Vitamin B6 deficiency with normal plasma levels of pyridoxal 5'-phosphate in perinatal hypophosphatasia

Markedly discordant hypophosphatasia in a young girl

Hypophosphatasia (HPP) is the inborn-error-of-metabolism from deactivating mutation(s) of ALPL, the gene that encodes the cell surface "tissue-nonspecific" isoenzyme of alkaline phosphatase (TNSALP). HPP's "biochemical signature" comprises low serum alkaline phosphatase activity together with elevated plasma levels of the TNSALP natural substrates phosphoethanolamine (PEA), pyridoxal 5'-phosphate (PLP), and inorganic pyrophosphate (PPi). Excess extracellular PPi (ePPi) inhibits mineralization and affected children prematurely shed deciduous teeth and often suffer weakness and rickets. Yet, HPP severity is greatest among all dento-osseous disorders and not fully explained by autosomal dominant versus autosomal recessive inheritance involving >470 ALPL mutations. Discordance of HPP phenotype sometimes manifests even among full siblings sharing an identical ALPL genotype. Herein, a girl's markedly discordant HPP featured at presentation life-threatening hypercalcemia, failure-to-thrive, and renal compromise. Subsequent pseudotumor cerebri syndrome caused blindness, and then craniosynostosis required cranial vault reconstruction. However, she was not deformed, had moderate hypophosphatasemia, normal plasma PLP level, and mild radiographic features of HPP rickets. Elevated plasma N-terminal parathyroid hormone-related protein (PTHrP) suggested malignancy, but corrected after kidney transplantation. HPP was diagnosed when whole exome sequencing revealed heterozygous ALPL c.1034C>T, p.A345V reported in mild pediatric HPP and transmitted by her mother who considered herself well. Genes conditioning ePPi formation and underlying other skeletal diseases were intact. Hypercalcemia, unresponsive to bone antiresorptive drugs, corrected promptly with asfotase alfa TNSALP supplementation therapy. Her markedly discordant findings highlight genotype/phenotype plasticity for pediatric HPP, and her clinical course importance for early diagnosis.

Vitamin B6 Status in Hypophosphatasia: Association With Clinical Severity, Diagnostic Utility, and Effects on Vitamin B6 Metabolism by Supplementation and Enzyme Replacement Therapy

This study reports the concentrations of major vitamin B6 (VB6) vitamers (pyridoxal 5'-phospate [PLP], pyridoxal [PL], and 4-pyridoxic acid [PA]) in over 100 hypophosphatasia (HPP) cases to explore clues for clinical severity, to improve diagnostic sensitivity, and to examine VB6 changes by supplementation and enzyme replacement therapy. Serum samples were collected from HPP and non-HPP patients. When available, cerebrospinal fluid (CSF) samples were also analyzed to determine PLP, PL, and PA concentrations. Serum PLP concentrations, PLP-to-PL ratios, and PLP-to-PA ratios were higher in untreated HPP patients compared to non-HPP patients and reflected clinical severity. Perinatal severe HPP showed lower PL concentrations than perinatal benign HPP and the lowest PA concentrations among all clinical forms. Combining PLP-to-PL ratios with PLP concentrations improved diagnostic sensitivity. Under VB6 therapy, PLP concentrations and PLP-to-PL ratios remained higher in HPP patients than in non-HPP patients. VB6 changes produced by ERT were most clearly reflected by the reduction of PLP-to-PL ratios. CSF PLP concentrations and PLP-to-PL ratios were higher in untreated HPP patients than in non-HPP patients, while CSF PL concentrations were higher in patients on ERT compared to those not on ERT. Co-evaluation of serum PLP, PL, and PA concentrations may help understand clinical severity, can improve diagnostic sensitivity, and can demonstrate the effect of ERT on VB6 metabolism more effectively than the assay of PLP concentrations alone. Altered VB6 status in the CSF suggests that tissue-nonspecific alkaline phosphatase plays a key role in VB6 transport from peripheral blood to the brain via the blood-CSF barrier.

Disease burden by ALPL variant number in patients with non-life-threatening hypophosphatasia in the Global HPP Registry

BACKGROUND

Hypophosphatasia (HPP) is a rare metabolic disease caused by autosomal dominant or recessive inheritance of ALPL variants resulting in low alkaline phosphatase activity. The objective of this analysis was to compare HPP disease burden between patients with non-life-threatening disease in the Global HPP Registry who have one ALPL variant versus two or more ALPL variants.

METHODS

Patients were included if they had one or more ALPL variants identified through genetic testing and first HPP manifestations after 6 months of age. Assessments included history of HPP manifestations, Brief Pain Inventory-Short Form (BPI-SF), Health Assessment Questionnaire-Disability Index (HAQ-DI), 6-Min Walk Test (6MWT), Paediatric Quality of Life Inventory (PedsQL) and 36-Item Short-Form Survey V.2 (SF-36v2).

RESULTS

Of 685 included patients, 568 (82.9%) had one ALPL variant, 116 (16.9%) had two variants, and one (0.1%) had three variants. Patients with two or more ALPL variants had higher proportions of skeletal (52.1% vs 32.6%), dental (73.5% vs 56.0%), muscular (36.8% vs 23.6%) and neurological (22.2% vs 8.8%) manifestations at last assessment. BPI-SF, HAQ-DI, PedsQL and SF-36v2 scores were similar between groups. Distances walked on the 6MWT were similar between groups for children. Distance walked was lower among adults with two or more variants (293 m (n=8)) than adults with one variant (466 m (n=103)), although the former group was very small.

CONCLUSION

HPP disease burden is high in patients with HPP, regardless of ALPL variant number. While prevalence of HPP-specific manifestations was higher in patients with two or more variants than those with one variant, patient-reported outcomes were similar between groups.

TRIAL REGISTRATION NUMBER

NCT02306720; EUPAS13514.

Pyridoxine challenge reflects pediatric hypophosphatasia severity and thereby examines tissue-nonspecific alkaline phosphatase's role in vitamin B6 metabolism

Alkaline phosphatase (ALP) is detected in most human tissues. However, ALP activity is routinely assayed using high concentrations of artificial colorimetric substrates in phosphate-free laboratory buffers at lethal pH. Hypophosphatasia (HPP) is the inborn-error-of-metabolism caused by loss-of-function mutation(s) of the ALPL gene that encodes the ALP isoenzyme expressed in bone, liver, kidney, and elsewhere and is therefore designated "tissue-nonspecific" ALP (TNSALP). Consequently, HPP harbors clues concerning the biological function of this phosphohydrolase that is anchored onto the surface of cells. The biochemical signature of HPP features low serum ALP activity (hypophosphatasemia) together with elevated plasma levels of three natural substrates of TNSALP: i) phosphoethanolamine (PEA), a component of the linkage apparatus that binds ALPs and other proteins to the plasma membrane surface; ii) inorganic pyrophosphate (PPi), an inhibitor of bone and tooth mineralization; and iii) pyridoxal 5'-phosphate (PLP), the principal circulating vitameric form of vitamin B6 (B6). Autosomal dominant and autosomal recessive inheritance involving several hundred ALPL mutations underlies the remarkably broad-ranging expressivity of HPP featuring tooth loss often with muscle weakness and rickets or osteomalacia. Thus, HPP associates the "bone" isoform of TNSALP with biomineralization, whereas the physiological role of the "liver", "kidney", and other isoforms of TNSALP remains uncertain. Herein, to examine HPP's broad-ranging severity and the function of TNSALP, we administered an oral challenge of pyridoxine (PN) hydrochloride to 116 children with HPP. We assayed both pre- and post-challenge serum ALP activity and plasma levels of PLP, the B6 degradation product pyridoxic acid (PA), and the B6 vitamer pyridoxal (PL) that can enter cells. Responses were validated by PN challenge of 14 healthy adults and 19 children with metabolic bone diseases other than HPP. HPP severity was assessed using our HPP clinical nosology and patient height Z-scores. PN challenge of all study groups did not alter serum ALP activity in our clinical laboratory. In HPP, both the post-challenge PLP level and the PLP increment correlated (Ps < 0.0001) with the clinical nosology and height Z-scores (Rs = +0.6009 and + 0.4886, and Rs = -0.4846 and - 0.5002, respectively). In contrast, the plasma levels and increments of PA and PL from the PN challenge became less pronounced with HPP severity. We discuss how our findings suggest extraskeletal TNSALP primarily conditioned the PN challenge responses, and explain why they caution against overzealous B6 supplementation of HPP.

Cronobacter sakazakii Pyridoxal Kinase PdxY Mediated by TreR and pESA3 Is Essential for Vitamin B6 (PLP) Maintenance and Virulence

Cronobacter sakazakii is an opportunistic pathogen capable of causing severe infections, particularly in neonates. Despite the bacterium's strong pathogenicity, the pathogenicity of C. sakazakii is not yet well understood. Using a comparative proteomic profiling approach, we successfully identified pdxY, encoding a pyridoxal kinase involved in the recycling of pyridoxal 5'-phosphate (PLP), as a gene essential for the successful pathogenesis of C. sakazakii. Knocking out the pdxY gene resulted in slower growth and reduced virulence. Our study sheds light on the fundamental importance of pyridoxal kinase for the survival and virulence of C. sakazakii. The identification of pdxY as gene essential for successful pathogenesis provides a potential target for the development of new antibiotic treatments. IMPORTANCE The opportunistic pathogen Cronobacter sakazakii is known to cause severe infections, particularly in neonates, and can result in high mortality rates. In this study, we used a comparative proteomic profiling approach to identify genes essential for the successful pathogenesis of C. sakazakii. We successfully identified pdxY, encoding a pyridoxal kinase involved in the salvage pathway of pyridoxal 5'-phosphate (PLP), as a gene essential for the successful pathogenesis of C. sakazakii. Knocking out the pdxY gene resulted in impaired growth and reduced virulence. This study sheds light on the fundamental importance of pyridoxal kinase for the survival and virulence of C. sakazakii, which can be a potential target for the development of new antibiotic treatments. This study highlights the importance of comparative proteomic profiling in identifying virulence factors that can be targeted for the development of new antibiotics.

Diagnostic Approach to Patients with Low Serum Alkaline Phosphatase

Increased serum levels of alkaline phosphatase (ALP) are widely recognized as a biochemical marker of many disorders affecting the liver or bone. However, the approach for patients with low ALP phosphatase is not well-established. Low serum ALP is an epiphenomenon of many severe acute injuries and diseases. Persistently low serum ALP may be secondary to drug therapy (including antiresorptives) or a variety of acquired disorders, such as malnutrition, vitamin and mineral deficiencies, endocrine disorders, etc. Hypophosphatasia, due to pathogenic variants of the ALPL gene, which encodes tissue non-specific ALP, is the most common genetic cause of low serum ALP. Marked bone hypomineralization is frequent in severe pediatric-onset cases. However, adult forms of hypophosphatasia usually present with milder manifestations, such as skeletal pain, chondrocalcinosis, calcific periarthritis, dental problems, and stress fractures. The diagnostic approach to these patients is discussed. Measuring several ALP substrates, such as pyrophosphate, pyridoxal phosphate, or phosphoethanolamine, may help to establish enzyme deficiency. Gene analysis showing a pathogenic variant in ALPL may confirm the diagnosis. However, a substantial proportion of patients show normal results after sequencing ALPL exons. It is still unknown if those patients carry unidentified mutations in regulatory regions of ALPL, epigenetic changes, or abnormalities in other genes.

Urine phosphoethanolamine is a specific biomarker for hypophosphatasia in adults

OBJECTIVES

We investigated the utility of urine phosphoethanolamine (PEA) as a marker to aid in diagnosing and/or confirming hypophosphatasia (HPP) in adults and for monitoring patients on enzyme replacement therapy (ERT).

METHODS

Data was collected from seventy-eight adults who were referred to the Vanderbilt Program for Metabolic Bone Disease for evaluation of a possible or confirmatory HPP diagnosis between July 2014 through December 2019. Fifty-nine patients were diagnosed with HPP and nineteen were excluded from a diagnosis of HPP. The urine PEA results of those patients with a confirmed diagnosis of HPP and those patients with a diagnosis of HPP excluded were captured and compared to other laboratory and clinical parameters consistent with HPP, including alkaline phosphatase (ALP) activity, plasma pyridoxal 5'-phosphate (PLP), the presence of musculoskeletal abnormalities, and genetic testing for pathogenic mutations in ALPL.

RESULTS

Initial urine PEA values in patients in our HPP cohort and not on ERT were significantly higher (median = 150.0 nmol/mg creatinine, IQR = 82.0-202.0) compared patients in our HPP negative group (median 18.0 nmol/mg creatinine, IQR = 14.0-30.0, p < 0.0001) and higher than patients on ERT (median 65.0 nmol/mg creatinine, IQR = 45.3-79.8). Patients who began ERT had a decline in urine PEA levels after treatment with a mean decrease of 68.1 %. Plasma ALP levels were significantly lower in the group of patients with HPP and not on ERT group (median = 24.0 U/L, IQR = 15.0-29.50) compared to the patients without HPP (median = 45.50 U/L, IQR = 34.0-62.0;) and plasma PLP levels were significantly higher in the HPP non-ERT group (median = 284.0 nmol/L, IQR = 141.0-469.4) compared to the patients without HPP (median = 97.5 nmol/L, IQR = 43.7-206.0;). The area under the curve (AUC) of urine PEA, ALP, and PLP to distinguish between HPP and non-HPP patients is 0.968, 0.927 and 0.781, respectively, in our cohort. Urine PEA had 100 % specificity (95 % CI of 83.2 % to 100.0 %) for diagnosing HPP at a value >53.50 nmol/mg creatinine with a sensitivity of 88.4 %; 95%CI 75.5 to 94.9 %. ALP had a 100 % specificity (95 % CI of 82.4 % to 100.0 %) for diagnosing HPP at a value <30.5 U/L with a sensitivity of 77.2 %; (95%CI 64.8 to 86.2 %). PLP had a 100 % specificity (95 % CI of 81.6 % to 100.0 %) for diagnosing HPP at a value >436 nmol/L with a sensitivity of 26.9 %; (95%CI 16.8 to 40.3 %). The most common pathogenic or likely pathogenic mutations in our cohort were c.1250A>G (p.Asn417Ser), c.1133A>T (p.Asp378Val), c.881A>C (p.Asp294Ala), c.1171C>T (p.Arg391Cys), and c.571G>A, (p.Glu191Lys).

CONCLUSIONS

Urine PEA is a promising diagnostic and confirmatory marker for HPP in patients undergoing investigation for HPP. Urine PEA also has potential use as a marker to monitor ERT compliance. Future studies are necessary to evaluate the association between PEA levels and clinical outcomes.

Hypophosphatasia: Vitamin B6 status of affected children and adults

Hypophosphatasia (HPP) is the heritable dento-osseous disease caused by loss-of-function mutation(s) of the gene ALPL that encodes the tissue-nonspecific isoenzyme of alkaline phosphatase (TNSALP). TNSALP is a cell-surface homodimeric phosphomonoester phosphohydrolase expressed in healthy people especially in the skeleton, liver, kidneys, and developing teeth. In HPP, diminished TNSALP activity leads to extracellular accumulation of its natural substrates including inorganic pyrophosphate (PPi), an inhibitor of mineralization, and pyridoxal 5'-phosphate (PLP), the principal circulating form of vitamin B₆ (B₆). Autosomal dominant and autosomal recessive inheritance involving >450 usually missense defects scattered throughout ALPL largely explains the remarkably broad-ranging severity of this inborn-error-of-metabolism. In 1985 when we identified elevated plasma PLP as a biochemical hallmark of HPP, all 14 investigated affected children and adults had markedly increased PLP levels. However, pyridoxal (PL), the dephosphorylated form of PLP that enters cells to cofactor many enzymatic reactions, was not low but often inexplicably elevated. Levels of pyridoxic acid (PA), the B₆ degradation product quantified to assess B₆ sufficiency, were unremarkable. Canonical signs or symptoms of B₆ deficiency or toxicity were absent. B₆-dependent seizures in infants with life-threatening HPP were later explained by their profound deficiency of TNSALP activity blocking PLP dephosphorylation to PL and diminishing gamma-aminobutyric acid synthesis in the brain. Now, there is speculation that altered B₆ metabolism causes further clinical complications in HPP. Herein, we assessed the plasma PL and PA levels accompanying previously reported elevated plasma PLP concentrations in 150 children and adolescents with HPP. Their mean (SD) plasma PL level was nearly double the mean for our healthy pediatric controls: 66.7 (59.0) nM versus 37.1 (22.2) nM (P < 0.0001), respectively. Their PA levels were broader than our pediatric control range, but their mean value was normal; 40.2 (25.1) nM versus 39.3 (9.9) nM (P = 0.7793), respectively. In contrast, adults with HPP often had plasma PL and PA levels suggestive of dietary B₆ insufficiency. We discuss why the B₆ levels of our pediatric patients with HPP would not cause B₆ toxicity or deficiency, whereas in affected adults dietary B₆ insufficiency can develop.