Impact of homozygous loss-of-function mutations in the LARP7 gene on the pA+ transcriptome of a B-lymphoblastoid cell line (B-LCL)

The La-related protein LARP7 has been mainly described as a component of the 7SK small nuclear ribonucleoprotein (snRNP) complex, which negatively regulates RNA polymerase II by sequestering the positive transcription elongation factor b (P-TEFb). In our studies, we discovered a novel, 7SK snRNP-independent function of LARP7. We show that LARP7 interacts with the U6 spliceosomal RNA as well as with the small nucleolar RNAs (snoRNAs) directing the 2'-O-methylations of U6. Importantly, in the absence of LARP7, significantly less 2'-O-methylations are deposited on U6 affecting splicing fidelity. Mutations in the LARP7 gene have been associated with the Alazami syndrome, a form of primary dwarfism characterized by intellectual disability. We describe a novel loss-of-function mutation of LARP7 occurring in Alazami patients and detect reduced 2'-O-methylations of U6 in patient-derived samples. Thus, aberrant posttranscriptional RNA modifications of the spliceosomal U6 snRNA may contribute to the development of this severe disease. Overall design: Analysis of the impact of homozygous loss-of-function mutations in the LARP7 gene on the expression and the alternative splicing of pA+ RNA transcripts. Two replicates each of a B-LCL derived from a boy diagnosed for the Alazami syndrome and carrying an homozygous mutations in the LARP7 gene (cell line AAAS14, established from the donor at the age of 4 years) and a B-LCL derived from the healthy father, heterozygous for the LARP7 mutation (cell line AASD72, established from the donor at the age of 46 years)