Renaud Legouis

Characterization of the two zebrafish orthologues of the KAL-1 gene underlying X chromosome-linked Kallmann syndrome

The gene underlying X chromosome-linked Kallmann syndrome, KAL-1, has been identified for several years, yet its role in development is still poorly understood. In order to take advantage of the zebrafish as a model in developmental genetics, we isolated the two KAL-1 orthologues, kal1.1 and kal1.2, in this species. Comparison of deduced protein sequences with the human one shows 75.5 and 66.5% overall homology, respectively. The most conserved domains are the whey acidic protein-like domain and the first of four fibronectin-like type III repeats. However, kal1.2 putative protein lacks the basic C-terminal domain (20 residues) found in kal1.1 and KAL-1. The expressions of kal1.1 and kal1.2 were studied in the embryo between 6 and 96 hours post fertilization using whole-mount in situ hybridization. Although a few structures express both genes, kal1.1 and kal1.2 expression patterns are largely non-overlapping. Taken together, these patterns match fairly well those previously reported for human KAL-1 and chicken kal1. As regards the olfactory system, kal1.1 is expressed, from 37 h.p.f. onward, in the presumptive olfactory bulbs, whereas kal1.2 transcript is only detected, from 48 h.p.f., in the epithelium of the nasal cavity. The relevance of the zebrafish as an animal model for studying both the function of KAL-1 in normal development and the developmental failure leading to the olfactory defect in Kallmann syndrome, is discussed.

Early expression of the KAL gene during embryonic development of the chick.

The human KAL gene is responsible for the X chromosome-linked Kallmann syndrome, which consists of the association of hypogonadotropic hypogonadism and anosmia. The human and chicken KAL genes have been isolated. Using in situ hybridization, we studied KAL gene expression during development of the chick. We have previously reported that, from embryonic day 8, the expression is almost restricted to definite neuronal populations in the central nervous system, most of which still express the gene after hatching. Here we report that the KAL gene is also expressed during early embryonic development (days 2–8) in various endodermal, mesodermal, and neurectodermal derivatives. In most endodermal and mesodermal derivatives, the expression is transient and precedes cell differentiation. In contrast, the expression in the nervous system concerns postmitotic central neuroblastic populations, most of which still express the gene after differentiation. In accordance with such a spatio-temporal pattern of expression, we suggest that the KAL gene is involved both in morphogenetic events and in neuronal late differentiation. In addition, the absence of detectable expression of the KAL gene either in the embryonic olfactory epithelium or in the surrounding nasal mesenchyme reinforces the hypothesis that Kallmanns syndrome results from a central olfactory target cell defect.