Somatic mosaicism of PIK3CA mutation in PIK3CA-related overgrowth disorders

Abstract

Background and aims Somatic mosaicism of phosphatidylinositol-4,5-bisphosphate 3-kinase (PIK3CA) mutation, one of the genes involved in the PI3K/AKT/mTOR pathway, is associated with a group of rare asymmetrical overgrowth syndrome that is collectively named as PIK3CA-related overgrowth disorder. This group of disorder includes CLOVES syndrome, fibroadipose hyperplasia, and facial infiltrating lipomatosis. The general abnormalities associated with somatic mutation of PIK3CA include abnormal overgrowth of skeletal and fibroadipose tissues, vascular malformations and skin abnormalities. The PI3K/AKT/mTOR pathway is an intracellular signaling pathway which is involved in cellular processes like cell proliferation and apoptosis, and mutations of components of this pathway result in overactivation the pathway, leading to various overgrowth- related diseases such as tuberous sclerosis and cancers. Mutation of PIK3CA also over-activates the PI3K/AKT/mTOR pathway, therefore leading to abnormal overgrowth in patients. We identify seven suspected cases of PIK3CA-related overgrowth disorder, including three suspected cases of CLOVES syndrome, one with cystic hygroma, one with isolated macrodactyly, one with asymmetric limb overgrowth, and one with multiple lipomatosis. We aim to identify the mutations of PIK3CA in each case. Methods For the cases with fresh tissues, tissue DNA extraction was performed by QIAamp DNA Mini (Qiagen) according to the manufacturer's protocol. For cases with formalin-fixed, paraffin-embedded (FFPE) fixed tissues, DNA extraction was performed by QIAamp DNA FFPE Tissue (Qiagen). Since four mutation hotspots have been identified in patients of PIK3CA-related overgrowth disorder, we carried out sanger sequencing on these mutation hotspots. Results In one suspected case of CLOVES syndrome, somatic mosaicism of PIK3CA c.3297A>G (p.His1047Arg) mutation was identified in the patient's affected cartilage and affected lipomatous tissues, whereas mutation was not found in her unaffected skin tissue. Molecular analysis of the other suspected PIK3CA-related overgrowth disorders is in progress.

Conclusion Somatic muation of PIK3CA is the cause of PIK3CA-related overgrowth disorder, where mutations can only be found in affected tissues but not in unaffected tissues, suggesting somatic mosaicism. This implies that when genetic test is considered, the correct choice of tissues is important for molecular confirmation of the disease. Further experiment will be carried out on identifying the mutation on other cases, and also quantifying the percentage of somatic mosaicism using pyrosequencing.