To investigate the prenatal manifestation and genetic basis for two fetuses suspected for Osteocraniostenosis (OCS). Two fetuses undergoing invasive prenatal diagnosis at Cangzhou People's Hospital in April and August 2021 for short long bones and abnormal skull morphology were selected as the study subjects. Clinical data were collected and analyzed. Genomic DNA was extracted from amniotic fluid and peripheral blood samples of the two couples. Candidate variants were validated by Sanger sequencing. Literature was retrieved from CNKI, Wanfang Data Knowledge Service Platform and PubMed using keywords including "FAM111A gene", "gracile bone dysplasia", "FAM111A" and "osteocraniostenosis" from January 1, 2000 to June 30, 2025. This study was approved by the Medical Ethics Committee of the hospital (Ethics No.: K2020-049). Fetus 1 was found to have short limbs, abnormal skull morphology and shallow cerebral sulci. Fetus 2 showed short limbs, irregular skull halo, prominent forehead and bilateral frontal narrowing. Trio-WES revealed that fetus 1 has carried a heterozygous missense variant c.1582G>C (p.Asp528His) in exon 4 of the FAM111A gene, which was unreported previously. Fetus 2 has harbored a heterozygous in-frame deletion c.1020_1022delTTC (p.Ser343del) in exon 6 of the FAM111A gene, which has been recorded as likely pathogenic by the ClinVar and HGMD databases. Sanger sequencing confirmed that the parents of both fetuses were wild-type for the variant sites. A total of 9 previously reported patients with FAM111A-related gracile bone dysplasia/OCS from 4 publications were retrieved. The main clinical features included intrauterine growth restriction, hypomineralized skull, gracile long bones with narrow medullary cavities and characteristic facial anomalies, which were in large in keeping with the prenatal features of the two fetuses. Both fetuses were diagnosed with FAM111A-related OCS based on the characteristic prenatal findings and identification of the FAM111A variants. Above finding expanded the phenotypic spectrum of FAM111A-associated disorders and provided clues for the prenatal diagnosis and genetic counseling.

Kenny-Caffey syndrome (KCS) is a rare genetic disorder characterized by extreme short stature, cortical thickening and medullary stenosis of tubular bones, facial dysmorphism, abnormal T cell function, and hypoparathyroidism. Biallelic loss-of-function variants in TBCE cause autosomal recessive type 1 KCS (KCS1). By contrast, heterozygous missense variants in a restricted region of the FAM111A gene have been identified in autosomal dominant type 2 KCS (KCS2) and a more severe lethal phenotype, osteocraniostenosis (OCS); these variants have recently been shown to confer a gain of function. In this study, we describe 2 unrelated children with KCS and OCS who were homozygous for different FAM111A variant alleles that result in replacement of the same residue, Tyr414 (c.1241A>G, p.Y414C and c.1240T>A, p.Y414N), in the mature FAM111A protein. Their heterozygous relatives are asymptomatic. Functional studies of recombinant FAM111AY414C demonstrated normal dimerization and a mild gain-of-function effect. This study provides evidence that both biallelic and monoallelic variants of FAM111A with varying degrees of activation can lead to dominant or recessive KCS2 and OCS.

FAM111A (family with sequence similarity 111 member A) is a serine protease and removes covalent DNA-protein cross-links during DNA replication. Heterozygous gain-of-function variants in FAM111A cause skeletal dysplasias, such as the perinatal lethal osteocraniostenosis and the milder Kenny-Caffey syndrome (KCS). We report two siblings born to consanguineous parents with dysmorphic craniofacial features, postnatal growth retardation, ophthalmologic manifestations, hair and nail anomalies, and skeletal abnormalities such as thickened cortex and stenosis of the medullary cavity of the long bones suggestive of KCS. Using exome sequencing, a homozygous synonymous FAM111A variant, NM_001312909.2:c.81 G > A; p.Pro27=, that affects the last base of the exon and is predicted to alter FAM111A pre-mRNA splicing, was identified in both siblings. We identified aberrantly spliced FAM111A transcripts, reduced FAM111A mRNA levels, and near-complete absence of FAM111A protein in fibroblasts of both patients. After treatment of patient and control fibroblasts with different concentrations of camptothecin that induces covalent DNA-protein cross-links, we observed a tendency towards a reduced proportion of metabolically active cells in patient compared to control fibroblasts. However, under these culture conditions, we did not find consistent and statistically significant differences in cell cycle progression and apoptotic cell death between patient and control cells. Our findings show that FAM111A deficiency underlies an autosomal recessive form of FAM111A-related KCS. Based on our results and published data, we hypothesize that loss of FAM111A and FAM111A protease hyperactivity, as observed for gain-of-function patient-variant proteins, may converge on a similar pathomechanism underlying skeletal dysplasias.

Background/Objectives: Kenny-Caffey syndrome 2 (KCS2) is a rare cause of hypoparathyroidism, inherited in an autosomal dominant mode, resulting from pathogenic variants of the FAM111A gene, which is implicated in intracellular pathways regulating parathormone (PTH) synthesis and skeletal and parathyroid gland development. Methods: The case of a boy is reported, presenting with the characteristic and newly identified clinical, biochemical, radiological, and genetic abnormalities of KCS2. Results: The proband had noticeable dysmorphic features, and the closure of the anterior fontanel was delayed until the age of 4 years. Biochemical evaluation at several ages revealed persistent hypocalcemia, high normal phosphorous, and inappropriately low normal PTH. To exclude other causes of short stature, the diagnostic approach revealed low levels of IGF-1, and on CNS MRI, small pituitary gland and empty sella. Nocturnal levels of growth hormone were normal. MRI also revealed bilateral symmetrical microphthalmia and torturous optic nerves. Skeletal survey was compatible with cortical thickening and medullary stenosis of the long bones. Genomic data analysis revealed a well-known pathogenic variant of the FAM111A gene (c.1706G>A, p. R569H), which is linked with KCS2 or nanophthalmos. Conclusions: KCS2, although a rare disease, should be included in the differential diagnosis of hypoparathyroidism and short stature. Understanding the association of pathogenic variants with KCS2 phenotypic variability will allow the advancement of clinical genetics and personalized long-term follow-up and will offer insights into the role of the FAM111A gene in the disease pathogenesis and normal embryogenesis of implicated tissues and organs.

FAM111A gene mutations cause Kenney-Caffey syndrome (KCS) and Osteocraniostenosis (OCS), conditions characterized by short stature, low serum ionized calcium (Ca2+), low parathyroid hormone (PTH), and bony abnormalities. The molecular mechanism mediating this phenotype is unknown. The c-terminal domain of FAM111A harbors all the known disease-causing variations and encodes a domain with high homology to serine proteases. However, whether this serine protease domain contributes to the maintenance of Ca2+ homeostasis is not known. We hypothesized the disruption of the serine protease domain of FAM111A would disrupt Ca2+ homeostasis. To test this hypothesis, we generated with CRISPR/Cas9, mice with a frameshift insertion (c.1450insA) or large deletion (c.1253-1464del) mutation in the Fam111a serine protease domain. Serum-ionized Ca2+ and PTH levels were not significantly different between wild type, heterozygous, or homozygous Fam111a mutant mice. Additionally, there were no significant differences in fecal or urine Ca2+ excretion, intestinal Ca2+ absorption or overall Ca2+ balance. Only female homozygous (c.1450insA), but not heterozygous mice displayed differences in bone microarchitecture and mineral density compared to wild-type animals. We conclude that frameshift mutations that disrupt the c-terminal serine protease domain do not induce a KCS or OCS phenotype in mice nor alter Ca2+ homeostasis.