巨指(趾)畸形是一种以单/多指(趾)局限性过度生长为核心特征的先天性肢体发育异常，常伴有骨骼、软组织及神经组织的病理改变，大多数属于体细胞变异。既往研究已证实，PIK3CA基因致病性变异是该疾病发生的关键遗传基础，但具体致病机制尚不清楚。本研究共纳入13例临床诊断为单纯性巨指(趾)畸形的患者，经全外显子测序分析技术结合Sanger测序，发现其中1例患者携带PIK3CA基因嵌合新发框内缺失突变(c.307_312del/p.Glu103_Pro104del)(其父母均未携带该变异)。根据美国医学遗传学与基因组学学会的遗传变异分类标准与指南，该变异被判定为意义未明变异(PM4+PM2_Supporting+PM6_Supporting)。蛋白结构预测分析表明，该框内缺失突变改变了蛋白局部构象和表面静电势。进一步通过构建野生型和突变型表达载体，以致病性明确的c.353G>A/p.Gly118Asp突变作对照，转染HEK293T细胞，对该突变进行了功能分析。结果表明，虽然该突变对PIK3CA基因mRNA和蛋白表达水平较野生型无明显变化，但是却促进了其下游AKT蛋白第308位(Thr308)与第473位(Ser473)磷酸化。这提示，该突变可能通过激活PI3K/AKT信号通路，进而导致患者局部组织过度生长的表型。本研究不仅拓展了巨指(趾)畸形的遗传变异谱，也为进一步阐明PI3K/AKT通路在PIK3CA相关过度生长谱系中的作用提供了实验依据。

Macrodactyly is a congenital abnormality characterized by overgrowth of single or multiple digits, frequently accompanied by pathological changes in bone, soft tissue, and nerve tissue, and is caused mostly by somatic variants. Previous studies have demonstrated that macrodactyly is predominantly caused by pathogenic variants in the PIK3CA gene, yet the underlying molecular mechanism remains unclear. In this study, we analyzed 13 patients clinically diagnosed with isolated macrodactyly. Through whole-exome and Sanger sequencing, we identified one pediatric patient carrying a novel mosaic de novo in-frame deletion mutation (c.307_312del/p.Glu103_Pro104del) in the PIK3CA gene, which was not detected in either parent. According to the American College of Medical Genetics and Genomics (ACMG) guidelines, we classified this mutation as a variant of uncertain significance (PM4+PM2_Supporting+PM6_Supporting). Protein structure prediction analysis indicated that this in-frame deletion altered the protein’s local conformation and surface electrostatic potential. Furthermore, the wild-type and mutant expression vectors were constructed, including the well-established pathogenic mutation c.353G>A/p.Gly118Asp in PIK3CA gene as a control. We transfected these vectors into HEK293T cells for functional analyses, respectively. The results showed that although this mutation did not significantly affect PIK3CA mRNA or protein expression levels, it promoted the phosphorylation of downstream AKT protein at both Thr308 and Ser473. Therefore, the mutation may express the localized overgrowth phenotype by activating the PI3K/AKT signaling pathway. This study not only expands the genetic variant spectrum of macrodactyly, but also further clarifies the role of the PI3K/AKT pathway in the PIK3CA-related overgrowth.