• Arg276X
• SALL1
• Townes–Brocks syndrome
• congenital heart defects

• TBS, Townes–Brocks syndrome

Townes–Brocks syndrome (TBS, OMIM #104780) is a rare autosomal dominant malformation syndrome characterised by anal, renal, limb, and ear anomalies.¹ TBS is caused by mutations in SALL1, a human putative zinc finger transcription factor gene related to the developmental regulator sal of Drosophila melanogaster.² The SALL1 gene product is a zinc finger protein thought to act as a transcription factor. It contains four highly conserved C2H2 double zinc finger domains that are evenly distributed. A single C2H2 motif is attached to the second domain, and at the amino terminus SALL1 contains a C2HC motif.³ The protein is exclusively found in the nucleus and localises to pericentromeric heterochromatin, acting as a transcriptional repressor.⁴

Twenty three of 24 SALL1 mutations known to date are located in exon 2.[2, 5–11 (and unpublished results)] These are short insertions and short deletions as well as one large intraexonic deletion, and nonsense mutations, one of which (c.826C→T, Arg276X) was found in seven independent families with sporadic TBS.^5–6,12 One mutation within intron 2 creates an aberrant splice site.⁷ All mutations lead to premature stop codons and have been thought to cause the phenotype via haploinsufficiency. However, neither heterozygous nor homozygous Sall1 knockout mice show the TBS phenotype, but homozygotes die from kidney malformations like those commonly seen in TBS,¹³ raising the possibility that SALL1 mutations in humans might have a dominant or dominant negative effect.

In a previous report, Marlin et al⁶ described the nucleotide 826C of the SALL1 cDNA as a mutational hotspot. They noted that the c.826C→T (Arg276X) mutation never occurred in a case of inherited TBS and speculated that this might be explained by infertility in carriers. In our cohort, we detected SALL1 mutations in 24 cases with sporadic TBS. Among these, 11 had the Arg276X mutation; three published⁵ and eight new cases. Combining these findings together with detection of this mutation in three out of four families with sporadic TBS by Marlin et al⁶ and the single case reported by Keegan et al,¹² the Arg276X mutation was found in 15 out of 29 (51.7%) families with sporadic TBS. However, it was not detected in 20 familial cases with TBS and detectable SALL1 mutation^{2,5–8,10,11}(and unpublished results).

One of the eight new patients with the Arg276X mutation was a 19 year old woman who presented in the 7+2 gestational week of her first pregnancy. She was diagnosed with TBS due to the presence of the following anomalies: preaxial polydactyly and triphalangeal thumbs on the right hand, broad, short thumb on the left, dysplastic ears, anteriorly placed anus, severe sensorineural hearing loss, overlapping toes on the right foot. A combined atrial septum defect/ventricular septum defect had been diagnosed previously. Her parents were unaffected. Molecular analysis of the SALL1 gene was performed as described,⁵ and the mutation Arg276X was detected in the patient. She was informed of the 50% risk that her child would also be affected with Townes–Brocks syndrome, bearing in mind the fact that this mutation had never been seen in a familial case. The patient opted for prenatal diagnostics. Amniocentesis was performed at 15+1 gestational weeks. Karyotype was normal female (46,XX) on the 300 band level. DNA was prepared from cultured amniocytes by routine procedures, the 5′ part of exon 2 of the SALL1 gene was PCR amplified from this DNA as described,⁵ and direct sequencing of DNA fragments was performed with primers S2 and TR1.1.⁵ On both strands, the mutation Arg276X was confirmed. DNA was prepared from the father of the fetus, and genotyping of informative polymorphic markers on DNA of mother, father, and fetus excluded that the result reflected contamination of amniocytes by maternal cells.

At 33+2 gestational weeks the patient was submitted to the hospital because of polyhydramnion and cervix insufficiency. Owing to a pathological cardiotocogram, a primary caesarian section was performed at 34+1 gestational weeks and the child transferred to the intensive care unit. APGAR was 6/8/8, weight 1980 g, head circumference 29.9 cm. The following abnormalities were noted: bilateral preaxial polydactyly with triphalangeal thumbs, bilateral dysplastic ears with preauricular tags, imperforate anus with rectovaginal fistula, bilateral split feet with 1–2 syndactyly and aplasia of the third ray (fig 1A). Cardiac evaluation revealed pulmonary atresia and a ventricular septal defect with right aortic arch (fig 1B). Multiple major aortopulmonary collateral arteries were seen, and aplasia of the central pulmonary arteries was noted. The initial hearing test, renal ultrasound, and urea/creatinine measurements were normal.

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Figure 1

Phenotype of the newborn girl carrying the R276X mutation in the second generation. (A) Bilateral syndactyly 1/2 with aplasia of the third ray on both feet. (B) Echocardiography at 29 days of age showing a severe tetralogy of Fallot with pulmonary atresia, ventricular septum defect and overriding aorta. LV, left ventricle; RV, right ventricle; RCA, right coronary artery; LA, left atrium; OA, overriding aorta. Pictures courtesy of the Department of Pediatrics, Würzburg University.