Estimated prevalence of moderate to severely elevated total homocysteine levels in the United States: A missed opportunity for diagnosis of homocystinuria?

Classical homocystinuria (HCU) is a genetic disorder caused by mutations in the cystathionine beta synthase gene, which results in impaired metabolism of the sulfur-bearing amino acid homocysteine and its accumulation in blood and tissues. Classical HCU can be detected via newborn screening in the United States, but the test is widely acknowledged to miss many patients. While severely elevated homocysteine levels (>100 µmol /L) frequently lead to a classical HCU diagnosis, intermediate levels (>30 to 100 µmol /L), though linked to many of the known complications of HCU, are not always recognized as associated with HCU. We aimed to identify and describe potentially undiagnosed classical HCU patients using a nationally-representative database of administrative claims and laboratory results. We estimated the national prevalence of patients with homocysteine >30 µmol /L, and compared their demographic and clinical characteristics to those of patients with homocysteine levels ≤30 µmol/L. Among 57,580 patients with a homocysteine test result, 1.8% had a value >30 µmol /L. Patients with homocysteine >30 µmol /L were more frequently diagnosed with hypothyroidism (39.2% vs. 20.7%, p < .001) and renal disease (9.7% vs. 5.5%, p < .001), and were more likely to have a prescription for an anxiolytic/antidepressant (44.5% vs. 38.9%), opioid (58.4% vs. 53.1%), steroid (46.4% vs. 42.5%), or thyroid hormone (38.8% vs. 18.8%), compared to patients with homocysteine ≤30 µmol /L (all p < .05). Both groups were equally likely to have a diagnosis of homocystinuria or another disorder of sulfur-bearing amino acid metabolism (3.8% vs. 4.0%, p = .752). The age-adjusted national prevalence of homocysteine >30 µmol /L was estimated at 33,068 (95% CI: 1033 - 35,104). These findings suggest that thousands of people in the US may be living with intermediate to severely elevated homocysteine levels and may require further evaluation for the presence of classical HCU.

Prevalence, characteristics, and costs of diagnosed homocystinuria, elevated homocysteine, and phenylketonuria in the United States: a retrospective claims-based comparison.

BACKGROUND: Classical homocystinuria (HCU), an inborn error of homocysteine metabolism, has previously been estimated to affect approximately 1 in 100,000-200,000 people in the United States (US). HCU is poorly detected by newborn screening, resulting in underestimates of its prevalence. This study compared characteristics, healthcare use and costs, and projected prevalence between patients with diagnosed HCU, elevated total homocysteine (tHcy), and diagnosed phenylketonuria (PKU). METHODS: Patients in the MarketScan® Research Databases were identified with strictly-defined HCU (> 2 diagnoses, including 1 ICD-10), broadly-defined HCU (> 1 ICD-10), elevated tHcy (> 20 µmol/L) without an HCU diagnosis, or > 1 ICD-9/ICD-10 PKU diagnosis during 1/1/2010-12/31/2016 (first qualifying claim = index). Demographics and healthcare utilization and costs per patient per month (PPPM) were compared between all cohorts, frequencies of comorbidities and medications were compared between HCU and elevated tHcy patients, and healthcare provider types were assessed among HCU patients. The prevalence of patients meeting each cohort definition was projected to the United States (US) population. RESULTS: Patients with strictly-defined (N = 2450) and broadly-defined (N = 6613) HCU, and with elevated tHcy (N = 2017), were significantly older than PKU patients (N = 5120) (57 vs. 56 vs. 53 vs. 18 years; p < 0.05). Vitamin D deficiency, hyperlipidemia, folic acid/B vitamins, and lipid-lowering medications, among others, were more common among diagnosed HCU patients vs. those with elevated tHcy (all p < 0.05). Rates of healthcare utilization were generally higher among HCU and elevated tHcy patients, compared to PKU, though total healthcare costs were similar between groups. Most HCU patients (~ 38%) received their index diagnosis from a primary care physician; very few (~ 1%) had any claim from a geneticist during their enrollment. The age-adjusted national prevalence of HCU was projected at 31,162 (95% CI: 30,411 - 31,913; ~ 1 in 10,000 of the US population) using the broad definition. CONCLUSIONS: The actual prevalence of HCU may be > 10 times prior estimates, at 1 in 10,000 in the US, and this study suggests that HCU is not being diagnosed until later in life. Improvements to newborn screening, detection in young children, and physician education regarding HCU among patients may be necessary to alleviate the burden of this genetic disease.

Erythrocyte Encapsulated Thymidine Phosphorylase for the Treatment of Patients with Mitochondrial Neurogastrointestinal Encephalomyopathy: Study Protocol for a Multi-Centre, Multiple Dose, Open Label Trial.

Mitochondrial neurogastrointestinal encephalomyopathy (MNGIE) is an autosomal recessive disorder which primarily affects the gastrointestinal and nervous systems. This disease is caused by mutations in the nuclear TYMP gene, which encodes for thymidine phosphorylase, an enzyme required for the normal metabolism of deoxynucleosides, thymidine, and deoxyuridine. The subsequent elevated systemic concentrations of deoxynucleosides lead to increased intracellular concentrations of their corresponding triphosphates, and ultimately mitochondrial failure due to progressive accumulation of mitochondrial DNA (mtDNA) defects and mtDNA depletion. Currently, there are no treatments for MNGIE where effectiveness has been evidenced in clinical trials. This Phase 2, multi-centre, multiple dose, open label trial without a control will investigate the application of erythrocyte-encapsulated thymidine phosphorylase (EE-TP) as an enzyme replacement therapy for MNGIE. Three EE-TP dose levels are planned with patients receiving the dose level that achieves metabolic correction. The study duration is 31 months, comprising 28 days of screening, 90 days of run-in, 24 months of treatment and 90 days of post-dose follow-up. The primary objectives are to determine the safety, tolerability, pharmacodynamics, and efficacy of multiple doses of EE-TP. The secondary objectives are to assess EE-TP immunogenicity after multiple dose administrations and changes in clinical assessments, and the pharmacodynamics effect of EE-TP on clinical assessments.

Validation of an Immunoassay for Anti-thymidine Phosphorylase Antibodies in Patients with MNGIE Treated with Enzyme Replacement Therapy.

Erythrocyte encapsulated thymidine phosphorylase is recombinant Escherichia coli thymidine phosphorylase encapsulated within human autologous erythrocytes and is under development as an enzyme replacement therapy for the ultra-rare inherited metabolic disorder mitochondrial neurogastrointestinal encephalomyopathy. This study describes the method validation of a two-step bridging electrochemiluminescence immunoassay for the detection of anti-thymidine phosphorylase antibodies in human serum according to current industry practice and regulatory guidelines. The analytical method was assessed for screening cut point, specificity, selectivity, precision, prozone effect, drug tolerance, and stability. Key findings were a correction factor of 129 relative light units for the cut-point determination; a specificity cut point of 93% inhibition; confirmed intra-assay and inter-assay precision; assay sensitivity of 356 ng/mL; no matrix or prozone effects up to 25,900 ng/mL; a drug tolerance of 156 ng/mL; and stability at room temperature for 24 hr and up to five freeze-thaws. Immunogenicity evaluations of serum from three patients who received erythrocyte encapsulated thymidine phosphorylase under a compassionate treatment program showed specific anti-thymidine phosphorylase antibodies in one patient. To conclude, a sensitive, specific, and selective immunoassay has been validated for the measurement of anti-thymidine phosphorylase antibodies; this will be utilized in a phase II pivotal clinical trial of erythrocyte encapsulated thymidine phosphorylase.

Sulfatide levels correlate with severity of neuropathy in metachromatic leukodystrophy.

OBJECTIVE: Metachromatic leukodystrophy (MLD) is an autosomal recessive lysosomal storage disorder due to deficient activity of arylsulfatase A (ASA) that causes accumulation of sulfatide and lysosulfatide. The disorder is associated with demyelination and axonal loss in the central and peripheral nervous systems. The late infantile form has an early-onset, rapidly progressive course with severe sensorimotor dysfunction. The relationship between the degree of nerve damage and (lyso)sulfatide accumulation is, however, not established. METHODS: In 13 children aged 2-5 years with severe motor impairment, markedly elevated cerebrospinal fluid (CSF) and sural nerve sulfatide and lysosulfatide levels, genotype, ASA mRNA levels, residual ASA, and protein cross-reactive immunological material (CRIM) confirmed the diagnosis. We studied the relationship between (lyso)sulfatide levels and (1) the clinical deficit in gross motor function (GMFM-88), (2) median and peroneal nerve motor and median and sural nerve sensory conduction studies (NCS), (3) median and tibial nerve somatosensory evoked potentials (SSEPs), (4) sural nerve histopathology, and (5) brain MR spectroscopy. RESULTS: Eleven patients had a sensory-motor demyelinating neuropathy on electrophysiological testing, whereas two patients had normal studies. Sural nerve and CSF (lyso)sulfatide levels strongly correlated with abnormalities in electrophysiological parameters and large myelinated fiber loss in the sural nerve, but there were no associations between (lyso)sulfatide levels and measures of central nervous system (CNS) involvement (GMFM-88 score, SSEP, and MR spectroscopy). INTERPRETATION: Nerve and CSF sulfatide and lysosulfatide accumulation provides a marker of disease severity in the PNS only; it does not reflect the extent of CNS involvement by the disease process. The magnitude of the biochemical disturbance produces a continuously graded spectrum of impairments in neurophysiological function and sural nerve histopathology.

Development of a panel of highly sensitive, equivalent assays for detection of antibody responses to velaglucerase alfa or imiglucerase enzyme replacement therapy in patients with Gaucher disease.

Anti-drug antibodies are elicited by virtually all therapeutic proteins, and standardized assays are required for clinical monitoring of patients as well as for comparing antibody response to different therapeutic proteins in clinical trials. Velaglucerase alfa and imiglucerase are enzyme replacement therapies for the long-term treatment of type 1 Gaucher disease, a lysosomal storage disease resulting from an inherited deficiency of the enzyme glucocerebrosidase. We used state-of-the-art tools to develop a panel of assays for detection and characterization of antibody responses to velaglucerase alfa and imiglucerase. Highly-sensitive, direct bridging electrochemiluminescence screening assays were developed using samples from treatment-naïve individuals with type 1 Gaucher disease to set cut points. A mouse anti-glucocerebrosidase monoclonal antibody used as a calibrator was shown to have similar affinity and binding kinetics for anti-velaglucerase alfa and anti-imiglucerase antibodies. A quantitative radioimmunoprecipitation assay for IgG antibodies was developed to eliminate false-positives from the highly sensitive screening assay. Using 59 samples from treatment-naïve individuals with type 1 Gaucher disease, the confirmatory cut points were calculated to be 1.42 ng/mL for anti-velaglucerase alfa antibodies and 3.23 ng/mL for anti-imiglucerase antibodies. Isotype-specific indirect electrochemiluminescence assays were developed for IgE, IgA, and IgM subclasses. The IgE subclass assay was shown to be more sensitive than the confirmatory assay using sheep anti-glucocerebrosidase polyclonal antibody cross-linked with fragments specific to human IgE, with cut points for anti-velaglucerase alfa or anti-imiglucerase antibodies determined to be 0.53 and 0.55 ng/mL, respectively. An assay that detects inhibition in vitro of velaglucerase alfa and imiglucerase hydrolysis of a synthetic substrate in the presence of antibodies was developed to test for neutralizing antibodies. Using 52 individual healthy human donor samples and 35 samples from treatment-naïve individuals with type 1 Gaucher disease, cut points for the velaglucerase alfa and imiglucerase neutralizing antibody assays were determined to be 20%, such that a sample with greater than 20% inhibition of enzyme activity in the presence of antibodies was considered positive for neutralizing antibodies. In conclusion, highly sensitive and equivalent methods were developed and validated to directly compare antibody response to velaglucerase alfa and imiglucerase treatments in patients with Gaucher disease, and may contribute to future internationally standardized assays for antibody detection in patients with Gaucher disease.