Background Mutations in MAB21L2 result in severe ocular defects including microphthalmia, anophthalmia, coloboma, microcornea, and cataracts. The molecular and cellular underpinnings of these defects are unknown, as is the normal cellular function of MAB21L2. Zebrafish mab21l2 au10 mutants possess ocular defects resembling those in humans with MAB21L2 mutations, providing an excellent model to characterize mab21l2 functions during eye development. Results mab21l2 −/− mutants possessed a host of ocular defects including microphthalmia and colobomas as well as small, disorganized lenses and cornea dysgenesis. Decreased proliferation, increased cell death, and defects in marker gene expression were detected in the lens. Cell death in the optic stalk was elevated in mab21l2 −/− mutants and the basement membrane between the edges of the choroid fissure failed to break down. Neuronal differentiation in the retina was normal, however. mab21l2 −/− mutant corneas were disorganized, possessed an increased number of cells, some of which proliferated ectopically, and failed to differentiate the corneal stroma. Conclusions mab21l2 function is required for morphogenesis and cell survival in the lens and optic cup, and basement membrane breakdown in the choroid fissure. mab21l2 function also regulates proliferation in the lens and cornea; in its absence, the lens is small and mispatterned, and corneal morphogenesis and patterning are also disrupted.

Background Pathogenic variants in human MAB21L2 result in microphthalmia, anophthalmia, and coloboma. The exact molecular function of MAB21L2 is currently unknown. We conducted a series of yeast two‐hybrid (Y2H) experiments to determine protein interactomes of normal human and zebrafish MAB21L2/mab21l2 as well as human disease‐associated variant MAB21L2‐p.(Arg51Gly) using human adult retina and zebrafish embryo libraries. Results These screens identified klhl31, tnpo1, TNPO2/tnpo2, KLC2/klc2, and SPTBN1/sptbn1 as co‐factors of MAB21L2/mab21l2. Several factors, including hspa8 and hspa5, were found to interact with MAB21L2‐p.Arg51Gly but not wild‐type MAB21L2/mab21l2 in Y2H screens. Further analyses via 1‐by‐1 Y2H assays, co‐immunoprecipitation, and mass spectrometry revealed that both normal and variant MAB21L2 interact with HSPA5 and HSPA8. In situ hybridization detected co‐expression of hspa5 and hspa8 with mab21l2 during eye development in zebrafish. Examination of zebrafish mutant hspa8hi138Tg identified reduced hspa8 expression associated with severe ocular developmental defects, including small eye, coloboma, and anterior segment dysgenesis. To investigate the effects of hspa8 deficiency on the mab21l2Arg51_Phe52del allele, corresponding zebrafish double mutants were generated and found to be more severely affected than single mutant lines. Conclusion This study identifies heat shock proteins as interacting partners of MAB21L2/mab21l2 and suggests a role for this interaction in vertebrate eye development.

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