LAUSR

Karyotype has been a standard test for many decades in the context of multiple fetal or neonatal anomalies. This technique, however, has limited resolution, and deletions or duplications smaller than 5–7 Mb are not usually detectable. Array comparative genomic hybridization (array CGH) provides much higher resolution and has replaced karyotype as firstline test in the investigation of children with developmental delay and congenital malformations. It is now increasingly employed, where resources are available, in the prenatal and in the neonatal setting as well. We have recently encountered two instances where an apparently balanced chromosomal translocation on karyotype provided the initial clue to the underlying ophthalmic diagnosis. Patient 1 was a 20-month-old boy. The antenatal scans were normal, but at birth, bilateral anophthalmia and glandular hypospadias were noted. An MRI brain scan showed hypogenesis of corpus callosum and confirmed the clinical suspicion of anophthalmia (Figure 1a and b). The postnatal karyotype was 46,XY,t(3;14)(q27;q22) and reported as a possibly balanced reciprocal translocation (Figure 2a). Parental karyotype was normal. Due to the clinical suspicion of the 3q breakpoint being close to a known anophthalmia gene, array CGH was undertaken. This showed loss of a 2 Mb region at 3q26.33 encompassing 5 protein coding genes including SOX2 (Figure 3). Patient 2 was a female fetus in a pregnancy terminated at 23 weeks. The antenatal scan at 20 weeks showed bilateral cleft lip and palate, enlarged cerebral ventricles, and growth restriction. Amniocentesis showed an apparently balanced translocation 46,XX,t(3;12)(q27;q13.1) (Figure 2b). The pregnancy was terminated, and on postmortem, in addition to the scan findings, bilateral microphthalmia and 13 pairs of ribs were noted. Stored DNA was unavailable, so array CGH could not be performed, but in view of the chromosome 3 breakpoint being close to the genomic location of SOX2, fluorescent in situ hybridization analysis of fixed material was requested which was consistent with the deletion of SOX2 (Figure 4). Parental karyotype was normal, indicating de novo origin of the fetal translocation. Heterozygous loss-of-function mutations in SOX2 are the commonest cause of severe microphthalmia and anophthalmia. Eye involvement is usually bilateral and severe. Other features of SOX2 mutations include variable degree of learning disability, brain malformations including mesial temporal lobe malformations, corpus callosum dysgenesis and pituitary hypoplasia, esophageal atresia, male genital tract abnormalities, and numerical rib anomalies. Our experience is notable because in both cases SOX2 deletion occurred because of a de novo unbalanced translocation. There are only two previous instances of translocations involving 3q26.3 or 3q27 in patients with microphthalmia or anophthalmia. In a further case, there was a de novo 2.7 Mb deletion involving SOX2 and a balanced de novo translocation with breakpoints at 3q28 and 7p21.3. The deletion and translocation breakpoints on 3q were at least 8.6Mb apart in this patient. Although array CGH use is now increasing, karyotype may be the only test performed in some healthcare settings. If a de novo “balanced” translocation is observed in a fetus or newborn, it is worth checking whether heterozygous loss of function of genes located close to the breakpoints may be responsible for the phenotype(s) observed. Confirmation of the clinically suspected diagnosis and the de novo origin of the translocations meant that the parents in both of our cases could be reassured that the risk of recurrence in a subsequent pregnancy was negligible. We have also added two further cases to the existing literature of a unique mechanism (SOX2 deletion due to unbalanced translocation), resulting in microphthalmia or anophthalmia.