SAT-382 Pyridoxal 5’-Phosphate Cerebrospinal Fluid Abnormalities in Hyopophosphatasia Before and After Enzyme Replacement Therapy

Introduction: Hypophosphatasia (HPP) is an inborn error of metabolism due to deficiency of tissue non-specific alkaline phosphatase (TNSALP), characterized by a wide range of skeletal and neurological symptoms. Little is known about the mechanism of neurologic involvement, but it may result from inefficient hydrolysis of vitamin B6 (pyridoxal 5’-phosphate, PLP), which is involved in neurotransmitter synthesis and is markedly elevated in HPP. However, it remains unknown what PLP levels are in the cerebrospinal fluid (CSF) of patients with HPP. We report two cases: (1) one with pre- and post-treatment CSF PLP levels and (2) one with pre-treatment CSF PLP and neurotransmitter levels. Case 1: A 30-year-old woman with extensive fracture history presented with seizure-like activity. Laboratory evaluation while on no B6 supplementation was notable for low ALP (14 U/L, ref: 40-150), high plasma PLP (362 mcg/L, ref: 5-50), and high CSF PLP (16 mcg/L), with a low CSF/plasma PLP ratio (0.04, ref: 0.1-0.7[1]). Genetic testing showed a pathogenic variant of TNSALP (Het c.1133A>T, p.Asp378Val, rs 121918008). After initiation of enzyme replacement therapy (ERT), her plasma and CSF PLP levels normalized to 6.3 and 1.4 mcg/L, respectively, with a ratio of 0.23. She reported improvement in energy level and seizure-like activity after several months of ERT. Case 2: A 34-year-old man with no history of skeletal pathology presented with diffuse musculoskeletal pain, weakness, and loss of sensory function attributed to generalized peripheral neuropathy. Laboratory evaluation while on no B6 supplementation was notable for low ALP (23 U/L), high plasma PLP (496 mcg/L), and high CSF PLP (17 mcg/L) with a low CSF/plasma ratio (0.03). CSF neurotransmitter levels were also low, including 5-hydroxyindoleacetic acid (40 nmol/L, ref: 67-140) and homovanillic acid (115 nmol/L, ref: 145-324). Genetic testing showed heterozygous expression of a catastrophic frameshift mutation of TNSALP (c.662del, p.Gly221Valfs*56). ERT is being considered at this time. Conclusions: We report the novel findings that in these patients with HPP: 1. CSF PLP levels are markedly elevated, though to a much lesser extent than plasma levels. Elevated CSF/plasma ratios suggest that transport of PLP and its metabolites across the blood brain barrier may be impaired in HPP. 2. Decreased CSF neurotransmitter levels, as seen in our patient, may contribute to neurologic symptoms of HPP. 3. Enzyme replacement therapy normalized both CSF and plasma PLP in case 1. Further studies are needed to understand how these changes affect patient symptoms and prognosis. Citation: 1. Albersen, M., et al., Vitamin B-6 vitamers in human plasma and cerebrospinal fluid. Am J Clin Nutr, 2014. 100(2): p. 587-92