Tissue-nonspecific alkaline phosphatase (TNAP) has emerged as a crucial regulator of neuronal circuit formation and maintenance; however, the complexities of its sex- and cell type-specific roles within microglia remain largely unexplored. To address this critical knowledge gap, this study examined how TNAP deficiency differentially affects microglial morphology, function, and signaling in both male and female mice, and investigated its broader implications for neurodevelopment and disease susceptibility. Using Alpl+/+ (wild-type) and Alpl-/- (TNAP knockout) mice, we conducted behavioral assessments at postnatal Days 13-14 to evaluate early neurobehavioral outcomes. Microglia were subsequently isolated for molecular, metabolic, and morphological analyses. TNAP-deficient mice of both sexes exhibited profound physiological deficits, including stunted growth and significant sensorimotor impairments, confirming effective TNAP knockout and indicating that systemic TNAP loss affects multiple cell types beyond microglia. At the cellular level, TNAP loss induced notable morphological changes in microglia, characterized by enlarged cell soma and shortened processes, hallmarks of microglial activation. Molecular profiling revealed upregulation of neuroinflammatory and phagocytic markers, implicating TNAP as a modulator of the innate immune response. Furthermore, metabolic analyses uncovered a dramatic shift in tryptophan-kynurenine metabolism, with increased quinolinic acid production signifying a transition to a neurotoxic, pro-inflammatory state. Additionally, TNAP-deficient microglia displayed extensive dysregulation in purinergic signaling pathways, exemplified by increased expression of key purinergic receptors, and acquired a senescent phenotype evidenced by elevated canonical senescence gene expression. Given the influence of TNAP deficiency on multiple cell populations, some observed microglial phenotypes may result from altered intercellular signaling or indirect effects. To delineate cell-autonomous effects, siRNA-mediated TNAP knockdown was performed in primary microglia isolated from wild-type (WT) mice. TNAP depletion modulated inflammatory responses, suggesting an intrinsic role for TNAP in microglial regulation; however, these effects may not fully recapitulate the extent of deficiency observed in vivo. Overall, TNAP emerges as a key modulator of microglial structure and function, with its dysfunction potentially increasing susceptibility to neurodevelopmental and neurodegenerative disorders. This highlights the potential of TNAP as a therapeutic target for central nervous system health and disease.

We report a rare case of infantile hypophosphatasia associated with recurrent pneumonia, a condition with few similar cases documented globally. Clinical exome sequencing identified a heterozygous mutation in the alkaline phosphatase (ALPL) gene (c.69_74del; p.Glu23_Lys24del), the first such case reported in India and classified as 'likely pathogenic'. Its causality remains unproven due to limited evidence, including the absence of in vitro studies and pedigree analysis. Phenotypic variability may be influenced by factors such as incomplete penetrance, variable expressivity and environmental or epigenetic modifiers. This case highlights a rare but important cause of recurrent pneumonia in infants. Despite treatment, the child succumbed to severe pneumonia within 2 months. Clinicians should consider infantile hypophosphatasia in cases of recurrent pneumonia, motor delay, seizures, severe malnutrition and persistently low serum alkaline phosphatase. Further genetic and functional studies are needed to validate genotype-phenotype correlations and improve disease management.

Here we report a Chinese infant with hypophosphatasia (HPP) carrying alkaline phosphatase (ALPL) gene mutations. Genetic analysis of the patient's ALPL gene revealed a maternally inherited canonical splice-site variant (c.997+1G>T; pathogenic; PVS1 + PM2 + PP4) and a paternally inherited missense variant (c.1405C>T, p.His469Tyr; reclassified as pathogenic; PP4 + PM2 + PP3). Both variants have previously been reported in gnomAD with very low frequency in Chinese infants. Hypophosphatasia is characterized by defective mineralization of growing or remodeling bone, with or without root-intact tooth loss, in the presence of low activity of serum and bone alkaline phosphatase (ALP). Biallelic ALPL pathogenic variants often result in severe hypophosphatasia that can result in stillbirth without mineralized bone, while heterozygous ALPL pathogenic variants are more likely to manifest as modest, mild, or even asymptomatic disease. Regardless of the number of ALPL pathogenic variants, many individuals with hypophosphatasia suffer from pain, disability, and reduced quality of life. Variability of clinical manifestations is common in both childhood and adult forms of hypophosphatasia and even occurs within affected families. While the disease spectrum is a continuum, seven clinical forms of hypophosphatasia are usually recognized based on age at diagnosis and severity of features. Perinatal (severe): Characterized by restrictive lung disease, respiratory failure, vitamin B6-dependent seizures, hypercalcemia with high morbidity, and mortality Perinatal (benign): Prenatal skeletal manifestations that slowly resolve into one of the milder forms Infantile: Onset between birth and age six months of clinical features of rickets without elevated serum ALP activity Severe childhood (juvenile): Variable presenting features progressing to rickets Mild childhood: Present later in childhood without rachitic disease, low bone mineral density for age, increased risk of fracture, and premature loss of primary teeth with intact roots Adult: Characterized by osteomalacia and stress fractures and pseudofractures of the lower extremities in middle age, sometimes associated with early loss of adult dentition. Adults with hypophosphatasia may also have significant bone pain and pronounced non-skeletal disease, with muscle weakness, dental problems, and reduced quality of life. Odontohypophosphatasia: Characterized by premature exfoliation of primary teeth and/or severe dental caries without skeletal manifestations The clinical diagnosis of hypophosphatasia can be established in a proband based on clinical diagnostic criteria. The molecular diagnosis of hypophosphatasia can be established in a proband with one major or two minor criteria and biallelic loss-of-function ALPL pathogenic variants or a heterozygous ALPL pathogenic variant with dominant-negative effect identified by molecular genetic testing. Targeted therapy: Asfotase alfa enzyme replacement therapy (ERT) Supportive care: For the perinatal (severe) type: expectant management and family support; respiratory support; management of calcium homeostasis and bone health per endocrinologist and orthopedist; pain management; neurosurgical management of craniosynostosis; management of kidney disease per nephrologist; dental care. For the infantile and early childhood (juvenile) types: respiratory support; management of calcium homeostasis and bone health per endocrinologist and orthopedist; pain management; treatment of seizures with vitamin B6; neurosurgical management of craniosynostosis; management of kidney disease per nephrologist; dental care. For all other types: dental care starting at age one year; nonsteroidal anti-inflammatory drugs for osteoarthritis, bone pain, and osteomalacia; internal fixation for pseudofractures and stress fractures. In adult hypophosphatasia, there is limited experience in treating osteomalacia with teriparatide. For all types: psychological support, social work support, and referral to mental health professionals as needed. Surveillance: Monitor calcium homeostasis and bone health per endocrinologist, nephrologist, and orthopedist; physical medicine and rehabilitation, physical therapy, and occupational therapy evaluations as needed; monitor children with infantile type for increased intracranial pressure secondary to craniosynostosis; renal ultrasound annually and nephrology evaluations as needed for kidney disease; neurology evaluations as needed for seizures; dental visits twice yearly starting at age one year. Agents/circumstances to avoid: Bisphosphonates, denosumab, and excess vitamin D; teriparatide is contraindicated in children. Pregnancy management: The use of asfotase alfa ERT during human pregnancy has not been extensively studied; therefore, any potential risk to the fetus of a pregnant woman taking this therapy during pregnancy is unknown. Perinatal and infantile hypophosphatasia are typically inherited in an autosomal recessive manner. Milder forms of hypophosphatasia, especially adult and odontohypophosphatasia, may be inherited in an autosomal recessive or autosomal dominant manner depending on the effect that the ALPL pathogenic variant has on alkaline phosphatase, tissue-nonspecific isozyme (TNSALP) activity. Autosomal recessive hypophosphatasia: If both parents are known to be heterozygous for an ALPL pathogenic variant, each sib of an affected individual has at conception a 25% chance of inheriting biallelic pathogenic variants and being affected, a 50% chance of being heterozygous, and a 25% chance of inheriting neither of the familial pathogenic variants. Depending on the ALPL pathogenic variant, heterozygous sibs may be either clinically asymptomatic (manifesting only biochemical abnormality) or have milder clinical manifestations than the proband. Autosomal dominant hypophosphatasia: Unless an individual with autosomal dominant hypophosphatasia has children with an individual who has a heterozygous or biallelic ALPL pathogenic variant(s), offspring have a 50% chance of inheriting the ALPL pathogenic variant. Once the ALPL pathogenic variant(s) have been identified in an affected family member, heterozygote testing for at-risk relatives and prenatal/preimplantation genetic testing for hypophosphatasia are possible. Recurrence of perinatal hypophosphatasia may reliably be identified by prenatal ultrasound examination.

Loss-of-function mutations in the tissue-nonspecific alkaline phosphatase (TNAP) gene can result in hypophosphatasia (HPP), an inherited multi-systemic metabolic disorder that is well-known for skeletal and dental hypomineralization. However, emerging evidence shows that both adult and pediatric patients with HPP suffer from cognitive deficits, higher measures of depression and anxiety, and impaired sensorimotor skills. The cerebellum plays an important role in sensorimotor coordination, cognition, and emotion. To date, the impact of TNAP mutation on the cerebellar circuitry development and function remains poorly understood. The main objective of this study was to investigate the roles of TNAP in cerebellar development and function, with a particular focus on Purkinje cells, in a mouse model of infantile HPP. Male and female wild type (WT) and TNAP knockout (KO) mice underwent behavioral testing on postnatal day 13-14 and were euthanized after completion of behavioral tests. Cerebellar tissues were harvested for gene expression and immunohistochemistry analyses. We found that TNAP mutation resulted in significantly reduced body weight, shorter body length, and impaired sensorimotor functions in both male and female KO mice. These developmental and behavioral deficits were accompanied by abnormal Purkinje cell morphology and dysregulation of genes that regulates synaptic transmission, cellular growth, proliferation, and death. In conclusion, inactivation of TNAP via gene deletion causes developmental delays, sensorimotor impairment, and Purkinje cell maldevelopment. These results shed light on a new perspective of cerebellar dysfunction in HPP.

Hypophosphatasia (HPP) is a genetic disease caused by loss-of-function mutations in ALPL, which encodes tissue-nonspecific alkaline phosphatase (ALP). Early diagnosis and treatment of perinatal and infantile HPP are important because of their high mortality rates. Enzyme replacement therapy (ERT) using human recombinant tissue-nonspecific ALP asfotase alfa was introduced in Korea in 2016. We report the first experience of ERT over 6 years for perinatal lethal and infantile HPP in Korea. The first patient was a 6-week-old Korean boy with a failure to thrive. The second patient was an 8-day-old Korean-Uzbek body with generalized tonic-clonic seizure with cyanosis. HPP was suspected in both patients because of the very low level of ALP activity and rachitic findings on radiographs, and the disease was confirmed by Sanger sequencing of the ALPL gene. The first patient with infantile HPP started ERT at 21 months of age and the second patient with perinatal HPP started ERT at 30 days of age. Both patients received asfotase alfa (2 mg/kg 3 times per week subcutaneously, adjusted to 3 mg/kg 3 times per week if required) for 6 years. After 6 years of ERT, radiographic findings and growth standard deviation scores improved in both patients. The second patient showed no evidence of rickets after 3 years of ERT. Mechanical respiratory support and supplemental oxygen were not required after 4.5 years of treatment in the first patient and at 2 months after treatment in the second patient. Among the 2 patients, the patient who started ERT early had a much better prognosis despite a more severe initial clinical presentation. Our results suggest that early diagnosis and prompt treatment play an important role in improving long-term prognosis and avoiding morbidity and premature mortality in patients with perinatal and infantile HPP.