LTBP2 null mutations in an autosomal recessive ocular syndrome with megalocornea, spherophakia, and secondary glaucoma

Abstract

The latent TGFbeta-binding proteins (LTBPs) and fibrillins are a superfamily of large, multidomain proteins with structural and TGFbeta-signalling roles in the extracellular matrix. Their importance is underscored by fibrillin-1 mutations responsible for Marfan syndrome, but their respective roles are still incompletely understood. We report here on two families where children from healthy, consanguineous parents, presented with megalocornea and impaired vision associated with small, round, dislocated lenses (microspherophakia and ectopia lentis) and myopia, as well as a high-arched palate, and, in older children, tall stature with an abnormally large arm span over body height ratio, that is, associated features of Marfan syndrome. Glaucoma was not present at birth, but was diagnosed in older children. Whole genome homozygosity mapping followed by candidate gene analysis identified homozygous truncating mutations of LTBP2 gene in patients from both families. Fibroblast mRNA analysis was consistent with nonsense-mediated mRNA decay, with no evidence of mutated exon skipping. We conclude that biallelic null LTBP2 mutations cause the ocular phenotype in both families and could lead to Marfan-like features in older children. We suggest that intraocular pressures should be followed-up in young children with an ocular phenotype consisting of megalocornea, spherophakia and/or lens dislocation, and recommend LTBP2 gene analysis in these patients.

Figure 1

Clinical findings. (a) Proband (VI.4) of the first family, whose height was 184 cm and arm span 204 cm at the age of 14 years. (b) Ultrasound scan of the eye of patient VI.7 showing complete posterior luxation of the lens, as well as increased sphericity and short size of the lens. (c) The eye of the younger affected daughter (VI.8) of family 1 showing megalocornea. (d) Eye of proband (V4) showing inferonasal dislocation of the lens. (e–g) Hands of patients VI.7 and VI.8 from the first family and of proband from second family.

Figure 2

Linkage analysis in family 1. The parents were second cousins. The 14q linkage region was initially identified by a genome-wide 10 K GeneChip microarray of SNPs in three affected siblings, then further analyzed by microsatellite polymorphisms in affected and unaffected siblings and parents, as shown. A single linkage region was found where all patients and no unaffected siblings were homozygous, yielding a maximal multipoint LOD score of 2.87 at D14S1002.

Figure 3

LTBP2 gene sequencing and mRNA analysis. (a) Family 1. Top to bottom, direct sequencing of genomic DNA from father, proband, and an ethnically matched unrelated control, showing c.1796_1797insC (p.Val600GlyfsX2) mutation in exon 9 of LTBP2, heterozygous in father and homozygous in proband. (b) Family 2. Top to bottom, direct sequencing of genomic DNA from father, proband, and an ethnically matched unrelated control, showing c.895C>T (p.Arg299X) mutation in exon 4 of LTBP2, heterozygous in father and homozygous in proband. (c) Analysis of LTBP2 transcript in cultured fibroblasts using primers situated in exons 7 and 12. 1–3: RT-PCR of three aliquots of mRNA extracted from fibroblasts culture of Family 1's proband; 4–5: RT-PCR of two unrelated control fibroblast cultures, showing a band of expected size (625 bp); 6: Negative control with H2O used in place of mRNA template; 7: Lambda DNA/Hind III and PhiX 174 DNA/HaeIII Mix molecular weight marker.