Generalized disturbance of DNA methylation in the uterine decidua in the CBA/J x DBA/2 mouse model of pregnancy failure.

Nonchromosomal pregnancy failure is a common but poorly understood phenomenon. Because recent data have suggested that epigenetic abnormalities such as abnormal placental DNA methylation may play a role in human pregnancy failure, we undertook experiments to test whether decidual and/or placental DNA methylation abnormalities are present in a mouse model of pregnancy failure. A large number of studies have shown that crosses between CBA/J female mice and DBA/2 males result in pregnancies with a high rate of failure/resorption, whereas other crosses with CBA/J females produce normal pregnancies. Although the CBA/J × DBA/2 mouse has frequently been used as a model for miscarriage, a detailed explanation for the pregnancy failure phenotype is lacking. We performed timed matings between CBA/J female and DBA/2 male mice as well as between DBA/2 female and CBA/J male mice. Decidual caps were isolated at Embryonic Day (E) 9.5 from both crosses, and a microarray-based method was used to comparatively assess genomic methylation at approximately 16,000 loci on mouse chromosome 7. In comparison with decidual caps from DBA/2 × CBA/J pregnancies, CBA/J × DBA/2 decidual caps were characterized by widely and apparently randomly disturbed methylation. In another set of analogous experiments, genomic methylation of placental DNA from E8.5 pregnancies was assessed using the same microarray-based method. This analysis revealed that in contrast to the decidua, placental DNA methylation from CBA/J × DBA/2 pregnancies was indistinguishable from that of normal controls. We conclude that abnormal DNA methylation in the uterine decidua likely plays a role in the CBA/J × DBA/2 model of pregnancy failure. To our knowledge, these experiments are the first to demonstrate that epigenetic abnormalities of the decidua are associated with pregnancy failure, and they set the stage for future efforts to understand the role of DNA methylation at the maternal-fetal interface.

Alanine tracts: the expanding story of human illness and trinucleotide repeats.

Expansions of polyglutamine repeats are known to cause a variety of human neurodegenerative diseases. More recently, expansions of alanine tracts, particularly in transcription factor genes, have been shown to cause at least nine human conditions, including mental retardation and malformations of the brain, digits and other structures. Present knowledge suggests that alanine tract expansions generally, but not always, arise through unequal recombination as opposed to replication slippage, the most likely mechanism in other triplet repeat expansions. The function of alanine tracts is unknown but when alanine expansions occur in transcription factor genes, alanine tracts can result in either loss-of-function or gain of an abnormal function. Given the frequency of alanine tracts in proteins, it is likely that more alanine tract expansions will be discovered in disease genes.

Immunolocalization of Zic2 expression in the developing mouse forebrain.

The Zic genes are a family of zinc finger transcription factors defined by their homology with the Drosophila gene, odd-paired (opa). Zic2 has a critical role in forebrain development, as is evidenced by the fact that, in both mice and humans, diminished expression results in the severe forebrain malformation known as holoprosencephaly. Published information indicates that Zic2 expression is most prominent in the dorsal neural tube/spinal cord and in the hindbrain; however, there is no published description of the pattern of expression of Zic2 in the developing forebrain where the main Zic2 associated phenotype occurs. Using a Zic2-specific antiserum, we present new information about the expression of Zic2 in the developing mouse forebrain. In addition, we show that in sonic hedgehog (Shh) null mice, the expression of Zic2 is expanded ventrally in some structures while absent in others, suggesting that Shh has a role in regulating the expression of Zic2.

Possible association of NTDs with a polyhistidine tract polymorphism in the ZIC2 gene.

Neural tube defects (NTDs) and brain malformations represent a common finding in chromosome 13q deletion patients. Hemizygosity for ZIC2, which is located in the 13q32 critical deletion region, results in holoprosencephaly (HPE) in humans, and diminished expression of ZIC2 results in HPE as well as lumbosacral NTDs in mice. Taken together, these observations led us to hypothesize that ZIC2 mutations may be a cause of isolated NTD. To test this, we screened 192 NTD patients for mutations in ZIC2. While we did not find ZIC2 mutations in these patients, we did find some evidence of a possible association between a histidine tract polymorphism in ZIC2 and NTDs. Our sample was too small to reach definitive conclusions, but the evidence is sufficiently intriguing to encourage further research. If this association is confirmed, subtle alterations in ZIC2 activity may confer a risk of NTD.