Minigene-based splicing analysis and ACMG/AMP-based tentative classification of 56 ATM variants

The ataxia telangiectasia-mutated (ATM) protein is a major coordinator of the DNA damage response pathway. ATM loss-of-function variants are associated with 2-fold increased breast cancer risk. We aimed at identifying and classifying spliceogenic ATM variants detected in subjects of the large-scale sequencing project BRIDGES. A total of 381 variants at the intron-exon boundaries were identified, 128 of which were predicted to be spliceogenic. After further filtering, we ended up selecting 56 variants for splicing analysis. Four functional minigenes (mgATM) spanning exons 4-9, 11-17, 25-29, and 49-52 were constructed in the splicing plasmid pSAD. Selected variants were genetically engineered into the four constructs and assayed in MCF-7/HeLa cells. Forty-eight variants (85.7%) impaired splicing, 32 of which did not show any trace of the full-length (FL) transcript. A total of 43 transcripts were identified where the most prevalent event was exon/multi-exon skipping. Twenty-seven transcripts were predicted to truncate the ATM protein. A tentative ACMG/AMP (American College of Medical Genetics and Genomics/Association for Molecular Pathology)-based classification scheme that integrates mgATM data allowed us to classify 29 ATM variants as pathogenic/likely pathogenic and seven variants as likely benign. Interestingly, the likely pathogenic variant c.1898+2T>G generated 13% of the minigene FL-transcript due to the use of a noncanonical GG-5'-splice-site (0.014% of human donor sites). Circumstantial evidence in three ATM variants (leakiness uncovered by our mgATM analysis together with clinical data) provides some support for a dosage-sensitive expression model in which variants producing ¿30% of FL-transcripts would be predicted benign, while variants producing ¿13% of FL-transcripts might be pathogenic.
PD, MPGV, DFE, MdlH and EAV received funding from the European Union's Horizon 2020 research and innovation program under grant agreement no. 634935. The EAV lab is supported by grants from the Spanish Ministry of Science and Innovation, Plan Nacional de I+D+I 2013-2016, ISCIII (PI17/00227 and PI20/00225) co-funded by the FEDER from Regional Development European Funds (European Union) and from the Consejería de Educación, Junta de Castilla y León, ref. CSI242P18 (actuación cofinanciada P.O. FEDER 2014-2020 de Castilla y León). The MdlH lab is supported by a grant from the Spanish Ministry of Science and Innovation, Plan Nacional de I+D+I 2013-2016, ISCIII (PI20/00110) co-funded by FEDER from Regional Development European Funds (European Union). Programa Estratégico Instituto de Biología y Genética Molecular (IBGM), Escalera de Excelencia, Junta de Castilla y León (Ref. CLU-2019-2002). EB-M is a postdoctoral researcher funded by the University of Valladolid (POSTDOC-UVA05, 2022-2025). AV-P and IL-B are supported by predoctoral fellowships from the Consejería de Educación, Junta de Castilla y León. LS-M is supported by a predoctoral fellowship from the AECC-Scientific Foundation, Sede Provincial de Valladolid (2019–2023). AE-S is supported through the Operational Program for Youth Employment and Youth Employment Initiative (YEI), called by the Community of Madrid in 2020, and co-financed by the European Social Fund.