The brachydactylies: a molecular disease family.

Brachydactyly refers to shortening of the hands and/or feet due to missing, deformed, or shortened bones. It may occur as an isolated trait or as part of a syndrome. According to their pattern of skeletal involvement, the isolated brachydactyly forms have been categorized in the groups A-D including several subgroups. As in many other genetic conditions, there is considerable phenotypic overlap between the groups. The identification of the molecular causes of these conditions has offered insights into their pathogenesis. The generation of animal models has facilitated research on the pathogenic events during digit development that lead to the brachydactyly phenotype. These studies have shown that the BMP pathway plays a pivotal role in the normal development of digits and joints and that the majority of brachydactyly disease genes are directly or indirectly linked to this pathway. Together, these genes function in a regulatory network which is deregulated in the disease state. As a consequence of the close interactions within the network, overlapping phenotypes are generated that are, nevertheless, characterized by specific recognizable patterns. This principle does not only apply for the brachydactylies but is also valid for many other disease entities. Groups of diseases that show a common phenotypic pattern due to the deregulation of a molecular network are suggested to be called molecular disease families.