The U1 small nuclear RNA (U1 snRNA) as a component of the major U2-dependent 
spliceosome recognizes 5' splice sites (5'ss) containing GT as the canonical 
dinucleotide in the intronic positions +1 and +2. The c.165+1G>T germline 
mutation in the 5'ss of exon 2 of the Fanconi anemia C (FANCC) gene commonly 
predicted to prevent correct splicing was identified in nine FA patients from 
three pedigrees. RT-PCR analysis of the endogenous FANCC mRNA splicing pattern 
of patient-derived fibroblasts revealed aberrant mRNA processing, but 
surprisingly also correct splicing at the TT dinucleotide, albeit with lower 
efficiency. This consequently resulted in low levels of correctly spliced 
transcript and minute levels of normal posttranslationally processed FANCD2 
protein, indicating that this naturally occurring TT splicing might contribute 
to the milder clinical manifestations of the disease in these patients. 
Functional analysis of this FANCC 5'ss within splicing reporters revealed that 
both the noncanonical TT dinucleotide and the genomic context of FANCC were 
required for the residual correct splicing at this mutant 5'ss. Finally, use of 
lentiviral vectors as a delivery system to introduce expression cassettes for 
TT-adapted U1 snRNAs into primary FANCC patient fibroblasts allowed the 
correction of the DNA-damage-induced G2 cell-cycle arrest in these cells, thus 
representing an alternative transcript-targeting approach for genetic therapy of 
inherited splice-site mutations.