In the current study, we have identified a single de novo homozygous mutation in SEC16B
in exon 4 (NM_033127.3: c.424CàT; p.Arg142àTry) in a ten-year-old boy with typical clinical,
radiological and bone ultrastructural features of OI using whole exome sequencing. Growing these
patients? fibroblasts in our lab, we found that these cells accumulate collagen type I in the ER when
compared to healthy control cells with no mutation. This is supported by comparing the
immunofluorescence (IF) staining of patient and control cells using antibodies to identify the location
of collagen type I within the ER and Golgi in the cells. Pulse chase experiment indicated that patient
cells with homozygous SEC16B mutation had accelerated cellular collagen type I secretion
compared to healthy control cells with intact SEC16B. This may be explained by the fact that collagen
proteins do not retain in the Golgi but instead get transported directly to the extracellular matrix.
Interestingly, using transmission electron microscopy and western blotting, we found that SEC16B
mutant fibroblasts exihibit enhanced autophagosome formation combined with increased apoptosis.
Most interestingly, mass spectrometry demonstrated that the quality of COL1A1 in mutated SEC16B
cells is different than the healthy controls. Particularly; two regions of COL1A1 are not detected in
patient fibroblasts when compared to healthy fibroblasts. These defects in type I collagen can be
explained by altered post translational modification (PTMs) in SEC16B mutant cells.