Dear Editr

I read with great interest this article on the mapping of an isolated form of X-linked cataract on Xp22. As the authors state it is highly probable that the condition in this family is an allelic form of Nance- Horan syndrome (NHS) or cataract-dental syndrome. Although OMIM database distinguishes NHS (MIM 302350) from two forms of isolated X-linked congenital cataract - X-linked congenital cataract with posterior sutural opacities in heterozygotes (MIM 302200) and X-linked congenital cataract with microcornea or slight microphthalmia (MIM 302300) - there are some arguments suggesting that this distinction may not be justified. This classification was based on old publications, and these two forms of X- linked cataract without dental anomalies were described before NHS was recognized as a distinct entity. Microcornea is part of NHS and has also been reported in some patients within families affected with cataract from the first group (MIM 302200) and posterior lens opacities are usually present in NHS heterozygotes. It is highly probable that NHS was not recognized in some families as the dental anomalies may be easily missed. One of the NHS families included in our linkage analysis (family 2) [1] was initially thought to be affected with an isolated X-linked cataract as the affected males were not markedly dysmorphic and their teeth anomalies were mild. Moreover the family reported by Krill et al. in 1969 [2], and considered as the most convincing example of isolated X-linked congenital cataract, was later recognized as a NHS family [3] and was included in a linkage analysis [4]. Finally, it appears that the family described by Waardenburg et al. in 1961 [5] and registered in OMIM database as an isolated cataract belonging to the second group (MIM 302300), was the same as that reported eighteen years later by van Dorp and Delleman [6] and was ultimately recognized as a NHS family [7]. Considering this marked phenotypic overlap, it is tempting to say that these three groups of X- linked cataracts probably correspond to a single condition with variable expression. The report of a first locus for isolated X-linked cataract on Xp22 in a region overlapped by the NHS genetic interval strongly supports this hypothesis.

References
(1) Toutain A, Ronce N, Dessay B, Robb L, Francannet C, Le Merrer M, Briard ML, Kaplan J, Moraine C. Nance-Horan syndrome: linkage analysis in 4 families refines localisation to Xp22.31-p22.13. Hum Genet 1997;99:256-61.
(2) Krill AE, Woodbury G, Bowman JE. X-chromosomal-linked sutural cataracts. Am J Ophthal 1969;68:867-872.
(3) Lewis RA, Nussbaum RL, Stambolian D. Mapping X-linked ophthalmic diseases. IV. Provisional assignment of the locus for X-linked congenital cataracts and microcornea (the Nance-Horan syndrome) to Xp22.2-Xp22.3. Ophthalmology 1990;97:110-121.
(4) Stambolian D, Lewis RA, Buetow K, Bond A, Nussbaum R. Nance-Horan syndrome: localization within the region Xp21.1-Xp22.3 by linkage analysis. Am J Hum Genet 1990;47:13-19.
(5) Waardenburg PJ, Franceschetti A, Klein D. Genetics and Ophthalmology. Vol. 1. Springfield, III.: Charles C Thomas (pub.) 1961. Pp. 880.
(6) van Dorp DB, Delleman JW. A family with X-chromosomal recessive congenital cataract, microphthalmia, a peculiar form of the ear and dental anomalies. J Pediat Ophthal Strabismus 1979;16:166-171.
(7) Bergen AAB, ten Brink J, Schuurman EJM, Bleeker-Wagemakers EM. Nance- Horan syndrome: linkage analysis in a family from the Netherlands. Genomics 1994;21:238-240.