Preimplantation Diagnosis for Sonic Hedgehog Mutation Causing Familial Holoprosencephaly

he sonic hedgehog ( shh ) gene is a human homologue of the drosophila gene encoding inductive signals involved in patterning the early embryo and which is highly functionally conserved in many species. 1 The gene was mapped to chromosome 7 (7q36), the locus for the gene involved in holoprosencephaly ( HPE3 ). 2 SHH mutations may cause the failure of cerebral hemispheres to separate into distinct left and right halves, leading to holoprosencephaly, which is one of the most common developmental anomalies of the forebrain and midface in humans.3 Although the majority of cases of holoprosencephaly are sporadic, familial cases are not rare, with a clear pattern of autosomal dominant inheritance.There is great clinical variability of holoprosencephaly within families, ranging from alobar holoprosencephaly and cyclopia to cleft lip and palate, microcephaly, ocular hypotelorism, and even to a normal phenotype.This variability suggests an interaction between SHH and other genes expressed during craniofacial development and the possible involvement of environmental factors.Because almost one third of carriers of SHH mutations may be clinically unaffected, even in affected families, the prenatal detection of SHH mutations might not justify termination of the pregnancy.Preimplantation genetic diagnosis is thus a more attractive option for couples at risk for having a child with holoprosencephaly.We describe the use of preimplantation genetic diagnosis for SHH mutation in a family with holoprosencephaly.The use of this technique, followed by confirmation of mutation-free status by amniocentesis, resulted in the birth of a healthy girl.A couple who had had two children with clinical signs of holoprosencephaly presented for preimplantation genetic diagnosis (Fig. 1).Their second child, a girl with severe holoprosencephaly and cleft lip and palate, died shortly after birth.The results of chromosomal analysis of peripheral-blood lymphocytes from this child and the parents were normal.However, DNA analysis of samples obtained at autopsy showed that the girl had an SHH nonsense mutation -a change from GAG to TAG -leading to the premature termination of the protein at position 256 of exon 3 (Glu256stop) 3 (Fig. 2A).SHH protein is an intercellular signaling molecule.This precursor is cleaved internally into a highly conserved domain (SHH-N) with signaling activity and a more divergent domain (SHH-C), which in addition to precursor processing, acts as an intramolecular cholesterol transferase crucial for proper patterning activity in animal development.1 Although the effect on SHH function of the nonsense mutation identified in the child is unknown, the resulting protein may not have the expected signaling function in early morphogenesis.1,3 t case report