Bi-allelic Loss-of-Function Variants in DNMBP Cause Infantile Cataracts

Abstract

Infantile and childhood-onset cataracts form a heterogeneous group of disorders; among the many genetic causes, numerous pathogenic variants in additional genes associated with autosomal-recessive infantile cataracts remain to be discovered. We identified three consanguineous families affected by bilateral infantile cataracts. Using exome sequencing, we found homozygous loss-of-function variants in DNMBP: nonsense variant c.811C>T (p.Arg271^∗) in large family F385 (nine affected individuals; LOD score = 5.18 at θ = 0), frameshift deletion c.2947_2948del (p.Asp983^∗) in family F372 (two affected individuals), and frameshift variant c.2852_2855del (p.Thr951Metfs^∗41) in family F3 (one affected individual). The phenotypes of all affected individuals include infantile-onset cataracts. RNAi-mediated knockdown of the Drosophila ortholog still life (sif), enriched in lens-secreting cells, affects the development of these cells as well as the localization of E-cadherin, alters the distribution of septate junctions in adjacent cone cells, and leads to a ∼50% reduction in electroretinography amplitudes in young flies. DNMBP regulates the shape of tight junctions, which correspond to the septate junctions in invertebrates, as well as the assembly pattern of E-cadherin in human epithelial cells. E-cadherin has an important role in lens vesicle separation and lens epithelial cell survival in humans. We therefore conclude that DNMBP loss-of-function variants cause infantile-onset cataracts in humans.

Keywords: DNMBP; Drosophila; ERG; bristles; cataract; cornea; eye development; photoreceptors; pigment cells; still life.

Figure 1

Family Pedigrees and Segregation of Variants (A) Pedigree diagram of family F372 shows the segregation of DNMBP c.2947_2948del (p.Asp983^∗). (B) Pedigree diagram of family F385 shows the segregation of DNMBP nonsense mutation c.811C>T (p.Arg271^∗). (C) Pedigree diagram of family F3 shows the proband with the homozygous DNMBP mutation c.2852_2855del (p.Thr951Metfs^∗41). The consanguineous parents indicated that they are distantly related but are uncertain as to exactly how the branches of their families are linked (indicated by the dashed line).

Figure 2

sif^{MI02376-GFSTF.2} Is a Functional GFP Trap (A) sif^{MI02376-GFSTF.2} was inserted in a coding intron tagging most or all sif transcripts through the integration of a SA-GFP-SD artificial exon. (B) Developing larval eye discs stained with antibodies against GFP (green) and Elav, a pan-neuronal nuclear marker (red). Sif::GFP is enriched in a single photoreceptor. Right: larval CNS stained with GFP; Sif::GFP is present in most neuropils of third-instar larvae.

Figure 3

Sif::GFP Is Localized to Subsets of Cells in the Pupal Eye and Adult Eye (A) Schematic representation of the Drosophila pupal eye (45 hr APF). (B) Sif::GFP is present in all photoreceptors but was clearly more abundant in one of them. (C) Sif::GFP is enriched in the shaft of bristle cells in the sub-apical region (SAR) during their development. E-cadherin marks the membranes of pigment cells, cone cells, and bristle cells (red labels in A and C), whereas Cut (white) marks the apical area of cone cells. White arrows point to bristles. (D) Schematic representation of a longitudinal section of an adult ommatidium. (E and F) Sif::GFP is localized in secondary and tertiary pigment cells in the SAR (E) but is also localized in the cytoplasm and nuclei of photoreceptors (F). Elav (red) marks the nuclei of photoreceptors. White arrows point to the area where Sif::GFP and Elav colocalize.

Figure 4

Knockdown (K/D) of sif Causes Morphological and Functional Defects in Developing Eyes (A) Pupal eye stained at 45 hr APF for E-cad (white), Nrx-IV (green), and Cut (red). Yellow arrows highlight abnormal bristle and pigment cells. Red arrows point to the region of adjacent photoreceptors. (B) Adult eyes from GMR>+ (control) and GMR>sif RNAi. Expression of sif RNAi causes rough eyes. (C) Reduced ERG amplitude of GMR>sif RNAi flies and quantification of the amplitude of ERG traces (n = 10; error bars represent ± SEM; ^∗∗∗p < 0.001, t test).