Identification of small RNAs interacting with LARP7

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 primordial 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. Identification of small RNAs interacting with endogenous LARP7 by immunoprecipitations and comparison of the enrichment over an input sample.