Eukaryotic cells have a layer of heterochromatin at the nuclear periphery. To investigate mechanisms regulating chromatin distribution, we analyzed heterochromatin organization in different tissues and species, including mice with mutations in the lamin B receptor (Lbr) and lamin A (Lmna) genes that encode nuclear envelope (NE) proteins. We identified LBR- and lamin-A/C-dependent mechanisms tethering heterochromatin to the NE. The two tethers are sequentially used during cellular differentiation and development: first the LBR- and then the lamin-A/C-dependent tether. The absence of both LBR and lamin A/C leads to loss of peripheral heterochromatin and an inverted architecture with heterochromatin localizing to the nuclear interior. Myoblast transcriptome analyses indicated that selective disruption of the LBR- or lamin-A-dependent heterochromatin tethers have opposite effects on muscle gene expression, either increasing or decreasing, respectively. These results show how changes in NE composition contribute to regulating heterochromatin positioning, gene expression, and cellular differentiation during development.

WT1 is a tumor suppressor gene with a key role in urogenital development and the pathogenesis of Wilms' tumor. Two alternative splice sites in the WT1 transcript allow the gene to encode four proteins. These carry four Krüppel-type zinc fingers and to date have primarily been implicated in transcriptional control of genes involved in growth regulation. However, here we demonstrate colocalization of WT1 with splicing factors in the fetal kidney and testis and in expressing cell lines. Using immunoprecipitation, we show that two WT1 isoforms directly associate with one or a limited number of components in the spliceosomes and coiled bodies. Moreover, COS cell expression studies suggest that alternative splicing within the WT1 zinc finger region determines whether the protein localizes mainly with splicing factors or with DNA in transcription factor domains in the nucleus. We propose that WT1 plays roles in posttranscriptional processing of RNA as well as in transcription.

Aniridia in man and Small eye in mice are semidominant developmental disorders caused by mutations within the paired box gene PAX6. Whereas heterozygotes suffer from iris hypoplasia, homozygous mice lack eyes and nasal cavities and exhibit brain abnormalities. To investigate the role of gene dosage in more detail, we have generated yeast artificial chromosome transgenic mice carrying the human PAX6 locus. When crossed onto the Small eye background, the transgene rescues the mutant phenotype. Strikingly, mice carrying multiple copies on a wild-type background show specific developmental abnormalities of the eye, but not of other tissues expressing the gene. Thus, at least five different eye phenotypes are associated with changes in PAX6 expression. We provide evidence that not only reduced, but also increased levels of transcriptional regulators can cause developmental defects.

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