Duane radial ray syndrome (Okihiro syndrome) maps to 20q13 and results from mutations in SALL4, a new member of the SAL family

Abstract

Duane syndrome is a congenital eye movement disorder characterized most typically by absence of abduction, restricted adduction, and retraction of the globe on attempted adduction. Duane syndrome can be coinherited with radial ray anomalies as an autosomal dominant trait, referred to as "Okihiro syndrome" or "Duane radial ray syndrome" (DRRS). We ascertained three pedigrees with DRRS and mapped their disease gene to a 21.6-cM region of chromosome 20 flanked by markers D20S888 and D20S102. A new member of the SAL family of proposed C(2)H(2) zinc finger transcription factors, SALL4, falls within the region. Mutation analysis of SALL4 in the three pedigrees revealed one nonsense and two frameshift heterozygous mutations. SALL4 represents the first identified Duane syndrome gene and the second malformation syndrome resulting from mutations in SAL genes and likely plays a critical role in abducens motoneuron development.

Figure 1

DRRS pedigrees. A, Top, attempted right and left gaze in an affected member of pedigree V (V:3) with Duane syndrome; bottom, thenar hypoplasia and thumb anomaly in affected members of pedigree V (V:3, left; IV:3, right). B, pedigree V. C, pedigree FN. D, pedigree DA. Affected individuals are indicated by filled symbols. Below the symbol for each participant are genotypes for chromosome 20q13.12-q13.31 markers. The disease-associated haplotype in each family is in black.

Figure 2

SALL4 mutations and predicted SAL protein schematics. Forward (top) and reverse (bottom) strand direct sequence of an affected member of (a) pedigree V, showing the heterozygous 1904delT and of (b) pedigree FN, showing the heterozygous 2425delG. c, Direct sequence of an affected member of pedigree DA (top) and control (bottom) showing the heterozygous 2593C→T mutation. d, Predicted SALL1–4 protein structures and location of SALL4 DRRS mutations. SALL4 contains one NH₂-terminus C₂HC-type zinc finger (ZF; black circle) and 7 C₂H₂-type ZF (oval) domains. The latter are arranged as double ZFs (DZF), with a third finger associated with the second doublet (white oval). The first finger of each pair contains a somewhat variable sequence (yellow, orange, and blue ovals), whereas the second finger of each pair contains the conserved SAL-box sequence, FT/STKGNLK (red ovals). The homologies to SALL1–3 are shown. SALL2 also contains three DZF motifs; the first two are highly similar to the other proteins, but the third is divergent (gray ovals). In contrast, SALL1 and SALL3 each contain three DZF motifs highly similar to SALL4 with an additional fourth DZF motif located between the second and third DZF in SALL4. The positions of the SALL4 splice junctions and the three DRRS mutations are superimposed on the schematic protein. The three mutations are predicted to result in unstable mRNAs or truncated proteins disrupting the fifth and/or lacking the sixth and seventh DZF motifs. Protein structures and similarities were predicted by alignment in MacVector 6.5.3 and by the prediction programs SWISS-PROT and TrEMBL on the ExPASy Molecular Biology Server.