Oct-1, Rb, GCL and IPF-1/PDX-1 provide strong support for the hypothesis that nuclear lamina proteins play active roles in transcriptional repression and chromatin structure. Further support for this idea comes from the Drosophila Su(HW) and Mod(mdg4) proteins, which establish chromatin boundaries that regulate homeotic gene expression 45. Immunostaining of Drosophila cells showed that Su(HW)–Mod(mdg4) complexes are restricted to only 10–20 spots, most of which are located near the nuclear lamina 45.
7. Nuclear lamina and genetic diseases
Interactions between the nuclear lamina and transcription factors might explain how mutations in nuclear lamina proteins cause inherited diseases. For example, the X-linked form of Emery–Dreifuss muscular dystrophy (EDMD) is caused by mutations in the emerin gene 46, whereas autosomal–dominant EDMD is caused by mutations in LMNA ( 47 Fig. 6). Other mutations in LMNA cause cardiomyopathy and lipodystrophy 47. These diseases are proposed to result from defects in gene expression, due to loss of specific attachment sites on the nuclear lamina needed to establish or maintain particular patterns of gene expression 48. In this model, a missense mutation that disrupts the binding of just one factor to lamin filaments would cause a limited phenotype, whereas complete loss of LMNA would disrupt all proteins that depend on A-type lamin filaments. This prediction is supported by the severe combined phenotype seen in LMNA-knockout mice 49, which have defects in muscle, fat and possibly bone (B. Burke, pers. commun.), consistent with defects in tissue mesenchymal stem cells 48.