Horizontal Gaze Palsy With Progressive Scoliosis: Two Novel ROBO3 Mutations in a Compound Heterozygous Sporadic Case
A 34-year-old woman was born at full term from nonconsanguineous parents with normal psychological development and no family history of scoliosis or gaze palsy. At the age of 6 months, her parents and physicians noticed a lack of full ocular motility. At the age 9 years, physical examination demonstrated dorsal lumbar scoliosis and the absence of the horizontal eye movements without nystagmus. Brain MRI showed typical signs of HGPPS. Sequencing of all 28 coding exons of the ROBO3 gene revealed 2 polymorphisms in the exon 8 (rs4935898) and in the intron 22 (rs145185934 deletion), and 2 novel mutations: a heterozygous missense mutation and a splice-site mutation. The missense mutation was a c.1433 C>T transition in the exon 9. To our knowledge, this novel mutation (p.Pro478Leu) has not been reported and it is not present in 1000 genomes database nor in the Exome Variant Server. Furthermore, evidence support the pathogenicity of the mutation found: 1) clinical features were consistent with the HGPPS phenotype; 2) besides the P478L change, the sequence of the entire gene was normal; 3) the proline-478 is well conserved among ROBO3s as predicted by Clustal W2 software (EMBL-EBI, Hinxton, UK) 4) bioinformatic programs predicted that this variant likely has pathological functional effect (PolyPhen [Harvard, Cambridge, MA] with a score of 0.995; Sorting Intolerant From Tolerant software [J. Craig Venter Institute, Rockville, MD] P = 0.02, intolerant threshold of 0.05; SNAP [Rostlab, Munich, Germany] prediction: nonneutral, expected accuracy: 96%; Panther Classification System [Gene Ontology Consortium, Washington, DC] −5.72042, P = 0.938; SNPs & Go [Bologna Biocomputing Group, Bologna, Italy] effect: disease, Reliability Index: 7).
The splice-site mutation c.3321-G>A occurs in the acceptor splice site of the intron 22 and probably leads to a missplicing. The in silico analysis (NNNSplice09; Berkeley Drosophila Genome Project, Berkeley, CA) revealed that the transition G>A abolishes the acceptor splice site. This change probably generates a loss of splicing in this region that could lead to an aberrant protein.
HGPPS is characterized by a peculiar morphological pattern on MRI, diffusion tensor imaging, and functional MRI (fMRI) that directly or indirectly results in the absence or only partial decussation of some bundles of white matter in the brainstem (2,3). In this disorder, the internuclear nucleus axons of the sixth nerve nucleus do not cross the midline to ascend in the medial longitudinal fasciculi to synapse with the neurons within the third nerve nucleus complex innervating the medial rectus muscles. The failed decussation of the axons of the interneurons is the primary cause of the defective control on horizontal eye movements.
The etiology of progressive scoliosis in patients with HGPPS remains unknown. Neuroimaging and fMRI findings also have shown defective decussation within the brainstem, specifically involving the corticospinal tracts, superior cerebellar peduncles, and transverse fibers in the pons. Although a neurogenic mechanism for scoliosis has been postulated (4), it is unclear whether this is related to mutations of the ROBO3 gene because HGPPS with scoliosis has been described without detectable mutations in that gene (5). This raises epigenetic and/or environmental factors as potentially playing a role in development of scoliosis.