Three novel mutations in KIF21A highlight the importance of the third coiled-coil stalk domain in the etiology of CFEOM1

Abstract

Background: Congenital fibrosis of the extraocular muscles types 1 and 3 (CFEOM1/CFEOM3) are autosomal dominant strabismus disorders that appear to result from maldevelopment of ocular nuclei and nerves. We previously reported that most individuals with CFEOM1 and rare individuals with CFEOM3 harbor heterozygous mutations in KIF21A. KIF21A encodes a kinesin motor involved in anterograde axonal transport, and the familial and de novo mutations reported to date predictably alter one of only a few KIF21A amino acids--three within the third coiled-coil region of the stalk and one in the distal motor domain, suggesting they result in altered KIF21A function. To further define the spectrum of KIF21A mutations in CFEOM we have now identified all CFEOM probands newly enrolled in our study and determined if they harbor mutations in KIF21A.

Results: Sixteen CFEOM1 and 29 CFEOM3 probands were studied. Three previously unreported de novo KIF21A mutations were identified in three CFEOM1 probands, all located in the same coiled-coil region of the stalk that contains all but one of the previously reported mutations. Eight additional CFEOM1 probands harbored three of the mutations previously reported in KIF21A; seven had one of the two most common mutations, while one harbored the mutation in the distal motor domain. No mutation was detected in 5 CFEOM1 or any CFEOM3 probands.

Conclusion: Analysis of sixteen CFEOM1 probands revealed three novel KIF21A mutations and confirmed three reported mutations, bringing the total number of reported KIF21A mutations in CFEOM1 to 11 mutations among 70 mutation positive probands. All three new mutations alter amino acids in heptad repeats within the third coiled-coil region of the KIF21A stalk, further highlighting the importance of alterations in this domain in the etiology of CFEOM1.

Figure 1

Pedigrees with autosomal dominant inheritance of CFEOM1. Black circles/squares indicate clinically affected individuals, and plus signs denote individuals who participated in this study. (A) Pedigrees harboring KIF21A mutations: pedigrees KR and TG harbor 2860C>T; pedigrees NH and NJ harbor 2861G>A; pedigree PG harbors 1067T>C. (B) Haplotype analysis of pedigree LX at the KIF21A locus. Genotyping data and schematic segregating haplotype bars for chromosome 12cen markers are shown below the symbol for each participant. A black bar indicates the haplotype passed from the affected father to his affected daughter. This haplotype is also inherited by three of the four unaffected siblings who participated in the study. A KIF21A mutation was not detected in this pedigree.

Figure 2

Nucleotide sequence, amino acid positions, and conservation of the three new KIF21A mutations. (A) Heterozygous de novo KIF21A mutations in probands of pedigrees RY, SK and XF. Sequence chromatographs of the unaffected parents are normal (top 2 rows), while sequence chromatographs of the affected offspring with CFEOM1 (bottom row) each reveals a heterozygous KIF21A mutation. The normal sequence and corresponding amino acid residues are indicated under each father's sequence chromatograph (black), while the mutation and resulting amino acid substitution are denoted under each affected proband's sequence (red). (B) Portions of the human KIF21A amino acid sequence that surround each of the reported CFEOM1 mutations aligned with homologous/paralogous sequence. Positions a-g of heptad repeat sequence are denoted. Identical amino acid residues are highlighted in dark gray; similar residues are highlighted in light grey. The residues altered by the three new mutations are boxed in red, and the previously reported mutations are boxed in blue. (C) Predicted KIF21A protein structure. The amino acid residues altered by the 3 new heterozygous mutations are depicted in red with red arrows above the protein pointing to their predicted positions. The 8 previously reported KIF21A mutations are indicated in blue and their locations are indicated by blue arrows above the protein.