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Home | Pregnancy Timeline | News Alerts |News Archive March 14, 2014

 

“Epilepsy and cleft palate affect tens of thousands of children in the U.S. alone each year. Respiratory failure is a particular problem in premature and low birth weight babies.
Finding the genes causing these conditions could have clinically important implications.”


P. Jeremy Wang, senior author, professor, Department of Animal Biology,
University of Pennsylvania School of Veterinary Medicine.






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Scientists link missing DNA to birth defects

By clarifying a syndrome’s genetics, researchers have begun identifying the biological root of cleft palate, epilepsy and respiratory difficulties, a condition called human Xq22.1 deletion syndrome.

In 2010, scientists in Italy reported that a woman and her daughter showed a puzzling array of disabilities, including epilepsy and cleft palate. The mother had previously lost a 15-day-old son to respiratory failure, and the research team noted that the mother and daughter each were missing a large chunk of DNA on their X chromosome. But they were unable to definitively show that the problems were tied to that genetic deletion.

Now a team combined from the University of Pennsylvania and The Children’s Hospital of Philadelphia, has confirmed that the patients’ ailments resulted from a genetic anomaly. Creating mice that lacked the same region of DNA, the Penn and CHOP researchers showed that these animals suffered the same problems that afflicted the mother, daughter and son — cleft palate, epilepsy and respiratory difficulties, a condition called human Xq22.1 deletion syndrome.


“Deleting this region in mice causes them to respond like humans with the same deletion. Now that we had a mouse model, we could dissect and try to genetically pinpoint which genes were responsible.”

P. Jeremy Wang, senior author, professor, Department of Animal Biology, University of Pennsylvania School of Veterinary Medicine.


The study appears in the journal Human Molecular Genetics.

To investigate the effects of missing this portion of DNA, more than 1 million base pairs long, the Penn team cross bred mice with a deletion in their DNA, finally creating a new mouse with the entire gene delition reflecting the human patients' same area of delition. Quickly they observed that all male mice died at birth - due to respiratory failure. Female mice, with one normal X chromosome and one X chromosome missing this stretch of genetic material, survived but with varying degrees of epilepsy, cleft palate and other developmental problems.

“We believe this is because of our genetic manipulations skewing the X chromosomes being made inactive,” Wang said. “In females, we randomly 'silenced' one of the X chromosomes so that males and females now had an equal dose of genetic material — not like under normal circumstances. If more female cells had been able to silence the X chromosome with the deletion, the effects of the syndrome wouldn’t have been as severe.”

To further narrow down which part of the deleted genes were responsible for the birth defects, researchers genetically engineered one type of mouse lacking the first two-thirds of the genes deleted and another mouse lacking only the final third of the genes deleted.

Unexpectedly, the mice lacking two-thirds of the region on the X chromosome — which included 17 genes — did not display any respiratory failure, cleft palate or epilepsy. “These mice were fine,” Wang said. “It was very surprising to us that deleting this many genes on the X chromosome did not cause apparent problems for the mice.”

However, this was not the case for the mice missing the final third of the gene delition, 350 kilobase pairs. These mice had the same suite of problems as mice missing the entire region: males died after birth and females had cleft palates, higher rates of death soon after birth, developmental delays and seizures.

The team then ruled out genes in this smaller region with no human equivalent genes, and were left with only four genes. All four belong to the same family of genes which encode proteins involved in cell signaling.“These proteins are involved in neural circuitry and the activity of neurotransmitters,” Wang said. “Which is probably why females lacking one copy of these X-linked genes have epilepsy.”

Wang and colleagues will continue studying these four genes to determine which lead to the developmental problems of cleft palate and epilepsy when missing. Hopefully Wang said, the information gained from future studies could become prenatal testing giving doctors advance warning to treat possible respiratory and other problems in newborns.


Understanding how the lack of these genes leads to epilepsy could also guide treatments for the condition.


“Epilepsy and cleft palate affect tens of thousands of children in the U.S. alone each year,” Wang said, “and respiratory failure is a particular problem in premature and low birth weight babies. Finding the genes causing these conditions could have some very clinically important implications.”

Abstract
Chromosomal segmental deletion is a frequent cause of human diseases. A familial 1.1 Mb deletion of human chromosome Xq22.1 associates with epilepsy, cleft palate and developmental defects in heterozygous female patients. Here, we describe a mouse mutant with a targeted deletion of the syntenic segment of the mouse X chromosome that phenocopies the human syndrome. Male mice with a deletion of a 1.1 Mb Nxf2–Nxf3 X-chromosomal segment exhibit respiratory failure, neonatal lethality and cleft palate. In female mice, heterozygosity for the deletion manifests cleft palate, early postnatal lethality, postnatal growth delay and spontaneous seizures in surviving animals, apparently due to X-chromosome inactivation. Furthermore, loss of a 0.35 Mb subregion containing Armcx5, Gprasp1, Gprasp2 and Bhlhb9 is sufficient to cause the Xq22.1 syndrome phenotype. Our results support that the 1.1 Mb deletion of human Xq22.1 is the genetic cause of the associated syndrome.

Wang co-led the study with his postdoctoral researcher Jian Zhou. Additional coauthors included Penn Vet’s N. Adrian Leu and CHOP’s Ethan Goldberg, Lei Zhou and Douglas Coulter.

The study was supported by funding from the National Institutes of Health.