SATB1 collaborates with loss of p16 in cellular transformation.

Tumor progression is associated with invasiveness and metastatic potential. The special AT-rich binding protein 1 (SATB1) has been identified as a key factor in the progression of breast cancer cells to a malignant phenotype and is associated with progression of human tumors. In normal development, SATB1 coordinates gene expression of progenitor cells by functioning as a genome organizer. In contrast to progenitor and tumor cells, SATB1 expression in nontransformed cells is not compatible with proliferation. Here we show that SATB1 expression in mouse embryonic fibroblasts induces cell cycle arrest and senescence that is associated with elevated p16 protein levels. Deletion of p16 overcomes the SATB1-induced senescence. We further provide evidence for an interaction of SATB1 with the retinoblastoma (RB)/E2F pathway downstream of p16. A combined deletion of the RB proteins, RB, p107 and p130 (triple-mutant; TM), prevents SATB1-induced G1 arrest, which is restored upon the reintroduction of RB into SATB1-expressing TM fibroblasts. SATB1 interacts with the E2F/RB complex and regulates the cyclin E promoter in an E2F-dependent manner. These findings demonstrate that p16 and the RB/E2F pathway are critical for SATB1-induced cell cycle arrest. In the absence of p16, SATB1 causes anchorage-independent growth and invasive phenotype in fibroblasts. Our data illustrate that p16 mutations collaborate with the oncogenic activity of SATB1. Consistent with our finding, a literature survey shows that deletion of p16 is generally associated with SATB1 expressing human cell lines and tumors.

Pax5/BSAP maintains the identity of B cells in late B lymphopoiesis.

The B lineage commitment factor Pax5 (BSAP) is expressed throughout B cell development. To investigate its late function, we generated a mouse strain carrying a floxed Pax5 allele that was conditionally inactivated by CD19-cre or Mx-cre expression. Pax5 deletion resulted in the preferential loss of mature B cells, inefficient lymphoblast formation, and reduced serum IgG levels. Mature B cells radically changed their gene expression pattern in response to Pax5 inactivation. Most B cell antigens were downregulated on the cell surface, and the transcription of B cell-specific genes was decreased, whereas the expression of non-B lymphoid genes was activated in Pax5-deficient B cells. These data demonstrate that Pax5 is essential for maintaining the identity and function of B cells during late B lymphopoiesis.

Comparison of the glycosyl-phosphatidylinositol cleavage/attachment site between mammalian cells and parasitic protozoa.

It was previously hypothesised that the requirements for glycosyl-phosphatidylinositol (GPI) anchoring in mammalian cells and parasitic protozoa are similar but not identical. We have investigated this by converting the GPI cleavage/attachment site in porcine membrane dipeptidase to that found in the trypanosomal variant surface glycoprotein 117 and expressing the resulting mutants in COS-1 cells. Changing the entire (omega), (omega)+1 and (omega)+2 triplet in membrane dipeptidase from Ser-Ala-Ala to Asp-Ser-Ser resulted in efficient GPI anchoring of the mutant proteins, as assessed by cell-surface activity assays and susceptibility to release by phosphatidylinositol-specific phospholipase C. Immunoelectrophoretic blot analysis with antibodies recognising epitopes either side of the native (omega) residue in porcine membrane dipeptidase, and expression of a mutant in which potential alternative cleavage/attachment sites were disrupted, indicated that alternative GPI cleavage/attachment sites had not been used. These results indicate that the requirements for GPI anchoring between mammalian and protozoal cells are not as different as previously suggested, and that rules for predicting the probability of a sequence acting as a GPI cleavage/attachment site need to be applied with caution.

twin of eyeless, a second Pax-6 gene of Drosophila, acts upstream of eyeless in the control of eye development.

The Drosophila Pax-6 gene eyeless (ey) plays a key role in eye development. Here we show tht Drosophila contains a second Pax-6 gene, twin of eyeless (toy), due to a duplication during insect evolution. Toy is more similar to vertebrate Pax-6 proteins than Ey with regard to overall sequence conservation, DNA-binding function, and early expression in the embryo, toy and ey share a similar expression pattern in the developing visual system, and targeted expression of Toy, like Ey, induces the formation of ectopic eyes. Genetic and biochemical evidence indicates, however, that Toy functions upstream of ey by directly regulating the eye-specific enhancer of ey. Toy is therefore required for initiation of ey expression in the embryo and acts through Ey to activate the eye developmental program.

PAX8 mutations associated with congenital hypothyroidism caused by thyroid dysgenesis.

Permanent congenital hypothyroidism (CH) is a common disease that occurs in 1 of 3,000-4,000 newborns. Except in rare cases due to hypothalamic or pituitary defects, CH is characterized by elevated levels of thyroid-stimulating hormone (TSH) resulting from reduced thyroid function. When thyroid hormone therapy is not initiated within the first two months of life, CH can cause severe neurological, mental and motor damage. In 80-85% of cases, CH is associated with and presumably is a consequence of thyroid dysgenesis (TD). In these cases, the thyroid gland can be absent (agenesis, 35-40%), ectopically located (30-45%) and/or severely reduced in size (hypoplasia, 5%). Familial cases of TD are rare, even though ectopic or absent thyroid has been occasionally observed in siblings. The pathogenesis of TD is still largely unknown. Although a genetic component has been suggested, mutations in the gene encoding the receptor for the thyroid-stimulating hormone (TSHR) have been identified in only two cases of TD with hypoplasia. We report mutations in the coding region of PAX8 in two sporadic patients and one familial case of TD. All three point mutations are located in the paired domain of PAX8 and result in severe reduction of the DNA-binding activity of this transcription factor. These genetic alterations implicate PAX8 in the pathogenesis of TD and in normal thyroid development.