Genomic cloning and characterization of the human homeobox gene SIX6 reveals a cluster of SIX genes in chromosome 14 and associates SIX6 hemizygosity with bilateral anophthalmia and pituitary anomalies.

The Drosophila gene sine oculis (so), a nuclear homeoprotein that is required for eye development, has several homologues in vertebrates (the SIX gene family). Among them, SIX3 is considered to be the functional orthologue of so because it is strongly expressed in the developing eye. However, embryonic SIX3 expression is not limited to the eye field, and SIX3 has been found to be mutated in some patients with holoprosencephaly type 2 (HPE2), suggesting that SIX3 has wide implications in head development. We report here the cloning and characterization of SIX6, a novel human SIX gene that is the homologue of the chick Six6(Optx2) gene. SIX6 is closely related to SIX3 and is expressed in the developing and adult human retina. Data from chick and mouse suggest that the human SIX6 gene is also expressed in the hypothalamic and the pituitary regions. SIX6 spans 2567 bp of genomic DNA and is split in two exons that are transcribed into a 1393-nucleotide-long mRNA. Chromosomal mapping of SIX6 revealed that it is closely linked to SIX1 and SIX4 in human chromosome 14q22.3-q23, which provides clues about the origin and evolution of the vertebrate SIX family. Recently three independent reports have associated interstitial deletions at 14q22.3-q23 with bilateral anophthalmia and pituitary anomalies. Genomic analyses of one of these cases demonstrated SIX6 hemizygosity, strongly suggesting that SIX6 haploinsufficiency is responsible for these developmental disorders.

Differential effects of glycosaminoglycans on neurite outgrowth from hippocampal and thalamic neurones.

Chondroitin sulphate proteoglycans are expressed in a temporally restricted pattern from embryonic day 17 to postnatal day 0 in both the thalamus and the cortical subplate, to which thalamic neurones transiently project. To study whether chondroitin sulphate proteoglycans could be specifically involved in the modulation of thalamic axon outgrowth, we compared neurite outgrowth from cultured rat embryonic hippocampal and thalamic neurones, in the presence of chondroitin sulphate type C (isolated from shark cartilage) and chondroitin sulphate type B (dermatan sulphate; isolated from bovine mucosa). When added to the culture medium, both types of glycosaminoglycan lowered the adhesion to laminin and polylysine of both hippocampal and thalamic neurones. However, only chondroitin sulphate specifically modified the pattern of thalamic but not hippocampal neurone outgrowth, promoting axon growth. The morphological changes induced by chondroitin sulphate were concentration dependent and correlated with the selective binding of chondroitin sulphate to the neuronal plasma membrane and its subsequent internalisation. Chondroitin sulphate loosely bound to the surface of hippocampal neurones, but was not internalised. These results indicate that proteoglycans, and in particular the glycosaminoglycan component of these molecules, can differentially modulate neurite outgrowth, depending on their biochemical composition and on the type of neurones they bind to; this would be a possible mechanism of controlling axon guidance in vivo.