New insights in the genetic variant spectrum of SLC34A2 in pulmonary alveolar microlithiasis; a systematic review.

Pulmonary alveolar microlithiasis (PAM) is a rare autosomal recessive lung disease caused by variants in the SLC34A2 gene encoding the sodium-dependent phosphate transport protein 2B, NaPi-2b. PAM is characterized by deposition of calcium phosphate crystals in the alveoli. Onset and clinical course vary considerably; some patients remain asymptomatic while others develop severe respiratory failure with a significant symptom burden and compromised survival. It is likely that PAM is under-reported due to lack of recognition, misdiagnosis, and mild clinical presentation. Most patients are genetically uncharacterized as the diagnostic confirmation of PAM has traditionally not included a genetic analysis. Genetic testing may in the future be the preferred tool for diagnostics instead of invasive methods. This systematic review aims to provide an overview of the growing knowledge of PAM genetics. Rare variants in SLC34A2 are found in almost all genetically tested patients. So far, 34 allelic variants have been identified in at least 68 patients. A majority of these are present in the homozygous state; however, a few are found in the compound heterozygous form. Most of the allelic variants involve only a single nucleotide. Half of the variants are either nonsense or frameshifts, resulting in premature termination of the protein or decay of the mRNA. There is currently no cure for PAM, and the only effective treatment is lung transplantation. Management is mainly symptomatic, but an improved understanding of the underlying pathophysiology will hopefully result in development of targeted treatment options. More standardized data on PAM patients, including a genetic diagnosis covering larger international populations, would support the design and implementation of clinical studies to the benefit of patients. Further genetic characterization and understanding of how the molecular changes influence disease phenotype will hopefully allow earlier diagnosis and treatment of the disease in the future.

Impaired phosphate transport in SLC34A2 variants in patients with pulmonary alveolar microlithiasis.

BACKGROUND: Variants in SLC34A2 encoding the sodium-dependent phosphate transport protein 2b (NaPi-IIb) cause the rare lung disease pulmonary alveolar microlithiasis (PAM). PAM is characterised by the deposition of calcium-phosphate concretions in the alveoli usually progressing over time. No effective treatment is available. So far, 30 allelic variants in patients have been reported but only a few have been functionally characterised. This study aimed to determine the impact of selected SLC34A2 variants on transporter expression and phosphate uptake in cellular studies. METHODS: Two nonsense variants (c.910A > T and c.1456C > T), one frameshift (c.1328delT), and one in-frame deletion (c.1402_1404delACC) previously reported in patients with PAM were selected for investigation. Wild-type and mutant c-Myc-tagged human NaPi-IIb constructs were expressed in Xenopus laevis oocytes. The transport function was investigated with a 32Pi uptake assay. NaPi-IIb protein expression and localisation were determined with immunoblotting and immunohistochemistry, respectively. RESULTS: Oocytes injected with the wild-type human NaPi-IIb construct had significant 32Pi transport compared to water-injected oocytes. In addition, the protein had a molecular weight as expected for the glycosylated form, and it was readily detectable in the oocyte membrane. Although the protein from the Thr468del construct was synthesised and expressed in the oocyte membrane, phosphate transport was similar to non-injected control oocytes. All other mutants were non-functional and not expressed in the membrane, consistent with the expected impact of the truncations caused by premature stop codons. CONCLUSIONS: Of four analysed SLC34A2 variants, only the Thr468del showed similar protein expression as the wild-type cotransporter in the oocyte membrane. All mutant transporters were non-functional, supporting that dysfunction of NaPi-IIb underlies the pathology of PAM.

Pulmonary Alveolar Microlithiasis - A Review.

Pulmonary Alveolar Microlithiasis (PAM) is a rare genetic disorder causing widespread deposition of calcium-phosphate crystals in the alveolar space. A hallmark of the disease is the discrepancy between perceived symptoms upon diagnosis compared with the extensive, sandstorm-like appearance of the microliths on chest X-ray or HRCT. Caused by a defective sodium-dependent phosphate transport protein due to loss-of-function variants of the SLC34A2 gene, PAM is an autosomal recessive transmitted disorder, and as such has a high correlation to consanguinity. The most common variants of the SLC34A2 gene are single nucleotide biallelic changes, but larger deletions are described. Initial suspicion of PAM on radiological examination should be followed by genetic testing to verify the diagnosis and identify the disease-causing variant. When not available, the diagnosis can be made by means of invasive techniques, such as transbronchial forceps or cryobiopsy, or a surgical lung biopsy. In families with a history of PAM, genetic counseling should be offered, as well as preimplantation/prenatal testing if necessary. As of writing this review, no definitive treatment exists, and PAM may in some cases progress to severe pulmonary disease with respiratory failure and potential death. Patients with PAM should be offered preventative and symptomatic treatments such as vaccinations and oxygen therapy when needed. In some cases, lung transplantation may be required.

Pulmonary alveolar microlithiasis: no longer in the stone age.

Pulmonary alveolar microlithiasis (PAM) is a rare parenchymal lung disease caused by variants in the SCL34A2 gene and characterised by the accumulation of intra-alveolar microliths. PAM has been reported in fewer than 1100 cases throughout the world. It is an autosomal recessive hereditary disease and often associated with consanguinity. Progress with respect to the genetic background and pathophysiology has resulted in an increased understanding of the disease in recent years. Until now, 30 genetic different SLC34A2 variants have been reported, which all are considered significant for disease development. There is no sex difference and the majority of cases are diagnosed at the age of 30-40 years. Many patients are asymptomatic and the diagnosis is made at random. When symptomatic, dyspnoea, cough, chest pain and fatigue are common complaints. The diagnosis of PAM can confidently be based on typical radiographic findings and genetic testing proving rare biallelic SCL34A2 gene variants. Bronchoalveolar lavage and histopathology may show microliths. There is no disease-specific treatment and management is supportive. Lung transplantation should be considered in advanced cases.

Eight novel variants in the SLC34A2 gene in pulmonary alveolar microlithiasis.

BACKGROUND: Pulmonary alveolar microlithiasis (PAM) is caused by genetic variants in the SLC34A2 gene, which encodes the sodium-dependent phosphate transport protein 2B (NaPi-2b). PAM is characterised by deposition of calcium phosphate concretions (microliths) in the alveoli leading to pulmonary dysfunction. The variant spectrum of SLC34A2 has not been well investigated and it is not yet known whether a genotype-phenotype correlation exists. METHODS: We collected DNA from 14 patients with PAM and four relatives, and analysed the coding regions of SLC34A2 by direct DNA sequencing. To determine the phenotype characteristics, clinical data were collected and a severity score was created for each variant, based on type and localisation within the protein. RESULTS: We identified eight novel allelic variants of SLC34A2 in 14 patients with PAM. Four of these were nonsense variants, three were missense and one was a splice site variant. One patient was heterozygous for two different variants and all other patients were homozygous. Four patients were asymptomatic and 10 patients were symptomatic. The severity of the disease was associated with the variant severity. CONCLUSIONS: Our findings support a significant role for SLC34A2 in PAM and expand the variant spectrum of the disease. Thus, SLC34A2 variants were detected in all patients and eight novel allelic variants were discovered. An association between disease severity and the severity of the variants was found; however, this needs to be investigated in larger patient populations.

Pulmonary alveolar microlithiasis: two case reports and review of the literature.

Pulmonary alveolar microlithiasis is a rare diffuse lung disease characterised by deposition of calcium phosphate within the alveolar airspaces. The disease is usually discovered from birth up to 40 yrs of age and is often diagnosed incidentally during radiography of the chest for other reasons. Many patients are asymptomatic and the majority of patients either have normal or restrictive pulmonary function. The clinical course of the disease varies. While it remains static in some patients, it progresses into pulmonary fibrosis, respiratory failure and cor pulmonale in others. With the exception of lung transplantation, there is no known effective treatment for the disease. Although the aetiology remains unclear, mutations of the solute carrier family 34 (sodium phosphate), member 2 gene (the SLC34A2 gene), which encodes a sodium/phosphate co-transporter, are considered to be the cause of the disease. We present two cases of pulmonary alveolar microlithiasis with different mutations in the SLC34A2 gene that have not been previously described, and a review of the literature.