Activating PIK3CA postzygotic mutations in segmental overgrowth of muscles with bone involvement in the body extremities.

Segmental overgrowth of the skeletal muscles with bone involvement in body extremities, predominantly affecting the upper limb, is an extremely rare condition with only 40-50 affected children described clinically. The molecular pathogenesis of this disorder remains largely unclear except for the identification of a somatic PIK3CA mutation in each of the six patients genetically tested, all restricted to upper limbs in the literature. This study aimed to further characterize the molecular defects for patients affected with segmental overgrowth of the skeletal muscles by analyzing a 9-gene panel selected from the PI3K/AKT/mTOR pathway and genes associated with other related conditions. Nineteen unrelated patients were chosen for this study, comprising ten upper limb (nine unilateral and one bilateral) and nine lower limb (eight unilateral and one bilateral) cases with variable bone involvement. In each case, an activating PIK3CA mutation (p.E110del, p.N345K, p.E542K, p.E545K, p.H1047R, or p.H1047L) was identified in the affected muscle tissue with variant allele frequencies ranging from 13.88 to 30.43%, while no mutation was detected in the paired peripheral blood sample, indicating somatic mosaicism. All detected mutations were limited to PIK3CA and were previously reported in other overgrowth syndromes currently categorized under the PIK3CA-Related Overgrowth Spectrum (PROS). Our study provides strong molecular evidence that isolated segmental overgrowth of the skeletal muscle with bone involvement is a subtype of PROS. Our findings expand the PROS clinical presentations with a newly molecularly classified condition and can provide guidance in clinical and molecular diagnosis and treatment for patients with this condition.