Recently, two truncating mutations in the PHF8 (plant homeodomain finger protein 8) gene have been found to cause X-linked mental retardation associated with cleft lip/cleft palate (CL/P). One of the truncating mutations was found in the original family with Siderius-Hamel CL/P syndrome where only two of the three affected individuals had mental retardation (MR) with CL/P and one individual had mild MR. The second mutation was present in a family with four affected men, three of whom had MR and CL/P, while the fourth individual had mild MR without clefting. Here, we report a novel nonsense mutation (p.K177X) in a male patient who has MR associated with CL/P. The mutation results in a truncated PHF8 protein lacking the Jumonji-like C terminus domain and five nuclear localization signals. Our finding further supports the hypothesis that the PHF8 protein may play an important role in cognitive function and midline formation.

This is the first missense mutation identified in PQBP1 and the first mutation in the WW domain of the gene. The WW domain has been shown to play an important role in the regulation of transcription by interacting with the PPxY motif found in transcription factors. The p.Y65C mutation may affect the proper functioning of the PQBP1 protein as a transcriptional co-activator.