The Lymphedema Family Study aims to determine the causes and mechanisms for how lymphedema is inherited in families. Our hope is that identification of the genes responsible for primary lymphedema will provide a better understanding of its cause, and aid in the development of new and better methods for diagnosing and treating lymphedema. The Lymphedema Family Study uses genetic methods such as linkage and candidate gene studies. Linkage studies, also called linkage analysis, involve a comparison of genetic material between family members with and without lymphedema. In large families this allows the identification of markers on a chromosome that are associated with a gene responsible for lymphedema. These markers indicate the chromosomal location of that gene. Further research then allows the identification of the causative gene. In addition, candidate genes are selected and screened based on what is already known about their function, location, or other properties that make them a "candidate" for causing hereditary lymphedema. Other genetic techniques may also be used to identify causal genes. So far, seven genes have been identified that cause primary lymphedema:FLT4, FOXC2, HGF, MET, and in very rare cases SOX18 and CCBE1. More recently, our research suggests that an additional gene, GJC2, is also responsible for primary lymphedema in some families.
FLT4, which encodes for the protein vascular endothelial growth factor receptor 3 (VEGFR3), was the first lymphedema gene to be identified. FLT4 is located on chromosome 5 and is known to be involved in the formation of lymphatic vessels during fetal development. FLT4 causes congenital lymphedema (swelling which is present from birth). To date we have identified lymphedema-causing genetic changes (mutations) in at least 12 participating families. Our article entitled "Hereditary Lymphedema: Evidence for Linkage and Genetic Heterogeneity" which was published in the December, 1998 issue of the scientific journal Human Molecular Genetics, was the first publication to identify FLT4 as causative for congenital familial lymphedema, also known as Milroy's Disease. Another article written by our research group, entitled "Missense mutations interfere with VEGFR3 signalling in primary lymphoedema," describes the affect of 4 different mutations in the FLT4 gene. It was published in June, 2000 in the scientific journal Nature Genetics.
FOXC2 was the second lymphedema gene to be identified. It is responsible for causing the lymphedema-distichiasis syndrome (LD). Individuals with LD have lymphedema of pubertal or adult onset as well as distichiasis, which is the presence of extra eyelashes. These eyelashes may simply appear long and thick, or they may become ingrown and irritate the eye, in which case they are often removed. Less frequently, individuals with LD may be born with a heart defect, cleft palate (opening in the roof of the mouth), or ptosis (droopy eyelids). We have identified changes or mutations in the FOXC2 gene in 17 families participating in the Lymphedema Family Study.
Lymphedema-distichiasis was first localized to chromosome 16 in 1999 by Mangion et al. in an article entitled "A gene for lymphedema-distichiasis maps to 16q24.3." However, FOXC2 was not identified as the causative gene until 2001 by Fang at al in an article entitled "Mutations in FOXC2 (MFH-1), a forkhead family transcription factor, are responsible for the hereditary lymphedema-distichiasis syndrome." Our paper on FOXC2, entitled "Truncating mutations in FOXC2 cause multiple lymphedema syndromes," was published in Human Molecular Genetics also in the year 2001.
SOX18, a gene involved in fetal development and located on chromosome 20, was found to cause the rare Hypotrichosis-Lymphedema-Telangiectasia syndrome. Hypotrichosis is sparse or absent hair, on the head, face, or body. Telangiectasias are red dots on the skin caused by small dilated blood vessels close to the skin surface. The article entitled "Mutations in the Transcription Factor Gene SOX18 Underlie Recessive and Dominant Forms of Hypotrichosis-Lymphedema-Telangiectasia" was published in the American Journal of Human Genetics in 2003.
HGF/MET, a gene involved in fetal development and located on chromosome 20, was found to cause the rare Hypotrichosis-Lymphedema-Telangiectasia syndrome. Hypotrichosis is sparse or absent hair, on the head, face, or body. Telangiectasias are red dots on the skin caused by small dilated blood vessels close to the skin surface. The article entitled "Mutations in the Transcription Factor Gene SOX18 Underlie Recessive and Dominant Forms of Hypotrichosis-Lymphedema-Telangiectasia" was published in the American Journal of Human Genetics in 2003.
The human CCBE1 gene is similar to a zebrafish gene that is critical for that animal's lymphatic development. The collagen and calcium-binding EGF-domain-1 (CCBE1) gene on chromosome 18 causes the recessive (and rare) Hennekam syndrome, with features including lymphedema, lymphangiectasia, mental retardation, and unusual facial features. Lymphangiectasia are dilated lymphatic vessels, usually in the intestines, which cause poor fat absorption, diarrhea, and protein loss, requiring a special low fat-high protein diet. Because Hennekam syndrome is recessive, 2 copies of a mutation in CCBE1, one inherited from each parent, is required to cause the disease. The article describing this new finding about CCBE1 was published in Nature Genetics in 2009 and entitled: "Mutations in CCBE1 cause generalized lymph vessel dysplasia in humans."
Gap junction protein 2 (GJC2) on chromosome 1, which encodes for a protein called Connexin 47 (Cx47), was previously believed to be active only in the central nervous system. However, our paper entitled "GJC2 missense mutations cause human lymphedema" was published in the American Journal of Human Genetics in June, 2010. Individuals with a single (dominant) mutation in this gene typically had onset of lymphedema in childhood or adolescence and, as with all the other dominantly inherited lymphedemas, there were some non-penetrant individuals (people with the mutation but no signs of lymphedema), who were typically male. GJC2 represents the first lymphedema gene thought to affect the function of the lymphatic system, rather than its development, which opens the door for new research into drug therapies to treat individuals with a mutation in this gene.
Not all hereditary lymphedema is caused by VEGFR3, FOXC2, or HGF/MET indicating that there is at least one other gene (probably many more) responsible for hereditary lymphedema. This phenomenon, the existence of more than one gene responsible for the same condition, is called genetic heterogeneity. We are continuing to look for the locations of these additional lymphedema genes.
General results regarding the progress of the study will be made available to participants in several ways. Updates will be posted on this website, and articles will be published in the National Lymphedema Network's Newsletter whenever significant new developments have occured. In addition, if you are a participant in this study and a mutation was identified in your family, you will receive a letter with additional information about the discovery, and you will be offered the opportunity to receive your research results if you want them. Your individual research results will only be disclosed to you or, if applicable, your legal guardian, unless we are provided with written permission to release those results to another individual.
In most cases specific genetic information is not yet available for individual families or family members. However, as new genes are identified and proven to be responsible for causing lymphedema in specific families, we will continue to contact the participating family members to inform them of this finding and discuss the impact of this information on them and their families. At that time individual genetic information will be offered to those participants who are interested. However, any genetic information provided by this study will be in the form of research results only and should not be used for medical decision-making without confirmation by a laboratory offering clinical or diagnostic testing. Those individuals who receive research results may elect to have confirmatory testing from a certified diagnostic laboratory. This would involve an additional blood sample and possibly the signing of another consent form. We are currently negotiating with a local laboratory to arrange this testing at a reduced rate for study participants.
We would like to remind you that this is a research study, and therefore, we cannot guarantee that any information will be available to individual families or participants. This study does not provide diagnosis or treatment of lymphedema, and is not designed to result in any direct benefit to the participants. However, it is our hope that it will benefit lymphedema patients in the future. In addition, the participation of families and family members is voluntary and confidential. Advances in understanding the underlying cause of lymphedema are critically dependent on the participation of individuals and families in research such as ours, so for those families participating in our study, we would like to thank you once again. Should you have any questions or concerns about this information or the study in general, please don't hesitate to call Kara Levine at (412) 624-4659 or e-mail to . Please be sure to include the word "LYMPHEDEMA" in the subject line of any e-mail.