PCSK5 ^M452I is a recessive hypomorph exclusive to MCF10DCIS.com cells

The most widely used cell line for studying ductal carcinoma in situ (DCIS) premalignancy is the transformed breast epithelial cell line, MCF10DCIS.com . During its original clonal isolation and selection, MCF10DCIS.com acquired a heterozygous M452I mutation in the proprotein convertase PCSK5, which has never been reported in any human cancer. The mutation is noteworthy because PCSK5 matures GDF11, a TGFβ-superfamily ligand that suppresses progression of triple-negative breast cancer. We asked here whether PCSK5 ^M452I and its activity toward GDF11 might contribute to the unique properties of MCF10DCIS.com . Using an optimized in-cell GDF11 maturation assay, we found that overexpressed PCSK5 ^M452I was measurably active but at a fraction of the wildtype enzyme. In a PCSK5 ^−/− clone of MCF10DCIS.com reconstituted with different PCSK5 alleles, PCSK5 ^M452I was mildly defective in anterograde transport. However, the multicellular organization of PCSK5 ^M452I addback cells in 3D matrigel cultures was significantly less compact than wildtype and indistinguishable from a PCSK5 ^T288P null allele. Growth of intraductal MCF10DCIS.com xenografts was similarly impaired along with the frequency of comedo necrosis and stromal activation. In no setting did PCSK5 ^M452I exhibit gain-of-function activity, leading us to conclude that it is hypomorphic and thus compensated by the remaining wildtype allele in MCF10DCIS.com .