Dual Functions of ASCIZ in the DNA Base Damage Response and Pulmonary Organogenesis

Abstract

Zn2+-finger proteins comprise one of the largest protein superfamilies with diverse biological functions. The ATM substrate Chk2-interacting Zn2+-finger protein (ASCIZ; also known as ATMIN and ZNF822) was originally linked to functions in the DNA base damage response and has also been proposed to be an essential cofactor of the ATM kinase. Here we show that absence of ASCIZ leads to p53-independent late-embryonic lethality in mice. Asciz-deficient primary fibroblasts exhibit increased sensitivity to DNA base damaging agents MMS and H2O2, but Asciz deletion or knock-down does not affect ATM levels and activation in mouse, chicken, or human cells. Unexpectedly, Asciz-deficient embryos also exhibit severe respiratory tract defects with complete pulmonary agenesis and severe tracheal atresia. Nkx2.1-expressing respiratory precursors are still specified in the absence of ASCIZ, but fail to segregate properly within the ventral foregut, and as a consequence lung buds never form and separation of the trachea from the oesophagus stalls early. Comparison of phenotypes suggests that ASCIZ functions between Wnt2-2b/ß-catenin and FGF10/FGF-receptor 2b signaling pathways in the mesodermal/endodermal crosstalk regulating early respiratory development. We also find that ASCIZ can activate expression of reporter genes via its SQ/TQ-cluster domain in vitro, suggesting that it may exert its developmental functions as a transcription factor. Altogether, the data indicate that, in addition to its role in the DNA base damage response, ASCIZ has separate developmental functions as an essential regulator of respiratory organogenesis.

A breathtaking phenotype: Unexpected roles of the DNA base damage response protein ASCIZ as a key regulator of early lung development

“…Absence of ASCIZ led to already macroscopically obvious exencephaly (~25% penetrance in our mice) as a relatively generic developmental defect. 7,8 Most strikingly, detailed histological analyses revealed complete absence of lungs combined with absence of, or severely underdeveloped tracheas in, all of our Asciz-null embryos. 7 Complete absence of lungs is a very rare phenotype and shared only with Gli2/Gli3 mutations in the hedgehog pathway, the Wnt2-2b/b-catenin pathway and the FGF10/FGFR2b pathway.…”

“…7,8 Most strikingly, detailed histological analyses revealed complete absence of lungs combined with absence of, or severely underdeveloped tracheas in, all of our Asciz-null embryos. 7 Complete absence of lungs is a very rare phenotype and shared only with Gli2/Gli3 mutations in the hedgehog pathway, the Wnt2-2b/b-catenin pathway and the FGF10/FGFR2b pathway. 10 In contrast to Gli2/Gli3 and Wnt2/2b mutants, Asciznull embryos are able to specify Nkx2.1-positive respiratory precursors in the foregut endoderm, but these never seem to form lung buds.…”

“…Probably as a consequence of the pulmonary agenesis, Asciz-null embryos exhibit additional thoracic defects with severe thymic hypoplasia and complex cardiac alterations, which may be responsible for their general growth retardation from mid-gestation and death in utero. 7 No lung or other internal organ development defects were reported in the other Asciz-null mouse line. 8 Mice lacking the key enzyme of the BER pathway, DNA polymerase b, also exhibit lung maturation defects and midgestational teratogenic treatment with the DNA damaging agent Adriamycin has been reported to impair foregut separation.…”

“…Consistent with this, ASCIZ as well as its isolated SCD were able to strongly activate reporter gene expression in yeast one-hybrid assays and mammalian cell luciferase reporter assays. 7 Thus, it is very conceivable that ASCIZ regulates lung development as a transcription factor, and it will be interesting to identify its critical target genes in this process.…”

“…10 In contrast to Gli2/Gli3 and Wnt2/2b mutants, Asciznull embryos are able to specify Nkx2.1-positive respiratory precursors in the foregut endoderm, but these never seem to form lung buds. 7 Thus, ASCIZ appears to play a key role in regulating the differentiation of respiratory precursors required for lung bud formation and foregut separation. Probably as a consequence of the pulmonary agenesis, Asciz-null embryos exhibit additional thoracic defects with severe thymic hypoplasia and complex cardiac alterations, which may be responsible for their general growth retardation from mid-gestation and death in utero.…”