L1 elements are autonomous retrotransposons that can cause hereditary diseases. We have previously identified a full-length L1 insertion in the CHM (choroideremia) gene of a patient with choroideremia, an X-linked progressive eye disease. Because this L1 element, designated L1(CHM), contains two 3'-transductions, we were able to delineate a retrotransposition path in which a precursor L1 on chromosome 10p15 or 18p11 retrotransposed to chromosome 6p21 and subsequently to the CHM gene on chromosome Xq21. A cell culture retrotransposition assay showed that L1(CHM) is one of the most active L1 elements in the human genome. Most importantly, analysis of genomic DNA from the CHM patient's relatives indicated somatic and germ-line mosaicism for the L1 insertion in his mother. These findings provide evidence that L1 retrotransposition can occur very early in human embryonic development.

Choroideremia (CHM) is a progressive chorioretinal degeneration caused by mutations in the widely expressed CHM gene on chromosome Xq21. The product of this gene, Rab escort protein (REP)-1, is involved in the posttranslational lipid modification and subsequent membrane targeting of Rab proteins, small GTPases that play a key role in intracellular trafficking. We have searched for mutations of the CHM gene in patients with choroideremia by analysis of individual CHM exons and adjacent intronic sequences PCR-amplified from genomic DNA and by reverse transcription (RT)-PCR analysis of the coding region of the CHM mRNA. In 35 patients, at least 21 different causative CHM defects were identified. These included two partial CHM gene deletions and an insertion of a full-length L1 retrotransposon into the coding region of the CHM gene, a type of mutation that has not been previously reported as a cause of CHM. We also detected nine different nonsense mutations, five of which are recurrent, a small deletion, a small insertion, and at least five distinct splice site mutations, one of which has been described previously. Moreover, we report for the first time the identification of an intronic mutation remote from the exon-intron junctions that creates a strong acceptor splice site and leads to the inclusion of a cryptic exon into the CHM mRNA. Finally, in an affected male who did not have a mutation in any of the CHM exons or their splice sites, the deletion of a complete exon from the CHM mRNA was observed.

Positional cloning has previously resulted in the identification of a gene which is disrupted by deletions in patients with the classic choroideremia (CHM) phenotype. More subtle mutations had been identified in 4 exons of the 3' portion but not elsewhere in the CHM gene. We have now isolated and characterized the complete open reading frame of the CHM gene and determined its exon-intron structure. The CHM gene encodes a protein of 653 amino acids, which is highly homologous to the mouse and rat CHM proteins, and, to a slightly lesser extent, to the human CHM-like (CHML) protein. The open reading frame (ORF) of the human CHM gene consists of 15 exons, spanning at least 150 kb of Xq21.2, and it is possible that there is an additional exon corresponding to the 5' non-coding region of the gene. Cloning of the 5' end of the CHM gene and the elucidation of its intron-exon structure enabled us to localize the X-chromosomal breakpoint in a CHM female with an X;7 translocation between exons 3 and 4.

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