The goal of our research investigations is to identify the underlying molecular cause of CdLS. Many people drive this process. It involves the tireless work of Dr. Laird Jackson, the staff at my research laboratory here at The Children's Hospital of Philadelphia and at the research lab of Dr. Tom Strachan at The University of Newcastle upon Tyne in England, as well as the many clinicians who refer their patients to us. Most importantly, it involves all of the children and adults with CdLS, and their families who have generously contributed samples.
Many of you and your children have donated samples to help forward the research process. These samples form the foundation for the work involved in identifying the underlying gene difference that results in CdLS. This brief update will outline the process involved when a family donates a blood or skin sample to research and will highlight the projects currently underway and the progress being made.
When a sample is donated under informed consent, usually from the child/adult with CdLS as well as from their parents, it is sent to our research laboratory and is prepared in a number of ways. From one tube we are able to isolate DNA directly. DNA is the basic building block that makes up all of the genes in our bodies. By having a DNA sample on the individual with CdLS and on their parents we are able to search for, using molecular techniques, changes in genes that we think may be candidates for causing CdLS. By having a sample from the parents we are able to see if any changes found in the individual with CdLS is also present in the parents and if it is what we would term a disease-causing change.
Having these DNA samples also helps us to ask the question: Is there a particular region of DNA that is always present in children with CdLS and not in unaffected family members?. These type of studies are termed linkage analysis, as we are looking for regions of the genome (all of the DNA in an individual) that are "linked" with CdLS. Families that have more than one affected child with CdLS are the most useful for these types of studies, simply because the more affected and unaffected family members there are the more sure we can be of finding linked regions.
In addition to making DNA, a second tube of blood is usually drawn to establish a cell line. A cell line is created by taking the white blood cells out of the total blood sample then preparing them in a way that allows them to continually divide and grow. This allows us to have a renewable source of cells for future studies (i.e. to make more DNA), to look at the chromosomes (those structures that are visible under the microscope that are made up of DNA, and contain all the genes), and to have a source of cells to do specific functional studies once the CdLS gene has been found.
So where have we come in the last few years? We have been able to establish a collection of samples on over 100 individuals with CdLS and their family members. As mentioned above, these samples form the foundation of our research efforts, and no progress could be made towards identifying the molecular cause of CdLS without them.
We are in the process of performing high-resolution chromosome analysis on all individuals with CdLS, and have done so with over 30 samples to date. The reason for doing these analyses is to try to identify chromosomal abnormalities that may indicate regions where a candidate CdLS gene may be located. To date we have not identified any chromosomal abnormalities amongst our samples. There have been a handful of chromosomal abnormalities identified in the past by other physicians. Our collaborators in England, headed by Dr. Tom Strachan are investigating several of these. They are looking for candidate genes at the sites of these rearrangements. Any chromosomal rearrangements identified in our lab will be shared with Dr. Strachan's group.
We have also collected skin samples on a number of CdLS individuals to look at the chromosomes in the skin cells called fibroblasts. Some genetic disorders result from chromosomal differences that are not seen in the blood but are seen in other tissue samples such as the skin. We have not found any chromosomal abnormalities in the samples that we have examined.
Our labs are also in the process of looking at the chromosomes with different molecular techniques to specifically examine regions of the chromosomes at higher resolution than you can under the microscope. One such region is the long arm of chromosome 3 (called ‘3q'). Chromosome 3q has been implicated as a potential site for a CdLS gene for two reasons. One is that individuals with a duplication of this region (an extra copy) have a similar appearance to individuals with CdLS. There are some similarities between these two groups but there are also several differences. Geneticists have argued back and forth as to whether or not CdLS is related to individuals with duplication of 3q or not. In addition to this, one of the chromosomal abnormalities being studied in Dr. Strachan's lab involves the long arm of chromosome 3 in a child with typical features of CdLS. This added support to the idea that a gene may be localized there.
By studying our rare familial cases of CdLS (where there is more than one individual in the family with CdLS) that Dr. Laird Jackson has collected over the years, we have looked to see if the region on chromosome 3 is associated with CdLS in all the families. These linkage studies (as described above) indicate that this region is not involved in all families. This may mean that this region is not involved with CdLS or that it may be involved in some families and not in others indicating the possibility that more than one gene may result in CdLS when altered (termed heterogeneity). These results are being submitted for scientific publication. They suggest that we should not focus all of our resources on chromosome 3 in the search for the CdLS gene. Our laboratory is presently undertaking a genome-wide linkage analysis on the familial cases of CdLS in attempt to identify other regions that may contain the causative gene.
In addition to all of these studies we are involved in looking at specific candidate genes directly. As the scientific community heads toward sequencing the entire human genome (all of the genes in a human being), new genes are being identified every day. If any of these new genes map to a candidate region for CdLS, or if we know that disruption of these novel genes in lower organisms (such as mice), cause differences similar to what we see in CdLS (i.e. limb differences, growth failure, etc.) we will look for changes in these genes in our samples. We are currently examining three such candidate genes.
The identification of the CdLS gene is our ultimate and attainable goal. The foundation has been laid and the path is clear. What we don't know is if the identification of the CdLS gene will take years or months. We are committed to moving the process ahead at full speed and relating the status of our research back to the families as quickly as possible.
The identification of this gene, or genes, will allow for a specific diagnostic test. It will enable the development of a diagnostic test to confirm diagnosis, as well as assisting in counseling of families with regards to recurrence risk. Through the identification of the CdLS gene(s) we will be able to study the role that it plays in human development and how it acts on a cellular level. Our true ultimate goal would be to develop better therapeutic interventions for the children and adults with CdLS, through the identification of the CdLS gene, and by understanding its role in human development.