Person:

Andrews, Caroline

Purpose: Two Chinese families (XT and YT) with congenital fibrosis of the extraocular muscles (CFEOM) were identified. The purpose of this study was to determine if previously described Homo sapiens kinesin family member 21A (KIF21A) mutations were responsible for CFEOM in these two Chinese pedigrees. Methods: Clinical characterization and genetic studies were performed. Microsatellite genotyping for linkage to the CFEOM1 and CFEOM3 loci was performed. The probands were screened for KIF21A mutations by bidirectional direct sequencing. Once a mutation was detected in the proband, all other participating family members and 100 unrelated control normal individuals were screened for the mutation. Results: All affected individuals in family XT shared the common manifestations of CFEOM1. Family YT had two affected individuals, a mother and a daughter. The daughter had CFEOM1, while her mother never had congential ptosis but did have limited extraocular movements status post strabismus surgery. Haplotype analysis revealed that pedigree XT was linked to the 12q CFEOM1 locus and the affected memberes harbored the second most common missense mutation in KIF21A (2,861G>A, R954Q). Family YT harbored the most common missense de novo mutation in KIF21A (2,860C>T, R954W). Both of these mutations have been previously described. Conclusions: The observation of these two KIF21A mutations in a Chinese pedigree underscores the homogeneity of these mutations as a cause of CFEOM1 and CFEOM3 across ethnic divisions.

Purpose: To describe phenotypic characteristics of two pedigrees manifesting early onset crystalline cataract with mutations in the γD-crystallin gene (CRYGD). Methods: A detailed medical history was obtained from two Caucasian pedigrees manifesting autosomal dominant congenital cataracts. Genomic DNA was extracted from saliva (DNA Genotek). Single Nucleotide Polymorphism (SNP) based genome analysis of the larger pedigree revealed linkage to an 8.2 MB region on chromosome 2q33-q35 which encompassed the crystallin-gamma gene cluster (CRYG). Exons and flanking introns of CRYGA, CRYGB, CRYGC and CRYGD were amplified and sequenced to identify disease-causing mutations. Results: A morphologically unique cataract with extensive refractile “crystals” scattered throughout the nucleus and perinuclear cortex was found in the probands from both pedigrees. A heterozygous C→A mutation was identified at position 109 of the coding sequence (R36S of the processed protein) in exon 2 of CRYGD and this missense mutation was found to cosegregate with the disease in the larger family; this mutation was then identified in affected individuals of pedigree 2 as well. Conclusions: The heterozygous 109C→A CRYGD missense mutation is associated with a distinct crystalline cataract in two US Caucasian pedigrees. This confirms crystalline cataract formation with this mutation, as previously reported in sporadic childhood case from the Czech Republic and in members of a Chinese family.

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.

Key Clinical Message A patient with syndromic Duane retraction syndrome harbors a chromosome 811.1q13.2 inversion and 8p11.1-q12.3 marker chromosome containing subregions with differing mosaicism and allele frequencies. This case highlights the potential requirement for multiple genetic methods to gain insight into genotype–phenotype correlation, and ultimately into molecular mechanisms that underlie human disease.