In this study, we report the clinical features of a 7 year old male with both CFEOM1 and congenital aniridia. He inherited the c.2860C
T (p.Arg954Trp) mutation in KIF21A from his mother, who also had CFEOM1, and the c.745delC (p.Leu249TyrfsX22) mutation in PAX6 from his father who had congenital aniridia.
The KIF21A gene comprises 38 exons and encodes a 1674 amino acid protein. Structurally, the KIF21A protein includes an N-terminal motor domain, a coiled-coil domain, and a C-terminal WD-40 repeat domain [4
]. Functionally, the KIF21A protein is responsible for the transport of membranous organelles, protein complexes, and mRNAs to specific destinations within the cell in a microtubule-and ATP-dependent manner. These are essential for normal morphogenesis and functioning of the cell [5
]. To date, 13 mutations in the KIF21A gene are reported to be detected in the affected individuals with CFEOM, of which 12 are missense mutations [4
T (p.Arg954Trp) missense mutation in exon21 is the most common in the KIF21A gene and accounts for 84% of KIF21A mutations seen in CFEOM [4
]. It has been detected in different ethnic populations, including European, African, Arabian, and Asian [4
]. R954 is located in the coiled-coil domain which comprises a repeated heptad consensus sequences, and is regarded as an important site for KIF21A function including modulating the assembly and stability properties of the protein structure and interaction. Three mutations at Arg954 (p.Arg954Trp, Arg954Gln and Arg954Leu) [4
] were identified in the patients with CFEOM. The recurrent mutation of p.Arg954Trp in our study further supports the idea that R954 is a mutation “hotspot” in the KIF21A.
PAX6 is a member of the paired box gene family and encodes a transcriptional regulator that recognizes target genes through its highly conserved paired-box and homeobox domains [10
]. The human PAX6 gene consists of 14 exons and may produce at least three transcript isoforms by alternative splicing. The p.Leu249TyrfsX22 mutation in this study is located in the homeodomain and caused by a 1-bp deletion which results in a frameshift mutation and would be predicted to produce a truncated protein with the initial 248 amino acids similar to the wild type PAX6 protein followed by an additional 22 aberrant amino acids and then a premature termination codon (PTC). In mammalian cells, mRNAs containing a PTC due to a nonsense mutation or a frameshift mutation in an internal exon are usually recognized and degraded by nonsense-mediated mRNA decay (NMD), which is a preventive mechanism for eliminating the production of potentially deleterious truncated proteins interfering with normal cellular processes [11
]. As a general rule, mammalian transcripts that contain a PTC more than 50–55 nucleotides (nt) upstream of the last exon-exon junction will be subjected to NMD, while PTCs located within 50–55 nt or downstream of the last exon-exon junction are not recognized by NMD and a truncated protein may be formed. The p.Leu249TyrfsX22 mutation occurred in exon11 of the PAX6 gene and produced a PTC 369 nucleotides distant from last exon-exon junction, and would be predicted to be recognized by the NMD mechanism.
Functionally, the PAX6 protein is critical for the development of various tissues and organs, particularly the eye and the nervous system. Mutations in the PAX6 gene may cause aniridia, coloboma, hereditary keratitis (KERH, OMIM:148190), bilateral optic nerve hypoplasia [12
] (BONH), interhemispheric brain malformations [3
], Peters syndrome (PAN, OMIM 604229), Gillespie syndrome (OMIM:206700), and probably myopia [13
]. Mutations in the PAX6 have not yet been reported to be associated with congenital fibrosis of extraocular muscle, but may be associated with congenital ptosis [14
The boy in this study has the clinical features of CFEOM1 and congenital aniridia. By DNA sequence analysis, we found that he inherited the p.Arg954Trp mutation in the KIF21A from his mother and the p.Leu249TyrfsX22 mutation from his father. Although PAX6 mutations may cause the phenotype of congenital ptosis, comparing the feature of ptosis with his mother, we didn’t find his bilateral ptosis was more serious than his mother’s, which suggested that his bilateral ptosis with ophthalmoplegia was caused by the KIF21A mutation. As an offspring in the family with both CFEOM1 and aniridia, the boy has 50% risk to be an affected individual with either CFEOM1 or aniridia, and would be 25% risk of developing both CFEOM1 and aniridia. The probability of developing both CFEOM1 and aniridia for their additional child would be 25%, should his parents produce an additional child. However, since his aniridia is a consequence of an internal mutation within the PAX6 gene rather than a contiguous gene deletion including WT1, neither he nor his offspring will be at increased risk for the WAGR syndrome.