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Two microcephaly-associated novel missense mutations in CASK specifically disrupt the CASK–neurexin interaction


Deletion and truncation mutations in the X-linked gene CASK are associated with severe intellectual disability (ID), microcephaly and pontine and cerebellar hypoplasia in girls (MICPCH). The molecular origin of CASK-linked MICPCH is presumed to be due to disruption of the CASK–Tbr-1 interaction. This hypothesis, however, has not been directly tested. Missense variants in CASK are typically asymptomatic in girls. We report three severely affected girls with heterozygous CASK missense mutations (M519T (2), G659D (1)) who exhibit ID, microcephaly, and hindbrain hypoplasia. The mutation M519T results in the replacement of an evolutionarily invariant methionine located in the PDZ signaling domain known to be critical for the CASK–neurexin interaction. CASKM519T is incapable of binding to neurexin, suggesting a critically important role for the CASK–neurexin interaction. The mutation G659D is in the SH3 (Src homology 3) domain of CASK, replacing a semi-conserved glycine with aspartate. We demonstrate that the CASKG659D mutation affects the CASK protein in two independent ways: (1) it increases the protein’s propensity to aggregate; and (2) it disrupts the interface between CASK’s PDZ (PSD95, Dlg, ZO-1) and SH3 domains, inhibiting the CASK–neurexin interaction despite residing outside of the domain deemed critical for neurexin interaction. Since heterozygosity of other aggregation-inducing mutations (e.g., CASKW919R) does not produce MICPCH, we suggest that the G659D mutation produces microcephaly by disrupting the CASK–neurexin interaction. Our results suggest that disruption of the CASK–neurexin interaction, not the CASK–Tbr-1 interaction, produces microcephaly and cerebellar hypoplasia. These findings underscore the importance of functional validation for variant classification.

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The current study is supported by R01EY024712 from the NIH’s National Eye Institute to KM. We gratefully acknowledge the participation and cooperation of families of children with CASK mutation in our studies. We thank Prof. Thomas Südhof for providing pCMV5-CASK plasmid, Dr. Pelagia Derizioti for providing the TBR1 plasmid and Dr. Michael Fox for critically reading the manuscript.

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Correspondence to Konark Mukherjee.

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LaConte, L.E.W., Chavan, V., Elias, A.F. et al. Two microcephaly-associated novel missense mutations in CASK specifically disrupt the CASK–neurexin interaction. Hum Genet 137, 231–246 (2018). https://doi.org/10.1007/s00439-018-1874-3

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