The Conradi-Hünermann-Happle syndrome is caused by mutations in the gene that encodes a 8- 7 sterol isomerase and is biochemically related to the CHILD syndrome.

We here review the clinical and genetic features of the Conradi-Hünermann-Happle syndrome. The disease is characterized by chondrodysplasia punctata, linear ichthyosis, cataract, and short stature. The X-linked dominant mode of inheritance was first recognized by Rudolf Happle in the years 1977 to 1981, who also fully delineated the clinical spectrum of this clinico-genetic entity. In the past, linkage studies had firmly excluded the gene for this syndrome from the Xq28 region, but unfortunately had also failed to clearly map the gene elsewhere on the X-chromosome. Very recently, causative mutations were identified in a large number of patients in the gene for emopamil binding protein. This gene is located on the short arm of Xp11.22-23 and also acts as a D8-D7 sterol isomerase. This enzymatic function plays a crucial role in cholesterol biosynthesis. It is of note that very recent investigations by the Marburg group have disclosed that the CHILD syndrome is likewise caused by a similar metabolic defect, namely a deficiency of a 3b-hydroxysteroid dehydrogenase (NSDHL). In the pathway of cholesterol biosynthesis this enzyme functions "upstream" of D8-D7 sterol isomerase and was shown to underlie the mouse mutant bare patches. Molecular studies in these syndromes now allow us to determine which family members carry the mutation and have already provided evidence in the Conradi-Hünermann-Happle syndrome for both gonadal and somatic mosaicism. As gonadal mosaicism seems to be frequent in this disease, a recurrence risk for further pregnancies has to be considered when dealing with a seemingly sporadic case.