GERM LINE-SPECIFIC EXPRESSION OF INTRACISTERNAL A-PARTICLE RETROTRANSPOSONS IN TRANSGENIC MICE

Intracisternal A-particle (IAP) sequences are endogenous retrovirus-li ke mobile elements, or retrotransposons, present at 1,000 copies in th e mouse genome. These elements transpose in a replicative manner via a n RNA intermediate and its reverse transcription, and their transposit ion should therefore be tightly controlled by their transcription leve l. To analyze the in vivo pattern of expression of these retrovirus-li ke elements, we constructed several independent transgenic mice with e ither a complete IAP element marked with an intron or with the IAP pro moter, or long terminal repeat (LTR), alone controlling the expression of a lacZ reporter gene with a nuclear localization signal. For all t ransgenic lines analyzed, IAP expression as determined by reverse tran scription-PCR analysis was found to be essentially restricted to the m ale germ line. Furthermore, in situ -bromo-4-chloro-3-indolyl-beta-D-g alactopyranoside (X-Gal) staining of all organs disclosed specific bet a-galactosidase-positive blue cells only within the testis, found as p atches along the seminiferous tubules and often organized as assemblie s of 2, 4, 8, or 16 cells. Histochemical analyses of tissues from 13.5 -day-old embryos to adults demonstrated that this LTR activity is rest ricted to gonocytes and premeiotic undifferentiated spermatogonia. Fin ally, analysis of the methylation status of both transgenes and endoge nous IAP LTRs demonstrated identical patterns, with methylation in som atic tissues and hypomethylation in the testis. Transgenic mice theref ore reveal an intrinsic, highly restricted IAP expression which had es caped detection in previous global Northern (RNA) blot analyses and wi th possible strong biological relevance, as IAP activation specificall y within the germ line might be a way to generate diversity at the evo lutionary level without being deleterious to individuals.