Allelic Mutations of KITLG, Encoding KIT Ligand, Cause Asymmetric and Unilateral Hearing Loss and Waardenburg Syndrome Type 2

Celia Zazo Seco, Luciana Serrão de Castro, Josephine W. van Nierop, Matías Morín, Shalini Jhangiani, Eva J.J. Verver, Margit Schraders, Nadine Maiwald, Mieke Wesdorp, Hanka Venselaar, Liesbeth Spruijt, Jaap Oostrik, Jeroen Schoots, Baylor-Hopkins Center for Mendelian Genomics, Jeroen van Reeuwijk, Stefan H. Lelieveld, Patrick L.M. Huygen, María Insenser, Ronald J.C. Admiraal, Ronald J.E. Pennings, Lies H. Hoefsloot, Alejandro Arias-Vásquez, Joep de Ligt, Helger G. Yntema, Joop H. Jansen, Donna M. Muzny, Gerwin Huls, Michelle M. van Rossum, James R. Lupski, Miguel Angel Moreno-Pelayo, Henricus P.M. Kunst, Hannie Kremer

Linkage analysis combined with whole-exome sequencing in a large family with congenital and stable non-syndromic unilateral and asymmetric hearing loss (NS-UHL/AHL) revealed a heterozygous truncating mutation, c.286_303delinsT (p.Ser96Ter), in KITLG. This mutation co-segregated with NS-UHL/AHL as a dominant trait with reduced penetrance. By screening a panel of probands with NS-UHL/AHL, we found an additional mutation, c.200_202del (p.His67_Cys68delinsArg). In vitro studies revealed that the p.His67_Cys68delinsArg transmembrane isoform of KITLG is not detectable at the cell membrane, supporting pathogenicity. KITLG encodes a ligand for the KIT receptor. Also, KITLG-KIT signaling and MITF are suggested to mutually interact in melanocyte development. Because mutations in MITF are causative of Waardenburg syndrome type 2 (WS2), we screened KITLG in suspected WS2-affected probands. A heterozygous missense mutation, c.310C>G (p.Leu104Val), that segregated with WS2 was identified in a small family. In vitro studies revealed that the p.Leu104Val transmembrane isoform of KITLG is located at the cell membrane, as is wild-type KITLG. However, in culture media of transfected cells, the p.Leu104Val soluble isoform of KITLG was reduced, and no soluble p.His67_Cys68delinsArg and p.Ser96Ter KITLG could be detected. These data suggest that mutations in KITLG associated with NS-UHL/AHL have a loss-of-function effect. We speculate that the mechanism of the mutation underlying WS2 and leading to membrane incorporation and reduced secretion of KITLG occurs via a dominant-negative or gain-of-function effect. Our study unveils different phenotypes associated with KITLG, previously associated with pigmentation abnormalities, and will thereby improve the genetic counseling given to individuals with KITLG variants.