Treacher Collins syndrome 3 (TCS3)-associated POLR1C mutants are localized in the lysosome and inhibits chondrogenic differentiation

Treacher Collins syndrome (TCS) is a craniofacial developmental disorder whose key feature is a combination of symptoms. For example, a patient could have bilateral downward slanting of the palpebral fissures, colobomas of the lower eyelids, hypoplasia of the facial bones, cleft palate, malformation of the external ears, and atresia of the external auditory canals. TCS3 is caused by mutations of the polr1c gene, which encodes RNA polymerase I and III subunit C (POLR1C). There have been two known missense mutations (Arg279-to-Gln [R279Q] and Arg279-to-Trp [R279W]) at the Arg-279 position. However, it remains to be clarified whether or how both or each individual mutation affects the cellular properties of POLR1C. Here we show that TCS3-associated missense mutations cause aberrant intracellular localization of POLR1C, inhibiting chondrogenic differentiation. The wild type POLR1C is normally localized in the nuclei. The R279Q or R279W mutant is primarily found to be localized in the lysosome. Expression of the R279Q or R279W mutant in mouse chondrogenic ATDC5 cells decreases phosphorylation of 4E-BP1 and ribosomal S6 proteins, which belong to the mammalian target of rapamycin (mTOR) signaling involved in critical roles in the lysosome. Furthermore, expression of the R279Q or R279W mutant inhibits chondrogenic differentiation in ATDC5 cells. Taken together, TCS3-associated mutation leads to the localization of POLR1C into the lysosome and inhibits chondrogenic differentiation, possibly explaining a portion of the pathological molecular basis underlying Treacher Collins syndrome.