Mutations in the homeodomain of the human SIX3 gene cause holoprosencephaly

Holoprosencephaly (HPE) is a common, severe malformation of the brain that involves separation of the central nervous system into left and right halve s. Mild HPE can consist of signs such as a single central incisor, hypotelo rism microcephaly, or other craniofacial findings that can be present with or without associated brain malformations(1-3). The aetiology of HPE is ext remely:heterogeneous, with the proposed participation of a minimum of 12 HP E-associated genetic loci as well as the:causal. involvement of specific te ratogens acting: at the earliest stages of neurulation(4). The HPE2 locus w as recently characterized as a 1-Mb interval on human chromosome 2p21 that contained a gene associated with HPE: A minimal critical region was defined by a set of six overlapping deletions and three clustered translocations i n HPE patients(5). We:describe here the isolation and characterization of t he human homeobox-containing SIX3 gene from the HPE2 minimal critical regio n (MCR). We show that at least 2 of the HPE-associated translocation :break points in 2p21 are less than 200 kb from the 5' end of SIX3. Mutational ana lysis has identified four different mutations in the homeodomain of SIX3 th at are predicted to interfere with transcriptional activation and are assoc iated with HPE, We propose that SIX3 is the HPE2 gene, essential for the de velopment of the anterior neural plate and eye in humans.