Hb Tacoma: G>T or G>C, and Does It Matter?

Hb Tacoma [ß30(B12)Arg→Ser] is a missense variant that is caused by either an AGG>AGT or AGG>AGC substitution at codon 30 of the HBB gene. Currently, the latter is classified as a rare cause of ß0-thalassemia (ß0-thal). We propose that HBB: c.93G>C has been incorrectly assigned as ß0-thal and discuss whether HBB: c.93G>T or HBB: c.93G>C should be classified as ß+-thal instead, or as ß-globin variants without thalassemic effect. We present several subjects who are heterozygous for Hb Tacoma, one with HBB: c.93G>T and two with HBB: c.93G>C, to support our conclusions.

Hb Waikato [α127(H10)Lys→Gln; HBA1: c.382A>C]: A Novel High Oxygen Affinity Variant.

We report the identification of a novel, high oxygen affinity hemoglobin (Hb) variant [α127(H10)Lys→Gln; HBA1: c.382A>C]. The variant was detected in an adolescent male (proband) of Syrian descent by cation exchange high performance liquid chromatography (HPLC), during Hb A1c analysis. A complete blood count (CBC) showed elevated red blood cells (RBCs) (6.08 × 1012/L), Hb (16.1 g/dL) and packed cell volume (PCV) (0.48 L/L). Capillary electrophoresis (CE) revealed the variant was more negatively charged and represented 18.2% of total Hb. Isopropanol stability was normal. Cyanosis in the subject prompted investigation of oxygen affinity, with a reduced p50 of 20.8 mm Hg and a left shifted oxygen dissociation curve demonstrating increased oxygen affinity. We propose the novel variant be named Hb Waikato, which reflects the Hospital Laboratory where the variant was discovered and region where the proband was born and herein describe characterization.

Novel α0-Thalassemia Deletion Identified in an Indian Infant with Hb H Disease.

We report the identification of a large deletion of the α-globin gene cluster, which removed both HBA2 and HBA1 and included the region from HBZ to HBQ1 on chromosome 16 (16p13.3). The α0-thalassemia (α0-thal) deletion was discovered in an Indian family residing in New Zealand. The proband was a 3-month-old female, who presented with a Hb H disease of unknown molecular origin. Routine hematology showed marked hypochromic microcytic anemia, with numerous Hb H inclusion bodies. In the absence of iron deficiency, there was a strong clinical suspicion of α-thal. On initial screening using a multiplex gap polymerase chain reaction (gap-PCR), only the common rightward deletion (-α3.7) was detected. Investigation of the proband's mother and father revealed the mother was heterozygous for the -α3.7 deletion, while none of the seven most common pathogenic α-thal deletions were detected in the father. Multiplex ligation-dependent probe amplification (MLPA) was employed to detect the presence of a novel α0-thal deletion in both the proband and her father. For the proband, the α0-thal deletion in combination with the -α3.7 deletion, eliminated three copies of HBA consistent with a clinical diagnosis of Hb H disease.

An intact C-terminal end of albumin is required for its long half-life in humans.

Albumin has an average plasma half-life of three weeks and is thus an attractive carrier to improve the pharmacokinetics of fused therapeutics. The half-life is regulated by FcRn, a cellular receptor that protects against intracellular degradation. To tailor-design the therapeutic use of albumin, it is crucial to understand how structural alterations in albumin affect FcRn binding and transport properties. In the blood, the last C-terminal residue (L585) of albumin may be enzymatically cleaved. Here we demonstrate that removal of the L585 residue causes structural stabilization in regions of the principal FcRn binding domain and reduces receptor binding. In line with this, a short half-life of only 3.5 days was measured for cleaved albumin lacking L585 in a patient with acute pancreatitis. Thus, we reveal the structural requirement of an intact C-terminal end of albumin for a long plasma half-life, which has implications for design of albumin-based therapeutics.

Conjugation of urate-derived electrophiles to proteins during normal metabolism and inflammation.

Urate is often viewed as an antioxidant. Here, we present an alternative perspective by showing that, when oxidized, urate propagates oxidative stress. Oxidation converts urate to the urate radical and the electrophilic products dehydrourate, 5-hydroxyisourate, and urate hydroperoxide, which eventually break down to allantoin. We investigated whether urate-derived electrophiles are intercepted by nucleophilic amino acid residues to form stable adducts on proteins. When urate was oxidized in the presence of various peptides and proteins, two adducts derived from urate (Mr 167 Da) were detected and had mass additions of 140 and 166 Da, occurring mainly on lysine residues and N-terminal amines. The adduct with a 140-Da mass addition was detected more frequently and was stable. Dehydrourate (Mr 166 Da) also formed transient adducts with cysteine residues. Urate-derived adducts were detected on human serum albumin in plasma of healthy donors. Basal adduct levels increased when neutrophils were added to plasma and stimulated, and relied on the NADPH oxidase, myeloperoxidase, hydrogen peroxide, and superoxide. Adducts of oxidized urate on serum albumin were elevated in plasma and synovial fluid from individuals with gout and rheumatoid arthritis. We propose that rather than acting as an antioxidant, urate's conversion to electrophiles contributes to oxidative stress. The addition of urate-derived electrophiles to nucleophilic amino acid residues, a process we call oxidative uratylation, will leave a footprint on proteins that could alter their function when critical sites are modified.

Hb Amsterdam-A1 [α32(B13)Met→Ile; HBA1: c.99G>A]: A Hyperunstable Variant Due to a New Mutation on the α1 Gene.

Patients with hyperunstable α chain variants usually present with a thalassemic, rather than hemolytic, phenotype. Electrophoretic, ion exchange and reverse phase separations usually fail to detect the variant and when DNA sequencing identifies a 'silent' substitution it is usually presumed to be hyperunstable. We report the identification of such a variant, α32(B13)Met→Ile; HBA1: c.99G>A, arising from a new mutation on the α1 gene. The hemoglobin (Hb) was unequivocally detected by the isopropanol stability test and confirmed as hyperunstable by mass spectrometry (MS) of the precipitate and lysate, which showed proportions of 55% and 2.5% of α chains, respectively. The instability appears to be driven by perturbation of globin-heme, and possibly α1ß1 subunit, interactions.

Direct analysis of VLDL by TOF-MS allows rapid definition of Apo E genotypes and facilitates characterisation of post translational changes.

BACKGROUND: Apolipoprotein E (Apo E) is a glycoprotein which acts as a ligand facilitating the uptake of lipids. Three common isoforms of Apo E are recognised, E2, E3 and E4. E2 and E4 are associated with altered lipid metabolism and increased cardiovascular risk. We report a novel variant of Apo E (c.382G>A) predicting 110Asp→Asn identified by genotyping, we were prompted to investigate this further as the amino acid substitution produced a prospective N-glycosylation site in this novel variant. METHODS: We present a new rapid approach to genotyping Apo E performed by electrospray TOF-MS, on the same sample analysed by ultracentrifugation. The analysis can be performed in <10min and requires minimal sample volume. Control samples were used to verify the analysis. RESULTS: Spectra showed the expected mass for the E3 isoform at 34,237Da, E2 and E4 isoforms were identifiable by peaks at -53Da and +53Da respectively. Post translational glycosylation of the protein can also be identified. The novel isoform had a mass of 34,237Da without evidence of N-glycosylation. No significant effect on lipid metabolism was identified. CONCLUSION: The electrospray TOF-MS approach potentially provides a rapid alternative method for genotyping Apo E and for the investigation of novel isoforms.

Unique albumin with two silent substitutions (540Thr→Ala and 546Ala→Ser): Insights into how albumin is recycled.

OBJECTIVES: To determine the cause of an albumin abnormality detected by chance on electrospray time-of-flight mass spectrometry (TOF MS) of whole plasma, and to assess its physiological consequences. METHOD: Plasma was examined by TOF MS and tryptic mapping was used to locate mutation sites and determine the relative expression level of the variant and normal albumins. DNA sequencing was used to precisely define mutations. RESULTS: Whole protein electrospray TOF MS indicated a decrease of 14Da in the mass of albumin. Peptide mass mapping and DNA sequencing established the presence of two novel heterozygous point mutations (540Thr→Ala and 546Ala→Ser) whose combined mass changes (-30 and +16Da) indicated both mutations occurred on the same allele. Peptide ratios showed the variant albumin was present at a lower level than normal with an expression ratio of approximately 1:2 (variant:normal). Phylogenetic sequence alignments show Thr540 is highly conserved while Ala546 has wide species variation, suggesting 540Thr→Ala might compromise the protein. CONCLUSION: Both mutations occur close together in domain IIIB, a region involved in albumin scavenging and recycling. In particular, Thr540 is close to His535, a residue directly involved in pH-dependent binding and release of albumin from its recycling neonatal Fc receptor. Compromised receptor binding would explain the low albumin (34g/l) concentration and the diminished variant expression level.

Bisalbuminaemia due to novel mutation at a critical residue involved in recycling; Albumin Lyon (510His→Arg).

OBJECTIVES: To define the underlying cause of bisalbuminaemia in an individual presenting with spontaneous venous thrombosis. METHOD: Plasma was examined by electrospray time-of-flight mass spectrometry (TOF MS) to assess albumin mutations and to quantify variant expression level. Tryptic peptide mapping and DNA sequencing were used to precisely define the mutation. RESULTS: Whole protein MS indicated a 19Da increase in the mass of 50% of the albumin molecules suggesting a His→Arg substitution. A novel heterozygous 510His→Arg mutation was identified by peptide mass mapping and confirmed by DNA sequencing of exon 12 of the albumin gene. CONCLUSION: The nature and location of the mutation suggest it would have no direct influence on haemostasis through altered warfarin binding or increased fibrinogen attachment and it appears to be incidental to the thrombotic phenotype. However the highly conserved His510 residue is recognised as being of critical importance in albumin recycling through interaction with its savaging neonatal Fc receptor. The normal albumin level of 41.1g/l and the coequal expression of albumin Lyon demonstrate that the conservative 510His→Arg substitution does not interfere with the pH dependant capture and release of albumin by the receptor.

Acquired and congenital fast albumin bands; insights from electrospray TOF analysis of whole plasma into drug binding and albumin recycling.

OBJECTIVES: To define the underlying cause of unusual fast albumin bands detected on plasma protein electrophoresis of two patients. METHOD: Plasma was examined by electrospray time-of-flight mass spectrometry (TOF MS) to assess the possibility of congenital or acquired structural modifications. RESULTS: In one patient whole protein MS indicated a drop of 486Da in the mass of 5.1% of the albumin molecules. This and the presence of an additional minor product (65,806Da) lacking a C-terminal phenylalanine (-147Da) indicated that this was albumin Rugby Park; an electrophoretically fast albumin variant caused by a splice site mutation (GT>CT) in intron 13 of the albumin gene. The second patient had an acquired alteration with a drift of albumin mobility to the anode. This severely ill patient was on intra venous antibiotics and electrospray TOF MS showed a stuttered repetition of the 66,439/66,558Da albumin isoforms at multiples of 455-459Da corresponding to the covalent attachment of 1, 2, 3 and 4 molecules of flucloxacillin. This modification of +455Da was also detected in a control on a 1g/day oral dose of flucloxacillin. CONCLUSION: Both aberrations were associated with diminished albumin concentrations. The C-terminal truncation of Rugby Park (albumin, 29g/l) likely interferes with receptor binding and albumin scavenging, while the 20g/l albumin in the second patient was mostly due to renal disease. In both cases electrospray TOF MS proved a rapid (5min) sensitive (0.2µl plasma) and highly informative way of analysing whole plasma or serum.

Familial dysalbuminaemic hyperthyroxinaemia: a rapid and novel mass spectrometry approach to diagnosis.

BACKGROUND: Familial dysalbuminaemic hyperthyroxinaemia is an important cause of discordant thyroid function test results (due to an inherited albumin variant); however, the diagnosis can be challenging. A 51-year-old man had persistently elevated free thyroxine (T4), with discordant normal thyroid-stimulating hormone and normal free triiodothyronine. He was clinically euthyroid and had a daughter with similar thyroid function test results. We aimed to apply a whole protein mass spectrometry method to investigate this case of suspected familial dysalbuminaemic hyperthyroxinaemia. METHODS: Intact serum albumin was assessed directly using electrospray time-of-flight mass spectrometry. Results were confirmed using tryptic peptide m/z mapping and targeted DNA sequencing (exons 3 and 7 of the albumin gene). We also used this sequencing to screen 14 archived DNA samples that were negative for thyroid hormone receptor mutations (in suspected thyroid hormone resistance). RESULTS: Mass spectrometry analysis demonstrated heterozygosity for an albumin variant with a 19 Da decrease in mass, indicative of an Arg→His substitution. The familial dysalbuminaemic hyperthyroxinaemia variant was confirmed with peptide mapping (showing the precise location of the substitution, 218Arg→His) and DNA sequencing (showing guanine to adenine transition at codon 218 of exon 7). The same familial dysalbuminaemic hyperthyroxinaemia variant was identified in one additional screened sample. CONCLUSIONS: Time-of-flight mass spectrometry is a novel procedure for diagnosing familial dysalbuminaemic hyperthyroxinaemia. The test is rapid (<10 min), can be performed on <2 µL of serum and requires minimal sample preparation.

Variation of fibrinogen oligosaccharide structure in the acute phase response: Possible haemorrhagic implications.

BACKGROUND: Fibrinogen is an acute phase glycoprotein whose concentration increases in response to trauma. The newly synthesised protein is functionally enhanced and it is known that treatment with neuraminidase increases the rate of fibrin polymerisation. To explore this, we examined the differences between the oligosaccharide structures of quiescent and acute phase fibrinogen. METHODS: A series of plasma samples was obtained from two individuals suffering an acute phase response. Fibrinogen chains were examined directly by ESI mass spectrometry before and after digestion with N-glycosidase F and ß1,4 galactosidase. RESULTS: The Bß and γ chains of acute phase fibrinogen showed a mass decrease of 162 Da (Gal) in some 50% of the molecules, and the Bß chain showed an additional decrease corresponding to a further loss of NAcGlc. Incubation with N-glycosidase F normalised all isoform masses to that of the quiescent naked protein, confirming the N-linked oligosaccharide as the source of heterogeneity. ß1,4 galactosidase treatment showed the structural difference was the absence of the penultimate Gal from the biantennary oligosaccharides, and mapping of tryptic glycopeptides confirmed these results showing that approximately half the chains lacked Gal. CONCLUSIONS AND IMPLICATIONS: The failure of incorporation Gal excludes the possibility of the hepatic NAcNeu Gal transferase capping the oligosaccharides with sialic acid. This has two desirable haemostatic outcomes: fibrin monomers will polymerise and form clots more rapidly, and two galactose residues can never be exposed diminishing uptake of the protein by the asialoglycoprotein receptor and ramping up concentration at a time of challenge.