ETn insertion in the mouse Adcy1 gene: transcriptional and phylogenetic analyses.

Early retrotransposons (ETn) are murine transposable elements, bearing some structural similarity to integrated proviruses, and can be insertional mutagens. We have recently identified the causative mutation of the barrelless (Adcy1brl) phenotype as an integration of a 5.7-kb ETn in an intron of the adenylyl cyclase type I (Adcy1) gene. In the present study, Northern blot analysis shows that the ETn insertion results in loss of the normal Adcy1 transcript, a finding consistent with the loss-of-function Adcy1brl mutation, and generation of shorter transcripts. These aberrant transcripts are the products of abnormal RNA splicing and termination owing to the inserted sequence, and transcription initiation within the 3' long terminal repeat (LTR) of the ETn. The DNA sequences of the LTRs were compared in phylogenetic analyses with LTRs from 22 other ETn-related sequences. Three distinct families of ETn sequences can be identified on the basis of their LTRs. The ETn found in Adcy1brl is a member of a family that includes all classified ETn elements known to have recently transposed. Further, of the four known solitary (solo) LTRs, we have identified two that show evidence of recombination between LTRs from different ETn families.

Loss of adenylyl cyclase I activity disrupts patterning of mouse somatosensory cortex.

The somatosensory (SI) cortex of mice displays a patterned, nonuniform distribution of neurons in layer IV called the 'barrelfield' (ref. 1). Thalamocortical afferents (TCAs) that terminate in layer IV are segregated such that each barrel, a readily visible cylindrical array of neurons surrounding a cell-sparse center, represents a distinct receptive field. TCA arbors are confined to the barrel hollow and synapse on barrel-wall neurons whose dendrites are oriented toward the center of the barrel. Mice homozygous for the barrelless (brl) mutation, which occurred spontaneously in ICR stock at Université de Lausanne (Switzerland), fail to develop this patterned distribution of neurons, but still display normal topological organization of the SI cortex. Despite the absence of barrels and the overlapping zones of TCA arborization, the size of individual whisker representations, as judged by 2-deoxyglucose uptake, is similar to that of wild-type mice. We identified adenylyl cyclase type I (Adcy1) as the gene disrupted in brl mutant mice by fine mapping of proximal chromosome 11, enzyme assay, mutation analysis and examination of mice homozygous for a targeted disruption of Adcy1. These results provide the first evidence for involvement of cAMP signalling pathways in pattern formation of the brain.