Jürgen Kohlhase

We have recently shown that Okihiro syndrome results from mutation in the putative zinc finger transcription factor gene SALL4 on chromosome 20q13.13-13.2. There is considerable overlap of clinical features of Okihiro syndrome with other conditions, most notably Holt-Oram syndrome, a condition in part resulting from mutation of the TBX5 locus, as well as acro-renal-ocular syndrome. We analysed further families/patients with the clinical diagnosis of Holt-Oram syndrome and acro-renal-ocular syndrome for SALL4 mutations. We identified a novel SALL4 mutation in one family where the father was originally thought to have thalidomide embryopathy and had a daughter with a similar phenotype. We also found two novel mutations in two German families originally diagnosed as Holt-Oram syndrome and a further mutation in one out of two families carrying the diagnosis acro-renal-ocular syndrome. Our results show that some cases of “thalidomide embryopathy” might be the result of SALL4 mutations, resulting in an increased risk for similarly affected offspring. Furthermore we confirm the overlap of acro-renal-ocular syndrome with Okihiro syndrome at the molecular level and expand the phenotype of SALL4 mutations.

Okihiro syndrome refers to the association of forearm malformations with Duane syndrome of eye retraction. Based on the reported literature experience, clinical diagnosis of the syndrome can be elusive, owing to the variable presentation in families reported. Specifically, there is overlap of clinical features with other conditions, most notably Holt–Oram syndrome, a condition resulting from mutation of the TBX5 locus and Townes–Brocks syndrome, known to be caused by mutations in the SALL1 gene. Arising from our observation of several malformations in Okihiro syndrome patients which are also described in Townes–Brocks syndrome, we postulated that Okihiro syndrome might result from mutation of another member of the human SALL gene family. We have characterized the human SALL4 gene on chromosome 20q13.13–q13.2. Moreover, we have identified literature reports of forelimb malformations in patients with cytogenetically identifiable abnormalities of this region. We here present evidence in 5 of 8 affected families that mutation at this locus results in the Okihiro syndrome phenotype.

SALL4 is one out of four human homologues of the Drosophila region-specific homeotic gene spalt(sal). Heterozygous mutations of SALL4 on chromosome 20q13.13→ q13.2 cause the autosomal dominant Okihiro syndrome which is characterized by radial limb defects, Duane anomaly and hearing loss. We have partially cloned the murine homologue of this gene, named Sall4, and completed the coding sequence by comparison to available EST and genomic sequences in the GenBank database. This comparison also revealed the chromosomal location of Sall4 on mouse chromosome 2H3 and suggested that a predicted testis expressed gene Tex20 at the very same locus is most likely not a gene on its own but part of the Sall4 3′ UTR. We analyzed the expression of Sall4 during early embryogenesis by whole mount in situ hybridization and in the adult mouse by Northern blotting. In adult tissues, Sall4 expression is only found in testis and ovary. During embryonic development, Sall4 expression is widespread in early embryos and becomes gradually confined to the head region and the primitive streak. Prominent expression in the developing midbrain, branchial arches and the limbs suggests an important function of Sall4 during development of these structures as expected from the observation in Okihiro syndrome patients.

The existence of paternal uniparental disomy of chromosome 16 [upd(16)pat] has previously been suspected but has not been proven. We report prenatal detection and follow‐up of isodisomic upd(16)pat in a child with minimal defects but otherwise normal development. Our results indicate that isodisomic upd(16)pat is associated with a normal outcome if no recessive mutation is reduced to homozygosity. Am. J. Med. Genet. 91:190–191, 2000.

SALL1 and SALL2 have been identified as two human homologs of the region-specific homeotic gene spalt (sal) of Drosophila, which encodes a zinc finger protein of characteristic structure. SALL1 has recently been found to be mutated in patients with Townes-Brocks syndrome (TBS, OMIM No. 107480). Here we report the isolation and mapping of another sal-like human gene, named SALL1P, on chromosome Xp11.2. This intronless gene closely resembles SALL1 but displays several mutations, suggesting that SALL1P represents a sal-related pseudogene. The high similarity of SALL1P to SALL1 is of considerable importance for mutation analysis of SALL1 in TBS.

Townes-Brocks syndrome (IBS, OMIM #107480) is a rare autosomal-dominant malformation syndrome with a combination of anal, renal, limb and ear anomalies1. Cytogenetic findings2 suggested that the gene mutated in IBS maps to chromosome 16q12.1, where SALL1 (previously known as HSAL1), a human homologue of spa/t (sal), is located3. SAL is a developmental regulator in Drosophila melanogasfer4–8 and is conserved throughout evolution3,9–11. No phenotype has yet been attributed to mutations in vertebrate saMike genes. The expression patterns of saMike genes in mouse9, Xenopi/s10 and the fish Medaka11, and the finding that Medaka sal is regulated by Sonic hedgehog (Shh; ref. 11), prompted us to examine SALL1 as a TBS candidate gene. Here we demonstrate that SALL1 mutations cause TBS in a family with vertical transmission of TBS12 and in an unrelated family with a sporadic case of TBS. Both mutations are predicted to result in a prematurely terminated SALL1 protein lacking all putative DMA binding domains. TBS therefore represents another human developmental disorder caused by mutations in a putative C2H2 zinc–finger transcription factor.