Formation of highly organized neocortical structure depends on the production and correct placement of the appropriate number and types of neurons. POU homeodomain proteins Brn-1 and Brn-2 are coexpressed in the developing neocortex, both in the late precursor cells and in the migrating neurons. Here we show that double disruption of both Brn-1 and Brn-2 genes in mice leads to abnormal formation of the neocortex with dramatically reduced production of layer IV-II neurons and defective migration of neurons unable to express mDab1. These data indicate that Brn-1 and Brn-2 share roles in the production and positioning of neocortical neuron development. Received January 22, 2002; revised version accepted May 23, 2002. The mature neocortex is organized into six cell layers, each of which contains neurons with similar morphologies, molecular properties, and projection patterns. The development of this neocortical structure depends on a highly ordered pattern of neuronal production and migration. Cortical neurons that comprise each layer are sequentially produced in the ventricular zone of the dorsal telencephalon (Angevine and Sidman 1961;Takahashi et al. 1999). Although the regulatory factors that function in this sequential production of a variety of layer-specific neurons have not been identified in mammals, in Drosophila the successive production of different types of cells from neuroblasts has been found to require a temporally stereotyped pattern of expression of a set of transcription factors including the Drosophila POU transcription factors Pdm1 and Pdm2 (Isshiki et al. 2001). In mammals, newly produced neurons leave their birthplace, migrate toward the cortical surface, and form cortical layers in an inside-out pattern with respect to their time of birth (Angevine and Sidman 1961;Rakic 1972). Recent genetic studies have identified large numbers of functional molecules involved in the migration/ positioning of neocortical neurons (for review, see Rice and Curran 1999).Brn-1 and Brn-2, members of the mammalian class III POU transcription factor family, are prominently expressed in the embryonic brain, including the neocortex (He et al. 1989). Each single mutant, however, shows abnormalities only in limited brain regions. In Brn-2 mutant neonates, neuronal loss was observed only in the hypothalamic supraoptic and paraventricular nuclei, where Brn-1 is not expressed (Nakai et al. 1995;Schonemann et al. 1995). In Brn-1 mutants, remarkable changes in brain morphology were observed only in the hippocampus, where Brn-2 expression is barely detectable (data not shown). In the neocortex, where both Brn-1 and Brn-2 are expressed, no overt developmental defects were seen in either single mutant. These observations suggest functional complementation between Brn-1 and Brn-2 in neocortical development. Results and DiscussionTo explore their possible overlapping functions in neocortical development, we generated Brn-1/Brn-2 double homozygous mutants by intercrossing double heterozygotes that were healthy and fertile, wi...

The major mutagenic base lesion in DNA caused by exposure to reactive oxygen species is 8-hydroxyguanine or 7,8-dihydro-8-oxoguanine (8-OH-G). Products of the human MMH͞OGG1 gene are known to catalyze in vitro the reactions repairing this DNA lesion. To analyze the function of Mmh in vivo, we generated a mouse line carrying a mutant Mmh allele by targeted gene disruption. Mmh homozygous mutant mice were found to have a physically normal appearance, but to have lost nicking activity in liver extracts for substrate DNA containing 8-OH-G, exhibiting a 3-fold increased accumulation of this adduct at 9 weeks of age compared with wild-type or heterozygous mice. Further elevation to 7-fold was observed in 14-week-old animals. Substantial increase of spontaneous mutation frequencies was clearly identified in Mmh mutant mice bearing transgenic gpt genes. These results indicate that exposure of DNA to endogenous oxidative species continuously produces the mutagenic adduct 8-OH-G in mice, and Mmh plays an essential role in repair of this DNA damage.

We generated mice carrying a loss-of-function mutation in Brn-2, a gene encoding a nervous system specific POU transcription factor, by gene targeting in embryonic stem cells. In homozygous mutant embryos, migratory precursor cells for neurons of the paraventricular nuclei (PVN) and the supraoptic nuclei (SO) of the hypothalamus die at -E12.5. All homozygous mutants suffered mortality within 10 days after birth, possibly because of a complete deficiency of these neurons in the hypothalamus. Although neither developmental nor histological abnormalities were observed in heterozygous mice, the levels of expression of vasopressin and oxytocin in the hypothalamus of these animals were half these of wild-type mice. These results strongly suggest that Brn-2 plays an essential role in the determination and development of the PVN and SO neuronal lineages in the hypothalamus.

DFN3, an X chromosome-linked nonsyndromic mixed deafness, is caused by mutations in the BRN-4 gene, which encodes a POU transcription factor. Brn-4-deficient mice were created and found to exhibit profound deafness. No gross morphological changes were observed in the conductive ossicles or cochlea, although there was a dramatic reduction in endocochlear potential. Electron microscopy revealed severe ultrastructural alterations in cochlear spiral ligament fibrocytes. The findings suggest that these fibrocytes, which are mesenchymal in origin and for which a role in potassium ion homeostasis has been postulated, may play a critical role in auditory function.