X-linked situs abnormalities result from mutations in ZIC3

Vertebrates position unpaired organs of the chest and abdomen asymmetrically along the left-right (LR) body axis. Each structure comes to lie non-randomly with respect to the midline in an overall position designated sites solitus, exemplified in humans by placement of the heart, stomach and spleen consistently to the left. Aberrant LR axis development can lead to randomization of individual organ position (sites ambiguus) or to mirror-image reversal of all lateralized structures (sites inversus)(1). Previously we mapped a locus for sites abnormalities in humans, HTX1, to Xq26.2 by linkage analysis in a single family (LR1) and by detection of a deletion in an unrelated sites ambiguus male (Family LR2; refs 2,3). From this chromosomal region we have positionally cloned ZIC3, a gene encoding a putative zinc-finger transcription factor. One frameshift, two missense and two nonsense mutations have been identified in familial and sporadic sites ambiguus. The frameshift allele is also associated with sites inverses among some heterozygous females, suggesting that ZIC3 functions in the earliest stages of LR-axis formation. ZIC3, which has not been previously implicated in vertebrate LR-axis development, is the first gene unequivocally associated with human sites abnormalities.