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.

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.

Hypoparathyroidism, which can be sporadic or a component of an inherited syndrome, is the most common cause of hypocalcemia. If hypocalcemia is accompanied by other electrolyte disturbances, such as hypokalemia and hypomagnesemia, then the cause, such as renal tubular disease, should be carefully identified. An 18-year-old female visited our clinic because of short stature and facial deformities, including typical phenotypes, such as low ear position, depression of the nasal bridge, small hands and feet, and loss of dentition. The lab results suggested normal parathyroid hormone but hypocalcemia. In addition, multiple electrolyte disturbances were found, including hypokalemia, hypocalcemia and hypomagnesemia. The physical signs showed a short fourth metatarsal bone of both feet. The X-ray images showed cortical thickening of long bones and narrowing of the medulla of the lumen. Cranial computed tomography indicated calcification in the bilateral basal ganglia. Finally, the genetic investigation showed a de novo heterogenous mutation of "FAM111A" (c. G1706A:p.R569H). Through a review of previously reported cases, the mutation was found to be the most common mutation site in Kenny-Caffey syndrome type 2 (KCS2) cases reported thus far (16/23, 69.6%). The mutation was slightly more prevalent in females than in males (11/16, 68.8%). Except for hypocalcemia, other clinical manifestations are heterogeneous. As a rare autosomal dominant genetic disease of hypoparathyroidism, the clinical manifestations of KCS2 are atypical and diverse. This girl presented with short stature, facial deformities and skeletal deformities. The laboratory results revealed hypocalcemia as the main electrolyte disturbance. Even though her family members showed normal phenotypes, gene detection was performed to find the mutation of the FAM111A gene and confirmed the diagnosis of KCS2. FAM111A-related skeletal dysplasias include the milder phenotype of Kenny-Caffey syndrome (KCS) and a more severe lethal phenotype, osteocraniostenosis (OCS). KCS is characterized by proportionate short stature (typically postnatal onset), relative macrocephaly, large anterior fontanel with delayed closure, characteristic facial features, cortical thickening of the long bones with stenosis of the medullary cavity, and ophthalmologic and dental manifestations. OCS is characterized by intrauterine growth deficiency, microcephaly, characteristic facial features, decreased skull ossification, slender long bones with cortical thickening, stenosis of the medullary cavity of the long bones, flared metaphyses, and thin ribs with thoracic and pulmonary hypoplasia leading to respiratory insufficiency. Perinatal fractures may occur. Primary hypoparathyroidism with hypocalcemia and hyperphosphatemia can occur in individuals with KCS and OCS. The diagnosis of a FAM111A-related skeletal dysplasia is established in a proband with suggestive findings and a heterozygous pathogenic variant in FAM111A identified by molecular genetic testing. Treatment of manifestations: Survivors with OCS require aggressive respiratory support and management of restrictive lung disease with a respiratory specialist; for all affected individuals, supplemental calcium and activated forms of vitamin D per endocrinologist; management of refractive errors and cataracts; management of dental manifestations with a dental specialist / oral surgeon; environmental and/or occupational modifications as needed for short stature in those with KCS; conservative or surgical management per orthopedist and/or neurosurgeon for scoliosis; individualized developmental support by allied health clinicians; referral to psychologist as needed. Surveillance: For individuals with KCS and OCS, assess anthropometry (height, weight, growth velocity, limb proportions, and upper-to-lower segment proportions) at each visit; clinical examination for scoliosis at each visit, with referral to orthopedics and physical therapy as necessary; assess functional limitations and assessment with physical therapy and/or occupational therapy as needed; measurement of serum calcium, phosphate, and vitamin D every three months until calcium level is normalized on treatment and then subsequently every six months; clinical examination for manifestations of hypocalcemia; abdominal ultrasound to assess for nephrocalcinosis and/or nephrolithiasis annually while on treatment; ophthalmology examination annually or as indicated; dental examinations every six months; assess for clinical manifestations of anemia at each visit; monitor developmental progress and educational needs at each visit throughout childhood; assess for changes in mood, affect, and/or psychosocial stressors at each visit; assess care coordination needs and genetic counseling needs at each visit. FAM111A-related skeletal dysplasias (including KCS and OCS) are autosomal dominant disorders. KCS. Most individuals diagnosed with KCS have the disorder as the result of a de novo FAM111A pathogenic variant. Rarely, individuals diagnosed with KCS have an affected parent. If a parent of the proband is known to have the pathogenic variant identified in the proband, the risk to the sibs of inheriting the pathogenic variant is 50%. Each child of an individual with KCS has a 50% chance of inheriting the FAM111A pathogenic variant. OCS. With one possible exception, all probands reported to date with OCS whose parents have undergone molecular genetic testing have the disorder as the result of a de novo FAM111A pathogenic variant. Given that probands with OCS typically have the disorder as the result of a de novo FAM111A pathogenic variant, the risk to other family members is presumed to be low. Once the FAM111A pathogenic variant has been identified in an affected family member, prenatal and preimplantation genetic testing are possible.

Kenny-Caffey syndrome (KCS) is a rare autosomal recessive (AR)/dominant disease characterized by hypoparathyroidism, skeletal dysplasia, dwarfism, and dysmorphism. FAM111A or TBCE gene mutations are responsible for this syndrome. Osteocraniostenosis (OCS) is a lethal syndrome with similar features to KCS, and it can be a severe form of KCS type 2 which results from the FAM111A gene mutation. The FAM111A mutation is generally characterized by the autosomal dominant transition. We present a male case having compound heterozygous variants (c.976T>A and c.1714_1716del) in the FAM111A gene with an AR inheritance pattern. Hypocalcemia developed on the second day of life. The patient and his older sister had a dysmorphic face, skeletal dysplasia, and they were diagnosed with hypoparathyroidism. Both siblings died due to septicemia. He is the first reported patient with the FAM111A mutation in Turkey. The phenotype of the patient is compatible with OCS, and the detected variants may explain the disease genetically.

Kenny-Caffey syndrome type 2 (KCS2) is an extremely rare skeletal disorder involving hypoparathyroidism and short stature. It has an autosomal dominant pattern of inheritance and is caused by variants in the FAM111 trypsin-like peptidase A (FAM111A) gene. This disease is often difficult to diagnose due to a wide range of more common diseases manifesting hypoparathyroidism and short stature. Herein, we present the case of a 56-year-old female patient with idiopathic hypoparathyroidism and a short stature. The patient was treated for these conditions during childhood. Upon re-evaluating the etiology of KCS2, we suspected that the patient had the disorder because of clinical manifestations, such as cortical thickening and medullary stenosis of the bones, and lack of intellectual abnormalities. Genetic testing identified a heterozygous missense variant in the FAM111A gene (p.R569H). Interestingly, the patient also had bilateral sensorineural hearing loss and vestibular dysfunction, which have been rarely described in previous reports of pediatric cases. In KCS2, inner ear dysfunction due to Eustachian tube dysfunction may progress in middle age or later. However, this disease is now being reported in younger patients. Nevertheless, our case may be instructive of how such cases emerge chronically after middle age. Herein, we also provide a literature review of KCS2.