Haploinsufficiency of KPNA7 causes otosclerosis, likely due to the release of import inhibition of PTHrP and the reactivation of chondrogenesis in the globuli interossei

Otosclerosis is a genetic bone disorder restricted to the otic capsule and a common cause of conductive hearing loss with both familial and sporadic cases. To date, 14 genomic loci ( OTSC ) and four underlying OTSC genes ( MEPE , SERPINF1, FOXL1, SMARCA4) have been identified in autosomal dominant families. A combined genetic/genomics approach on five affected siblings of Northern European ancestry from the island of Newfoundland, Canada identified a premature stop mutation in Karyopherin subunit α7 ( KPNA7 , c.49C>T, p.R17X). KPNA7 maps to OTSC2 (7q22.1) and encodes the newest of the seven-member importin-α family of nuclear transporters and plays a critical role in early embryonic cleavage events and zygotic genome activation. Previous studies reveal that recessive KPNA7 variants cause skeletal abnormalities, including scoliosis and ocular hypertelorism in two sisters with Partial Corpus Callosum Agenesis-Cerebellar Vermis Hypoplasia With Posterior Fosa Cysts Syndrome and more recently, have been implicated in preimplantation embryo arrest (PREMBA) (OMIM 614107). Interestingly, KPNA7 is also a maternal factor with an exclusively embryonic role and likely inhibits non-classical NLS transport of PTHrP, a known activator of chondrogenesis. We propose that KPNA7 haploinsufficiency causes a failure in nuclear transport inhibition of PTHrP in the quiescent embryonic cells of the globuli interossei in the otic capsule and re-activates chondrogenesis. The KPNA7 discovery provides new insights into the pathogenesis of otosclerosis and potential for targeted therapies.