Expanding the clinical spectrum of COL1A1 mutations in different forms of glaucoma

Abstract

Background: Primary congenital glaucoma (PCG) and early onset glaucomas are one of the major causes of children and young adult blindness worldwide. Both autosomal recessive and dominant inheritance have been described with involvement of several genes including CYP1B1, FOXC1, PITX2, MYOC and PAX6. However, mutations in these genes explain only a small fraction of cases suggesting the presence of further candidate genes.

Methods: To elucidate further genetic causes of these conditions whole exome sequencing (WES) was performed in an Italian patient, diagnosed with PCG and retinal detachment, and his unaffected parents. Sanger sequencing of the complete coding region of COL1A1 was performed in a total of 26 further patients diagnosed with PCG or early onset glaucoma. Exclusion of pathogenic variations in known glaucoma genes as CYP1B1, MYOC, FOXC1, PITX2 and PAX6 was additionally done per Sanger sequencing and Multiple Ligation-dependent Probe Amplification (MLPA) analysis.

Results: In the patient diagnosed with PCG and retinal detachment, analysis of WES data identified compound heterozygous variants in COL1A1 (p.Met264Leu; p.Ala1083Thr). Targeted COL1A1 screening of 26 additional patients detected three further heterozygous variants (p.Arg253*, p.Gly767Ser and p.Gly154Val) in three distinct subjects: two of them diagnosed with early onset glaucoma and mild form of osteogenesis imperfecta (OI), one patient with a diagnosis of PCG at age 4 years. All five variants affected evolutionary, highly conserved amino acids indicating important functional restrictions. Molecular modeling predicted that the heterozygous variants are dominant in effect and affect protein stability and thus the amount of available protein, while the compound heterozygous variants act as recessive alleles and impair binding affinity to two main COL1A1 binding proteins: Hsp47 and fibronectin.

Conclusions: Dominant inherited mutations in COL1A1 are known causes of connective tissues disorders such as OI. These disorders are also associated with different ocular abnormalities, although recognition of the common pathology for both features is seldom being recognized. Our results expand the role of COL1A1 mutations in different forms of early-onset glaucoma with and without signs of OI. Thus, we suggest including COL1A1 mutation screening in the genetic work-up of glaucoma cases and detailed ophthalmic examinations with fundus analysis in patients with OI.

Keywords: COL1A1; Congenital glaucoma; Early onset glaucoma; Osteogenesis imperfecta; Whole exome sequencing.

Fig. 1

COL1A1 mutation identified in first patient. a Pedigree, genotypes and DNA sequence chromatograms of affected child (black colored symbol), his healthy parents and brother are reported. b Eyes of the patient at age 15: visible are Buphthalmus mainly present in the right eye and corneal edema

Fig. 2

Ophthalmic findings in patient CA-3: a–d Humphrey visual fields of the third patient demonstrated early nasal field defects in both eyes (a/c left eye and b/d right eye respectively). e–f Opthic nerve photographs of patient CA-3 shown glaucomatous optic nerve cupping in both eyes (e-left eye and f-right eye)

Fig. 3

Multiple sequence alignments of human COL1A1 region to orthologous. The alignment encompasses the affected amino acid residues (in Bold)

Fig. 4

Structural effect of detected mutations in COL1A1. a Gly154 is located in a regular triple-helical collagen segment. The two glycines belonging to the COL1A1-chains are shown in space-filled presentation and are labelled. b In the Gly154Val variant, the larger valine sidechain cannot be accommodated at this sequence positions resulting in several severe clashes with the adjacent amino acids (indicated by black arrows). c Gly767 is located in a triple-helical collagen segment. The two glycines belonging to the COL1A1-chains are shown in space-filled presentation and are labelled. d In the Gly767Ser variant, the larger serine sidechain cannot be accommodated at this sequence positions resulting in several severe clashes with the adjacent amino acids (indicated by black arrows). e Met264 belongs to a stretch of COL1A1 that is recognized by fibronectin. Met264 forms tight sidechain packing interactions with Trp553 of fibronectin. Both residues are shown in space-filled presentation; the remaining residues of COL1A1 are shown in stick presentation (atom-type coloring) and fibronectin is shown as blue ribbon. f In the Met264Leu variant, the Cγ-branched leucine sidechain forms steric clashes (black arrow) with Trp553, which are expected to decrease binding affinity. g Ala1083 is located immediately adjacent to Arg1084, which confers the consensus of a high-affinity recognition site for the chaperone Hsp47 that is essential for the proper assembly of the triple-helical procollagen molecules. The collagen triple-helix is shown in red, green, and blue; the two bound Hsp47 molecules are depicted in blue and cyan. Arg1084 of collagen and Asp385 of Hsp47 form a salt-bridge that is crucial for high-affinity binding. The residues are shown in stick and their interaction is highlighted by an orange circle. Ala1083 of collagen forms a weak sidechain interaction with Leu381 of Hsp47 (residues in space-filled presentation). h In the Ala1083Thr variant, the bulkier threonine sidechain causes steric clashes with Leu381 (black arrow). Further, Ala1083 sidechain hydroxyl group is positioned close to Arg1084 (orange arrow), which might interfere with the Arg1084-Asp385 hydrogen bond. Both effects are expected to decrease the collagen-Hsp47 interaction