNA sequencing as the primary assay for comprehensive NF1 mutation analysis, we uncovered in 18 unrelated index patients splicing alterations not readily explained at the genomic level by an underlying point-mutation or deletion. Improved PCR protocols avoiding allelic drop-out of the mutant alleles uncovered insertions of fourteen Alu elements, three L1 elements, and one poly(T) stretch to cause these splicing defects. Taken together, the 18 pathogenic L1 endonuclease-mediated de novo insertions represent the largest number of this type of mutations characterized in a single human gene. Our findings show that retrotransposon insertions account for as many as -0.4% of all NF1 mutations. Since altered splicing was the main effect of the inserted elements, the current finding was facilitated by the use of RNA-based mutation analysis protocols, resulting in improved detection compared to gDNA-based approaches. Six different insertions clustered in a relatively small 1.5-kb region (NF1 exons 21(16)-23(18)) within the 280-kb NF1 gene. Furthermore, three different specific integration sites, one of them located in this cluster region, were each used twice, i.e. NM_000267.3(NF1):c.1642-1_1642 in intron 14(10c), NM_000267.3(NF1):c.2835_2836 in exon 21(16), and NM_000267.3(NF1):c.4319_4320 in exon 33(25). Identification of three loci that each served twice as integration site for independent retrotransposition events as well as 1.5-kb cluster region harboring six independent insertions supports the notion of non-random insertion of retrotransposons in the human genome. Currently, little is known about which features make sites particularly vulnerable to L1 EN-mediated insertions. The here identified integration sites may serve to elucidate these features in future studies.

Dataset Information

De novo insertion of an intron into the mammalian sex determining gene, SRY.

Publications

De novo insertion of an intron into the mammalian sex determining gene, SRY.

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