Sex in birds is chromosomally based, as in mammals, but the sex chromosomes are different and the mechanism of avian sex determination has been a long-standing mystery. In the chicken and all other birds, the homogametic sex is male (ZZ) and the heterogametic sex is female (ZW). Two hypotheses have been proposed for the mechanism of avian sex determination. The W (female) chromosome may carry a dominant-acting ovary determinant. Alternatively, the dosage of a Z-linked gene may mediate sex determination, two doses being required for male development (ZZ). A strong candidate avian sex-determinant under the dosage hypothesis is the conserved Z-linked gene, DMRT1 (doublesex and mab-3-related transcription factor 1). Here we used RNA interference (RNAi) to knock down DMRT1 in early chicken embryos. Reduction of DMRT1 protein expression in ovo leads to feminization of the embryonic gonads in genetically male (ZZ) embryos. Affected males show partial sex reversal, characterized by feminization of the gonads. The feminized left gonad shows female-like histology, disorganized testis cords and a decline in the testicular marker, SOX9. The ovarian marker, aromatase, is ectopically activated. The feminized right gonad shows a more variable loss of DMRT1 and ectopic aromatase activation, suggesting differential sensitivity to DMRT1 between left and right gonads. Germ cells also show a female pattern of distribution in the feminized male gonads. These results indicate that DMRT1 is required for testis determination in the chicken. Our data support the Z dosage hypothesis for avian sex determination.

Immunosuppression after organ transplantation is an acknowledged risk factor for skin cancer and lymphoma. We examined whether there was also an excess of leukemia in patients after transplantation and whether this might be related to a particular immunosuppressive treatment. Data from more than 170 000 patients indicated that organ transplantation is associated with a significantly increased risk for acute myeloid leukemia (AML). AML was more frequent after heart transplantation and lung transplantation than after kidney transplantation and was associated with immunosuppression by azathioprine, a thiopurine prodrug. Cellular resistance to thiopurines is associated with DNA mismatch repair (MMR) deficiency. We demonstrate that thiopurine treatment of human cells in vitro selects variants with defective MMR. Consistent with a similar selection in patient bone marrow, in 7 of 7 patients, transplantrelated AML/myelodysplastic syndrome (MDS) exhibited the microsatellite instability (MSI) that is diagnostic for defective MMR. Because MSI occurs infrequently in de novo AML, we conclude that the selec- IntroductionThe immunosuppression required to prevent the rejection of a transplanted organ is associated with an increased risk for cancer. Skin carcinoma, non-Hodgkin lymphoma, and other cancers are more frequent among transplant recipients. 1-3 Part of the increased cancer risk reflects immunosuppression and is paralleled by a high incidence of malignancies in HIV-immunocompromised patients (for reviews see Penn 1 and Mueller 4 ). Long-term treatment with drugs may also be a risk factor for cancer. The ability of therapeutic drugs to inflict DNA damage may contribute to the increasing incidence of therapy-related malignancy, particularly therapyrelated acute myeloid leukemia/myelodysplastic syndrome (tAML/ MDS) (for a review, see Leone et al 5 ).Azathioprine is a thiopurine prodrug that is related to 6-mercaptopurine and 6-thioguanine (6-TG). 6 Azathioprine is widely used as an immunosuppressant in recipients of organ transplant, often in conjunction with cyclosporin A and steroids. Although the mechanism of action of the thiopurines is not completely understood despite more than 3 decades of clinical use, the cell's DNA mismatch repair (MMR) system is known to be an important cofactor. It is widely accepted that the formation of thiopurine nucleotides and the eventual incorporation of 6-TG into DNA underlie the cytotoxicity and therapeutic effect of thiopurines (for a review, see McLeod et al 7 ). Although substituting DNA with 6-TG is not, in itself, particularly detrimental, nonenzymatic methylation of DNA 6-TG generates DNA lesions. 8 The rare DNA 6-thiomethylguanine (6-meTG) bases are then processed by MMR. 9 MMR-dependent processing is linked to apoptosis, and inactivation of repair provides an escape from thiopurine-induced cell death. Thus, MMR-deficient cells can tolerate 6-meTG in their DNA, and their resistance to killing by 6-TG is well documented. MMR performs similar lethal processing on the structurall...

No abstract

No abstract