A study led by OHSU geneticist Cheryl Maslen finds mutations in the CRELD1 gene contribute to development of atrioventricular septal defects in Down syndrome individuals and raise the risk of them in the general population
One in every 100 children is born with a heart malformation, but the genetic causes are still largely a mystery. A research effort led by Cheryl L. Maslen, Ph.D., a professor of medicine and molecular and medical genetics at Oregon Health & Science University, has uncovered an important clue.
Maslen - who also is associate director of the OHSU Heart Research Center - and her collaborators at OHSU, Emory University, Sibley Heart Center Cardiology, and Johns Hopkins University reported in a paper to be published in the American Journal of Medical Genetics: Part A but now available on the journal's Web site (http://www3.interscience.wiley.com/cgi-bin/fulltext/113393142/HTMLSTART) that specific mutations of the CRELD1 gene identified in two of 39 Down syndrome (DS) individuals with complete atrioventricular septal defects (AVSD) - the most severe yet most common heart defect associated with Down syndrome - appear to play a role in development of those defects not only in DS individuals but also in the general population.
The study focused on DS individuals - whose condition is the result of being born with three copies of chromosome 21 instead of the normal pair - because they are 2,000 times more likely than the general population to develop an AVSD, making them the most highly sensitized population to congenital heart defects.
"We know that having that extra chromosome 21 makes Down syndrome individuals acutely sensitive to developing AVSD, but it doesn't cause it directly," said Maslen. "Given the incidence of congenital heart defects in DS there has been a focus on chromosome 21 as a source for congenital heart defect susceptibility genes. But so far no single gene or set of genes on chromosome 21 has been shown to contribute to the risk of heart defects either in DS individuals or the general population."
CRELD1- which is on chromosome 3 - encodes a cell surface protein that is thought to function as a cell adhesion molecule and makes possible a key mechanism of embryonic development. Previous studies in the Maslen lab had shown that CRELD1 mutations are a likely genetic risk factor for defects in the walls separating the heart's chambers. "We hypothesized that CRELD1 mutations may contribute to heart defects on the genetic background of Down syndrome. And when we looked at a population of individuals with DS and AVSD we did, indeed, find mutations. On that basis we concluded that CRELD1 mutations are the first specifically known genetic risk factor for developing an AVSD with other factors such as trisomy 21 (Down syndrome) and environmental factors further weighting the balance in that direction."
To test their hypothesis researchers isolated the CRELD1 gene in 39 individuals with DS and a complete AVSD. Thirty-two of them were part of a larger study of DS and seven were patients of Children's Healthcare of Atlanta's Sibley Heart Center Cardiology. Twenty-three were less than a year old. The rest ranged in age from 1 to 15, with a median age of 4. Blood samples were obtained from each as well as their parents. Demographic information, family and reproductive histories as well as health histories and environmental exposures were gathered from the parents through questionnaires. Medical records were abstracted and coded samples of genomic DNA was provided to the Maslen laboratory for mutation analyses.
Mutations were identified in two unrelated subjects, both infants. In one case the mutation was inherited from the mother who had the normal complement of chromosomes and no evidence of a heart defect. In the other case, the mutation was not detected in either parent both of whom had the normal number of chromosomes, indicating that the defect occurred either just after fertilization or was inherited from one of the parents.
Maslen believes that focusing on the Down syndrome population holds great promise as a way of uncovering other genes that are factors in causing heart defects in the general population.
"Because of the drastically higher incidence of congenital heart disease in the DS population, we think their sensitivity as a result of having three copies of chromosome 21 might help us unmask the presence of other genes that might be factors. Trisomy 21 (DS) takes the place of multiple modifiers so that fewer additional predisposing mutations are required to reach a heart defect in a person with trisomy 21 whereas it might take six or eight different modifier genes coming together to reach that threshold in a normal person.
"We think we might be able to simplify and reduce the cost of studying other genes that contribute to congenital heart defects, which are among the most frequent congenital anomalies among all live births, by looking at the Down syndrome population instead of the general population," said Maslen.
Besides Maslen, the co-authors of the study - titled "CRELD 1 Mutations Contribute to the Occurrence of Cardiac Atrioventricular Septal Defects in Down Syndrome" - were Stephanie Sherman, Ph.D., and Lora J.H. Bean, Ph.D., both of the Department of Human Genetics at Emory University School of Medicine; Kenneth J. Dooley, M.D., of Sibley Heart Center Cardiology, Children's Healthcare of Atlanta; Virginia L. Willour, Ph.D., of the Department of Psychiatry and Behavioral Sciences at Johns Hopkins University; and Darcie Babcock and Susan Robinson, research associates in the OHSU Division of Endocrinology, Department of Medicine.
Continued work on the research project that produced the study is being supported by a grant from the National Heart Lung and Blood Institute at the National Institutes of Health, which will provide OHSU about $700,000.