Limb reduction in an Esco2 cohesinopathy mouse model is mediated by p53-dependent apoptosis and vascular disruption.

Roberts syndrome (RBS) is an autosomal recessive disorder with profound growth deficiency and limb reduction caused by ESCO2 loss-of-function variants. Here, we elucidate the pathogenesis of limb reduction in an Esco2fl/fl;Prrx1-CreTg/0 mouse model using bulk- and single-cell-RNA-seq and gene co-expression network analyses during embryogenesis. Our results reveal morphological and vascular defects culminating in hemorrhage of mutant limbs at E12.5. Underlying this abnormal developmental progression is a pre-apoptotic, mesenchymal cell population specific to mutant limb buds enriched for p53-related signaling beginning at E9.5. We then characterize these p53-related processes of cell cycle arrest, DNA damage, cell death, and the inflammatory leukotriene signaling pathway in vivo. In utero treatment with pifithrin-α, a p53 inhibitor, rescued the hemorrhage in mutant limbs. Lastly, significant enrichments were identified among genes associated with RBS, thalidomide embryopathy, and other genetic limb reduction disorders, suggesting a common vascular etiology among these conditions.

Prenatal diagnosis of a skeletal disorder characterized by rhizomelic shortening of limbs caused by compound heterozygous variants in the PKDCC gene: Case report and literature review.

BACKGROUND: The protein kinase domain containing cytoplasmic (PKDCC) gene (OMIM#618821) is associated with bone development. Biallelic variants in the PKDCC gene can cause rhizomelic limb shortening with dysmorphic features. CASE REPORT: A fetus was found to be rhizomelic limb shortening at 16 weeks of gestation and amniocentesis was performed at 19 weeks of gestation. Genomic DNA extracted from the amniotic fluid was subjected to chromosomal microarray analysis (CMA), and Trio-total whole-exome sequencing (Trio-WES). Sanger sequencing was used to verify the candidate pathogenic variants. CMA was normal, while Trio-WES identified two compound heterozygous variants in the PKDCC gene, namely c.417_c.423delCGGCGCG insTCATGGGCTCAGTACAC(p.G140fs*35) and c.345G>A (p.W115*,379). Then the fetus was aborted and the development of its bone cells were compared with that of a normal fetus of similar gestational age by histopathological examination. Clinical findings of the fetus were shortening humerus and femur, synophrys, much hair on the side face, simian line on the right palm, etc. Histopathological examination showed that the affected fetus had increased proliferative chondrocytes, widened proliferative bands, and delayed bone mineralization. CONCLUSIONS: We reported a prenatal case of rhizomelic shortening of limbs caused by compound heterozygous variants in the PKDCC gene, which emphasized the important role of Trio-WES for diagnosis of skeletal dysplasia in fetuses.

Expanding the phenotypic spectrum of NOTCH1 variants: clinical manifestations in families with congenital heart disease.

Pathogenic variants in NOTCH1 are associated with non-syndromic congenital heart disease (CHD) and Adams-Oliver syndrome (AOS). The clinical presentation of individuals with damaging NOTCH1 variants is characterized by variable expressivity and incomplete penetrance; however, data on systematic phenotypic characterization are limited. We report the genotype and phenotype of a cohort of 33 individuals (20 females, 13 males; median age 23.4 years, range 2.5-68.3 years) from 11 families with causative NOTCH1 variants (9 inherited, 2 de novo; 9 novel), ascertained from a proband with CHD. We describe the cardiac and extracardiac anomalies identified in these 33 individuals, only four of whom met criteria for AOS. The most common CHD identified was tetralogy of Fallot, though various left- and right-sided lesions and septal defects were also present. Extracardiac anomalies identified include cutis aplasia (5/33), cutaneous vascular anomalies (7/33), vascular anomalies of the central nervous system (2/10), Poland anomaly (1/33), pulmonary hypertension (2/33), and structural brain anomalies (3/14). Identification of these findings in a cardiac proband cohort supports NOTCH1-associated CHD and NOTCH1-associated AOS lying on a phenotypic continuum. Our findings also support (1) Broad indications for NOTCH1 molecular testing (any familial CHD, simplex tetralogy of Fallot or hypoplastic left heart); (2) Cascade testing in all at-risk relatives; and (3) A thorough physical exam, in addition to cardiac, brain (structural and vascular), abdominal, and ophthalmologic imaging, in all gene-positive individuals. This information is important for guiding the medical management of these individuals, particularly given the high prevalence of NOTCH1 variants in the CHD population.

Al-Gazali Skeletal Dysplasia Constitutes the Lethal End of ADAMTSL2-Related Disorders.

Lethal short-limb skeletal dysplasia Al-Gazali type (OMIM %601356), also called dysplastic cortical hyperostosis, Al-Gazali type, is an ultra-rare disorder previously reported in only three unrelated individuals. The genetic etiology for Al-Gazali skeletal dysplasia has up until now been unknown. Through international collaborative efforts involving seven clinical centers worldwide, a cohort of nine patients with clinical and radiographic features consistent with short-limb skeletal dysplasia Al-Gazali type was collected. The affected individuals presented with moderate intrauterine growth restriction, relative macrocephaly, hypertrichosis, large anterior fontanelle, short neck, short and stiff limbs with small hands and feet, severe brachydactyly, and generalized bone sclerosis with mild platyspondyly. Biallelic disease-causing variants in ADAMTSL2 were detected using massively parallel sequencing (MPS) and Sanger sequencing techniques. Six individuals were compound heterozygous and one individual was homozygous for pathogenic variants in ADAMTSL2. In one of the families, pathogenic variants were detected in parental samples only. Overall, this study sheds light on the genetic cause of Al-Gazali skeletal dysplasia and identifies it as a semi-lethal part of the spectrum of ADAMTSL2-related disorders. Furthermore, we highlight the importance of meticulous analysis of the pseudogene region of ADAMTSL2 where disease-causing variants might be located. © 2023 The Authors. Journal of Bone and Mineral Research published by Wiley Periodicals LLC on behalf of American Society for Bone and Mineral Research (ASBMR).

A novel NSDHL variant in CHILD syndrome with gastrointestinal manifestations and localized skin involvement.

BACKGROUND: CHILD syndrome is an X-linked dominant disorder associated with pathogenic mutations in the NSDHL gene. The condition is predominantly found in females as it is lethal in males. Most cases present at birth with extensive unilateral ichthyosiform erythroderma involving the trunk and limbs. Milder and less extensive presentations have been reported, leading to misdiagnosis especially during early childhood. METHODS AND RESULTS: We report an adult female of Malay ancestry who presented with minimal skin and limb involvement. She was only diagnosed in adulthood when she presented with gastrointestinal symptoms and worsening of skin manifestations. The clinical diagnosis was suspected after a combination of clinical, pathological and immunohistochemistry correlation, and molecularly confirmed with the discovery of a frameshift variant in NSDHL. The novel variant was inherited from her mother who had some linear hypopigmented patches over the medial aspects of both her arms and right forearm. CONCLUSION: We uncovered a novel frameshift variant associated with presentations that cast a new light on the clinical features of CHILD syndrome.

Thanatophoric dysplasia type 1 with temporal lobe dysplasia: Report of a case along with differential diagnosis.

Thanatophoric dysplasia type 1 (TD1) is a lethal form of osteochondral dysplasia due to mutation of FGFR3 gene. In addition to severe shortening of the limbs there is temporo-occipital lobe dysplasia along with a range of other CNS anomalies. In this report we describe the radiological and anatomical features at autopsy in neonate with TD1 along with the CNS anomalies. We have also summarized the key distinguishing features of TD1 from other common types of osteochondral dysplasia. An accurate diagnosis is important for genetic counseling and impact on future pregnancies.

Developmental malformations resulting from high-dose maternal tamoxifen exposure in the mouse.

Tamoxifen is an estrogen receptor (ER) ligand with widespread use in clinical and basic research settings. Beyond its application in treating ER-positive cancer, tamoxifen has been co-opted into a powerful approach for temporal-specific genetic alteration. The use of tamoxifen-inducible Cre-recombinase mouse models to examine genetic, molecular, and cellular mechanisms of development and disease is now prevalent in biomedical research. Understanding off-target effects of tamoxifen will inform its use in both clinical and basic research applications. Here, we show that prenatal tamoxifen exposure can cause structural birth defects in the mouse. Administration of a single 200 mg/kg tamoxifen dose to pregnant wildtype C57BL/6J mice at gestational day 9.75 caused cleft palate and limb malformations in the fetuses, including posterior digit duplication, reduction, or fusion. These malformations were highly penetrant and consistent across independent chemical manufacturers. As opposed to 200 mg/kg, a single dose of 50 mg/kg tamoxifen at the same developmental stage did not result in overt structural malformations. Demonstrating that prenatal tamoxifen exposure at a specific time point causes dose-dependent developmental abnormalities, these findings argue for more considerate application of tamoxifen in Cre-inducible systems and further investigation of tamoxifen's mechanisms of action.

Early clinical and radiological improvement in a young boy with metaphyseal anadysplasia type 2.

Metaphyseal anadysplasia is a very rare hereditary skeletal dysplasia with onset occurring normally during the second and third years of life, but unlike many other dysplasias, symptoms appear to resolve by adolescence. Two types exist, the more severe form, type 1, with both autosomal dominant and recessive inheritance due to pathogenic variants in MMP13, whilst type 2, an even rarer form is due to biallelic MMP9 variants. To date, only two metaphyseal anadysplasia type 2 families have been reported. We describe a third family, a young boy, born to consanguineous parents, referred at 19 months old for abnormal gait due to bowed legs. Clinical and radiological examination revealed scoliosis, genu varum and metaphyseal abnormalities. A homozygous MMP9 nonsense variant, NM_004994.2:c.1764G>A; p.(Trp588*) was identified. By the age of 39 months, lower limb alignment and metaphyseal features had already significantly improved and scoliosis had disappeared. This case confirms that biallelic MMP9 variants cause this very rare skeletal dysplasia, metaphyseal anadysplasia type 2 but also shows that the skeletal manifestations can improve within a short period time and at an early age.

Terminal osseous dysplasia with pigmentary defects and cardiomyopathy caused by a novel FLNA variant.

Terminal osseous dysplasia with pigmentary defects (TODPD), also known as digitocutaneous dysplasia, is one of the X-linked filaminopathies caused by a variety of FLNA-variants. TODPD is characterized by skeletal defects, skin fibromata and dysmorphic facial features. So far, only a single recurrent variant (c.5217G>A;p.Val1724_Thr1739del) in FLNA has found to be responsible for TODPD. We identified a novel c.5217+5G>C variant in FLNA in a female proband with skeletal defects, skin fibromata, interstitial lung disease, epilepsy, and restrictive cardiomyopathy. This variant causes mis-splicing of exon 31 predicting the production of a FLNA-protein with an in-frame-deletion of 16 residues identical to the miss-splicing-effect of the recurrent TODPD c.5217G>A variant. This mis-spliced transcript was explicitly detected in heart tissue, but was absent from blood, skin, and lung. X-inactivation analyses showed extreme skewing with almost complete inactivation of the mutated allele (>90%) in these tissues, except for heart. The mother of the proband, who also has fibromata and skeletal abnormalities, is also carrier of the FLNA-variant and was diagnosed with noncompaction cardiomyopathy after cardiac screening. No other relevant variants in cardiomyopathy-related genes were found. Here we describe a novel variant in FLNA (c.5217+5G>C) as the second pathogenic variant responsible for TODPD. Cardiomyopathy has not been described as a phenotypic feature of TODPD before.

A unique case of progressive hemifacial microsomia or Parry-Romberg syndrome associated with limb and brain anomalies with normal neurological findings: A review of the literature.

In this report, we describe an unusual case of progressive hemifacial atrophy or Parry-Romberg syndrome in a 10-year-old girl with progressive hemifacial microsomia and limb anomalies who had brain magnetic resonance imaging (MRI) findings of white matter hyper-intensities. Patients typically present with neurological manifestations such as epilepsy, facial pain, and migraines and ophthalmological symptoms in conjunction with white matter lesions. The patient demonstrated normal cognition and psychomotor development despite the presence of white matter lesions in her frontal lobe that is commonly associated with neurological symptoms. This report brings attention to the complicated relationship between facial, limb and brain imaging findings in Parry-Romberg syndrome and differentiates it from hemifacial microsomia syndrome.

Feingold syndrome type 2 in a patient from China.

Feingold syndrome type 2 (FGLDS2, MIM614326) is a genetic congenital malformation syndrome, caused by germline heterozygous deletion of MIR17HG on chromosome 13q31, which is extremely rare worldwide. To date, less than 25 patients have been described in the literature. Here, we report on a 3-year-old girl presented with hip dysplasia, polysyndactyly of the left thumb, brachymesophalangy of the fifth digit, microcephaly, intellectual disability, and growth delay. This is likely to be the first case of Feingold syndrome type 2 ever discovered among Chinese population. Through genetic testing and pedigree analysis, she was identified to have a de novo 4.8-Mb microdeletion at chromosome 13q31.3-q32.1, encompassing MIR17HG, GPC5, and GPC6. Additionally, we detected two common compound heterozygous variants (c.919-2A>G and c.147C>G) in SLC26A4 encoding pendrin protein, as well as a novel heterozygous variant c.985_988del in COMP encoding cartilage oligomeric matrix protein. This case report aims to analyze the microdeletion and the three types of variant detected in the patient, and to explore the association between the genotype and phenotype in patients with Feingold syndrome type 2, which may contribute to further understanding and future diagnosis of this disorder.

Temperature-dependent autoactivation associated with clinical variability of PDGFRB Asn666 substitutions.

Ocular pterygium-digital keloid dysplasia (OPDKD) presents in childhood with ingrowth of vascularized connective tissue on the cornea leading to severely reduced vision. Later the patients develop keloids on digits but are otherwise healthy. The overgrowth in OPDKD affects body parts that typically have lower temperature than 37°C. We present evidence that OPDKD is associated with a temperature sensitive, activating substitution, p.(Asn666Tyr), in PDGFRB. Phosphorylation levels of PDGFRB and downstream targets were higher in OPDKD fibroblasts at 37°C but were further greatly increased at the average corneal temperature of 32°C. This suggests that the substitution cause significant constitutive autoactivation mainly at lower temperature. In contrast, a different substitution in the same codon, p.(Asn666Ser), is associated with Penttinen type of premature aging syndrome. This devastating condition is characterized by widespread tissue degeneration, including pronounced chronic ulcers and osteolytic resorption in distal limbs. In Penttinen syndrome fibroblasts, equal and high levels of phosphorylated PDGFRB was present at both 32°C and 37°C. This indicates that this substitution causes severe constitutive autoactivation of PDGFRB regardless of temperature. In line with this, most downstream targets were not affected by lower temperature. However, STAT1, important for tissue wasting, did show further increased phosphorylation at 32°C. Temperature-dependent autoactivation offers an explanation to the strikingly different clinical outcomes of substitutions in the Asn666 codon of PDGFRB.

Clinical and molecular characterizations of 11 new patients with type 1 Feingold syndrome: Proposal for selecting diagnostic criteria and further genetic testing in patients with severe phenotype.

Feingold Syndrome type 1 (FS1) is an autosomal dominant disorder due to a loss of function mutations in the MYCN gene. FS1 is generally clinically characterized by mild learning disability, microcephaly, short palpebral fissures, short stature, brachymesophalangy, hypoplastic thumbs, as well as syndactyly of toes, variably associated with organ abnormalities, the most common being gastrointestinal atresia. In current literature, more than 120 FS1 patients have been described, but diagnostic criteria are not well agreed upon, likewise the genotype-phenotype correlations are not well understood. Here, we describe 11 FS1 patients, belonging to six distinct families, where we have identified three novel MYCN mutations along with three pathogenetic variants, the latter which have already been reported. Several patients presented a mild phenotype of the condition and they have been diagnosed as being affected only after segregation analyses of the MYCN mutation identified in the propositus. We also describe here the first ever FS1 patient with severe intellectual disability having a maternally inherited MYCN variant together with an additional GNAO1 mutation inherited paternally. Mutations in the GNAO1 gene are associated with a specific form of intellectual disability and epilepsy, thus the finding of two different rare diseases in the same patient could explain his severe phenotype. Therein, a thorough investigation is merited into the possibility that additional variants in patients with a MYCN mutation and severe phenotype do exist. Finally, in order to guarantee a more reliable diagnosis of FS1, we suggest using both major and minor clinical-molecular diagnostic criteria.

Earlier detection of hypochondroplasia: A large single-center UK case series and systematic review.

Hypochondroplasia (HCH) is a rare autosomal dominant skeletal dysplasia condition caused by FGFR3 mutations leading to disproportionate short stature. Classically HCH presents in toddlers or school-age children, as limb-to-trunk disproportion and is often mild and easily overlooked during infancy. We report experiences from a single-center UK HCH-cohort of 31 patients, the rate of antenatal HCH detection in our cohort (13/31, 41.9%) and describe relevant case-data for this subset of 13 patients. Inclusion criteria were patients with confirmed molecular HCH diagnosis (by age 3 years) and presenting with short long-bones or large head size on antenatal ultrasound scan. We then conducted a systematic literature review using PUBMED and MEDLINE, analyzing patients with HCH and related antenatal findings. Antenatally suspected (with subsequent molecular confirmation) HCH has been reported 15 times in the literature (2004-2019). Key markers (consistent in both groups) included reduced; femur length, humeral length and increased; biparietal diameter and head circumference. HCH is increasingly detected both antenatally and in infancy, contrary to previous descriptions. This is likely due to greater HCH awareness, improved imaging, and easier molecular testing. Thus, one should consider HCH outside the classical presenting period. Studying the natural history of younger patients with HCH is important with the advent of several targeted FGFR3 therapies currently in trials for Achondroplasia, that may soon be trialed in HCH.

Lethal Cenani Lenz syndrome in two consecutive pregnancies: Further extension of phenotype from Maldives.

Cenani Lenz syndrome is a rare autosomal recessive disorder associated with variable degree of limb malformations, dysmorphism, and renal agenesis. It is caused due to pathogenic variants in the LRP4 gene, which plays an important role in limb and renal development. Mutations in the APC gene have also been occasionally associated with CLS. The phenotypic spectrum ranges from mild to very severe perinatal lethal type depending on the type of variant. We report a pathogenic variant, c.2710 del T (p.Trp904GlyfsTer5) in theLRP4 gene, in a fetus with lethal Cenani Lenz syndrome with antenatal presentation of tetraphocomelia and symmetrical involvement of hands and feet.

O-Fucosylation of ADAMTSL2 is required for secretion and is impacted by geleophysic dysplasia-causing mutations.

ADAMTSL2 mutations cause an autosomal recessive connective tissue disorder, geleophysic dysplasia 1 (GPHYSD1), which is characterized by short stature, small hands and feet, and cardiac defects. ADAMTSL2 is a matricellular protein previously shown to interact with latent transforming growth factor-ß binding protein 1 and influence assembly of fibrillin 1 microfibrils. ADAMTSL2 contains seven thrombospondin type-1 repeats (TSRs), six of which contain the consensus sequence for O-fucosylation by protein O-fucosyltransferase 2 (POFUT2). O-fucose-modified TSRs are subsequently elongated to a glucose ß1-3-fucose (GlcFuc) disaccharide by ß1,3-glucosyltransferase (B3GLCT). B3GLCT mutations cause Peters Plus Syndrome (PTRPLS), which is characterized by skeletal defects similar to GPHYSD1. Several ADAMTSL2 TSRs also have consensus sequences for C-mannosylation. Six reported GPHYSD1 mutations occur within the TSRs and two lie near O-fucosylation sites. To investigate the effects of TSR glycosylation on ADAMTSL2 function, we used MS to identify glycan modifications at predicted consensus sequences on mouse ADAMTSL2. We found that most TSRs were modified with the GlcFuc disaccharide at high stoichiometry at O-fucosylation sites and variable mannose stoichiometry at C-mannosylation sites. Loss of ADAMTSL2 secretion in POFUT2-/- but not in B3GLCT-/- cells suggested that impaired ADAMTSL2 secretion is not responsible for skeletal defects in PTRPLS patients. In contrast, secretion was significantly reduced for ADAMTSL2 carrying GPHYSD1 mutations (S641L in TSR3 and G817R in TSR6), and S641L eliminated O-fucosylation of TSR3. These results provide evidence that abnormalities in GPHYSD1 patients with this mutation are caused by loss of O-fucosylation on TSR3 and impaired ADAMTSL2 secretion.

Characterization of the Robinow syndrome skeletal phenotype, bone micro-architecture, and genotype-phenotype correlations with the osteosclerotic form.

Robinow syndrome (RS) is a genetically heterogeneous skeletal dysplasia with recent reports suggesting an osteosclerotic form of the disease. We endeavored to investigate the full spectrum of skeletal anomalies in a genetically diverse cohort of RS patients with a focus on the bone micro-architecture. Seven individuals with molecularly confirmed RS, including four with DVL1 variants and single individuals with variants in WNT5A, ROR2, and GPC4 underwent a musculoskeletal focused physical examination, dual-energy X-ray absorptiometry (DEXA) scan, and high-resolution peripheral quantitative computed tomography (HR-pQCT). Skeletal examination revealed variability in limb shortening anomalies consistent with recent reports. DEXA scan measures revealed increased total body bone mineral density (BMD) (3/7), cranial BMD (5/7), and non-cranial BMD (1/7). Cranial osteosclerosis was only observed in DVL1-RS (4/4) and GPC4-RS (1/1) subjects and in one case was complicated by choanal atresia, bilateral conductive hearing loss, and cranial nerve III, VI, and VII palsy. HR-pQCT revealed a unique pattern of low cortical BMD, increased trabecular BMD, decreased number of trabeculations, and increased thickness of the trabeculations for the DVL1-RS subjects. The spectrum of skeletal anomalies including the micro-architecture of the bones observed in RS has considerable variability with some osteosclerosis genotype-phenotype correlations more frequent with DVL1 variants.

A case of dwarfism in 6th century Italy: Bioarchaeological assessment of a hereditary disorder.

OBJECTIVE: The skeletal remains of a short-statured individual (T17) are described and a differential diagnosis performed to determine the etiology of the condition. MATERIALS: An individual considered pathologically short in stature was discovered in the burial site of Piazza XX Settembre, Modena (northern Italy). METHODS: Morphological and morphometric analyses were performed, and T17 was compared to dwarfs from other localities and periods and to the adult female population from the same site. A paleopathological survey was undertaken to assess the degree of the skeletal elements of T17 were affected. RESULTS: T17 was a female, 20-30 years of age at death, with a stature of 128 cm and disproportionate dwarfism associated with congenital skeletal dysplasia. CONCLUSIONS: T17 likely affected by a form of hypochondroplasia. SIGNIFICANCE: Anatomical consequences of hypochondroplasia are presented, and the timeframe and associated burial goods suggest a 6th-century Lombard short stature belonging to one of the earliest Lombard settlements in Italy. SUGGESTIONS FOR FURTHER RESEARCH: Future genetic analysis would resolve if the mutation in the type 3 fibroblast growth factor receptor (FGFR3) is present in the remains of T17; however, it is not exclusivly linked to hypochondroplasia.

Description of a family with X-linked oculo-auriculo-vertebral spectrum associated with polyalanine tract expansion in ZIC3.

Oculo-auriculo-vertebral spectrum (OAVS) [MIM:164210], or Goldenhar syndrome, is a developmental disorder associating defects of structures derived from the first and second branchial arches. The genetic origin of OAVS is supported by the description of rare deleterious variants in a few causative genes, and several chromosomal copy number variations. We describe here a large family with eight male members affected by a mild form of the spectrum, mostly auricular defects, harboring a hemizygous ZIC3 variant detected by familial exome sequencing: c.159_161dup p.(Ala55dup), resulting in an expansion of the normal 10 consecutive alanine residues to 11 alanines. Segregation analysis shows its presence in all the affected individuals, with a recessive X-linked transmission. Whole-genome sequencing performed in another affected male allowed to exclude linkage disequilibrium between this ZIC3 variant and another potential pathogenic variant in this family. Furthermore, by screening of a cohort of 274 OAVS patients, we found 1 male patient carrying an expansion of 10 to 12 alanines, a variant previously reported in patient presenting with VACTERL. Loss-of-function variants of ZIC3 are causing heterotaxy or cardiac malformations. These alanine expansion variants could have a different impact on the protein and thereby resulting in a different phenotype within the OAVS/VACTERL.

Activating PIK3CA mutation promotes osteogenesis of bone marrow mesenchymal stem cells in macrodactyly.

Macrodactyly is a disabling congenital disease characterized by overgrowth of soft tissues and bones, which leads to finger enlargement and joint deformity. The mechanism of bone overgrowth in macrodactyly was rarely understood. In our study bone manifestations of three macrodactyly patients were analyzed by micro-CT. PIK3CA mutation was detected by next-generation sequencing (NGS) of a tumor gene-panel. The PI3K/AKT/mTOR pathway activation and target genes were analyzed. The osteogenic potential of macrodactyly-derived bone marrow mesenchymal stem cells (MAC-BMSCs) was compared with polydactyly-derived bone marrow mesenchymal stem cells (PD-BMSCs). PIK3CA inhibitors were tested for proliferation and osteogenesis potential of MAC-BMSCs. Activating PIK3CA mutations and activation of PI3K/AKT/mTOR pathway were detected in all MAC-BMSCs. MAC-BMSCs had enhanced osteogenesis potential compared with PD-BMSCs. PIK3CA knockdown by shRNA or BYL719 treatment significantly reduced osteogenic differentiation capacity of MAC-BMSCs. RNA-Seq and qRT-PCR revealed the upregulation of distal-less homeobox 5 (DLX5) in MAC-BMSCs compared with PD-BMSCs. The osteogenic potential of MAC-BMSCs was inhibited by DLX5 knockdown, indicating that DLX5 is a downstream target of PIK3CA activation-mediated osteogenesis. This study revealed that osteogenic differentiation in MAC-BMSCs is enhanced by PIK3CA activation mutation through PI3K/AKT/mTOR signaling pathway and can be reversed by PIK3CA knockdown or drug inhibition.