Lipoprotein(a), cardiovascular events, and benefit of P2Y12 inhibition: insights from the PEGASUS-TIMI 54 trial

Background

Lp(a) plays a causal role in atherogenesis and may exert pro-thrombotic effects by inhibiting fibrinolysis owing to its structural homology with plasminogen. Patients with higher Lp(a) concentrations may derive greater benefit from anti-thrombotic therapy.

Purpose

We assessed whether patients with higher Lp(a) derive greater risk reduction from P2Y12 inhibition with ticagrelor vs. placebo on a background of aspirin therapy.

Methods

Lp(a) concentration was measured (Randox) in a prospective nested cohort of 8,967 pts enrolled in PEGASUS-TIMI 54, a randomized trial of ticagrelor vs. placebo in patients 1–3 years post MI (median follow-up: 2.7 y). Lp(a) was dichotomized at 200 nmol/L as an established threshold of risk. The prespecified MACE endpoint was CV death, MI or stroke, with KM rates reported at 3y. Cox proportional hazards were used to assess the relationship between Lp(a), MACE and treatment benefit. Models were adjusted for relevant baseline characteristics including apolipoprotein B.

Results

The median Lp(a) was 29 (25th-75th percentile: 12–137) nmol/L. A total of 1,053 pts (11.7%) had a high Lp(a) (≥200 nmol/L). In the pooled trial population, high Lp(a) concentration was associated with a 29% higher risk of MACE (9.1% vs 7.6%; adjusted hazard ratio [adj HR] 1.29, 95% confidence interval [CI] 1.02–1.62; p=0.03), including a 37% higher risk of MI (6.9% vs. 5.3%; adj HR 1.37, 95% CI 1.05–1.79; p=0.02). The hazard ratios for MACE with ticagrelor vs. placebo were 0.73 (95% CI 0.48–1.11) for patients with higher Lp(a) and 0.88 (95% CI 0.74–1.05) for patients with lower Lp(a) (p-interaction=0.41; Figure 1). The absolute risk reductions were 2.4% and 1.2%, respectively.

Conclusion

Lp(a) above 200 nmol/L identifies patients with prior MI at increased risk of MACE who may derive greater absolute risk reduction from treatment with ticagrelor. These exploratory observations provide insights for therapeutics that are evaluating the clinical benefit of Lp(a) reduction.

Funding Acknowledgement

Type of funding sources: Private company. Main funding source(s): The PEGASUS-TIMI 54 trial was funded by AstraZeneca

Interleukin-6 predicts recovery of systolic function after myocardial infarction

Funding Acknowledgements

Type of funding sources: Public grant(s) – National budget only. Main funding source(s): Ministry of Health of the Czech Republic

Background

Interleukin (IL)-6 is an important mediator of the inflammatory response to acute myocardial infarction (MI). Increased IL-6 levels are associated with adverse outcomes after MI. However, there are only limited data on the association of IL-6 levels with recovery of systolic function after MI.

Purpose

The aim of the present study was to evaluate the independent predictive value of IL-6 level for recovery of systolic function among patients hospitalized for their first MI.

Methods

Consecutive patients without previous history of MI or cardiomyopathy with left ventricular ejection fraction (LVEF) ≤40% measured by transthoracic echocardiography upon hospital discharge and available follow-up LVEF evaluated between 3 months and one year since hospital discharge were selected from a prospective registry of MI patients (n=1323).

Results

In total, data from 183 patients (mean age 64.2±11.2, 29% female, mean EF upon discharge 32.6±5.8) were analyzed. In the multivariate linear regression, IL-6 level measured within 24 hours from the hospital admission was positively associated with NT-proBNP, maximal hs-Troponin T a glycemia level and negatively with female gender, total cholesterol and admission systolic blood pressure. During follow-up, LVEF increased by 6.1±9.0% and LVEF recovered to value >50% in 43 (23.5%) patients. In the multivariate analysis, LVEF improvement was predicted by lower IL-6, lower maximal troponin level, female gender, lower heart rate at the time of hospital admission and Killip class I. In the ROC analysis (AUC 0.72, p<0.001), IL-6 ≤ 15.7ng/L had 51% sensitivity and 87% specificity to predict LVEF recovery.

Conclusion

The inflammatory response to MI, as assessed by IL-6, is associated with MI size, glucose and lipid metabolism and is lower in women. IL-6 independently predicts change in LVEF among patients with moderate to severe LV dysfunction. This supports previous findings that IL-6 inhibition can increase myocardial salvage after MI and identifies possible patients that may benefit from targeted therapy.

P6350Impact of diabetes on serum biomarkers in heart failure with preserved ejection fraction: insights from the spironolactone for heart failure with preserved ejection fraction (TOPCAT) trial

Background and purpose

Diabetes mellitus (DM) is common in heart failure with preserved ejection fraction (HFpEF). Patients with DM and HF with reduced ejection fraction have higher levels of prognostic biomarkers relative to non-diabetics. We sought to examine differences in biomarkers at baseline and over time in patients with HFpEF with and without DM (non-DM).

Methods

The Americas cohort of the TOPCAT trial included 248 subjects with baseline measurements of serum biomarkers and follow-up measures 12 months later. Subjects were identified as non-DM or DM at baseline. Baseline values were compared using non-parametric tests and 12-month changes were compared via linear regression after log-transformation and adjustment for baseline biomarker value, age, gender, randomization strata, and randomized treatment.

Results

At baseline, DM patients had significantly lower eGFR and higher hsCRP, PIIINP, TIMP1, and Gal-3 levels versus non-DM patients (Table). In addition, there was a significantly larger increase over time in levels of hs-TnT, a marker of myocyte death, in DM vs. non-DM patients (p=0.016).

Baseline and 12-Month Biomarkers Baseline % Change at 12 Months Non-DM (n=132) DM (n=116) p Non-DM (n=110) DM (n=94) p eGFR (mL/min/1.73m2) 67 [57, 77] 57 [46, 73] 0.003 −14% (−17, −10) −14% (−18, −10) 0.34 hsCRP (mg/L) 2.4 [1.1, 5.6] 3.1 [1.6, 7.5] 0.046 1% (−5, 3) −11% (−27, 7) 0.54 NT-proBNP (pg/mL) 624 [338, 1235] 629 [278, 1429] 0.80 −2% (−12, 10) −5% (−21, 13) 0.48 hs-TNT (ng/mL) 5.7 [3.1, 12.4] 7.1 [3.7, 14.2] 0.17 −1% (−14, 13) 11% (−3, 27) 0.016 Soluble ST2 (ng/mL) 28 [22, 32] 28 [21, 35] 0.36 −1% (−7, 5) −4% (−9, 2) 0.60 Aldosterone (ng/L) 149 [120, 202] 142 [113, 174] 0.09 17% (9, 26) 23% (14, 33) 0.76 PICP (ng/mL) 137 [101, 169] 127 [102, 155] 0.29 5% (−5, 17) 1% (−9, 12) 0.07 CITP (ng/mL) 1.6 [1.0, 2.8] 1.6 [0.9, 3.0] 0.93 −25% (−37, 10) −23% (−36, −8) 0.87 PIIINP (ng/mL) 23 [16, 30] 28 [21, 36] <0.001 5% (−4, 15) 5% (−3, 14) 0.12 TIMP-1 (ng/mL) 188 [170, 212] 212 [183, 245] <0.001 −1% (−4, 2) −2% (−5, 2) 0.59 Galectin-3 (ng/mL) 20 [16, 23] 22 [18, 28] <0.001 6% (3, 10) 9% (4, 14) 0.52 eGFR, estimated glomerular filtration rate; hsCRP, high-sensitivity CRP; NT-proBNP, N-terminal pro-B-type natriuretic peptide; hs-TnT, high-sensitivity troponin T; PICP, pro-collagen type I carboxy-terminal peptide; CITP, collage type I; PIIINP=pro-collagen type III amino-terminal peptide; TIMP-1=tissue inhibitor of MMP-1.

Conclusions

In comparison to patients without DM, those with DM had poorer renal function and higher baseline levels of markers of myocardial stretch, myocyte death, and pro-fibrotic biomarkers. Further, hs-TnT increased over 12 months only in patients with DM.

Filtration Markers, Cardiovascular Disease, Mortality, and Kidney Outcomes in Stable Kidney Transplant Recipients: The FAVORIT Trial

Cystatin C and beta-2-microglobulin (B2M) are filtration markers associated with adverse outcomes in nontransplant populations, sometimes with stronger associations than for creatinine. We evaluated associations of estimated glomerular filtration rate from cystatin C (eGFR_cys ), B2M (eGFR_B2M ), and creatinine (eGFR_cr ) with cardiovascular outcomes, mortality, and kidney failure in stable kidney transplant recipients using a case-cohort study nested within the Folic Acid for Vascular Outcome Reduction in Transplantation (FAVORIT) Trial. A random subcohort was selected (N = 508; mean age 51.6 years, median transplant vintage 4 years, 38% women, 23.6% nonwhite race) with enrichment for cardiovascular events (N = 306; 54 within the subcohort), mortality (N = 208; 68 within the subcohort), and kidney failure (N = 208; 52 within the subcohort). Mean eGFR_cr , eGFR_cys , and eGFR_B2M were 46.0, 43.8, and 48.8 mL/min/1.73m² , respectively. After multivariable adjustment, hazard ratios for eGFR_cys and eGFR_B2M <30 versus 60+ were 2.02 (95% confidence interval [CI] 1.09-3.76; p = 0.03) and 2.56 (1.35-4.88; p = 0.004) for cardiovascular events; 3.92 (2.11-7.31) and 4.09 (2.21-7.54; both p < 0.001) for mortality; and 9.49 (4.28-21.00) and 15.53 (6.99-34.51; both p < 0.001) for kidney failure. Associations persisted with additional adjustment for baseline eGFR_cr . We conclude that cystatin C and B2M are strongly associated with cardiovascular events, mortality, and kidney failure in stable kidney transplant recipients.

Multicenter analytical evaluation of a high-sensitivity troponin T assay

BACKGROUND

High-sensitivity cardiac troponin assays are being introduced clinically for earlier diagnosis of acute myocardial infarction (AMI). We evaluated the analytical performance of a high-sensitivity cardiac troponin T assay (hscTnT, Roche Diagnostics) in a multicenter, international trial.

METHODS

Three US and 5 European sites evaluated hscTnT on the Modular® Analytics E170, cobas® 6000, Elecsys 2010, and cobas® e 411. Precision, accuracy, reportable range, an inter-laboratory comparison trial, and the 99th percentile of a reference population were assessed.

RESULTS

Total imprecision (CVs) were 4.6-36.8% between 3.4 and 10.3 ng/L hscTnT. Assay linearity was up to 10,000 ng/L and the limit of blank and detection were 3 and 5 ng/L, respectively. The 99th percentile reference limit was 14.2 ng/L (n=533). No significant differences between specimen types, assay incubation time, or reagent lots existed. A substantial positive bias (76%) exists between the 4th generation and hscTnT assays at the low end of the measuring range (<50 ng/L). hscTnT serum pool concentrations were within 2SD limits of the mean of means in the comparison trial, indicating comparable results across multiple platforms and laboratories.

CONCLUSION

The Roche hscTnT assay conforms to guideline precision requirements and will likely identify additional patients with myocardial injury suspicious for AMI.

Rescue of a telomere length defect of Nijmegen breakage syndrome cells requires NBS and telomerase catalytic subunit

Nijmegen breakage syndrome (NBS) is a rare human disease displaying chromosome instability, radiosensitivity, cancer predisposition, immunodeficiency, and other defects [1, 2]. NBS is complexed with MRE11 and RAD50 in a DNA repair complex [3-5] and is localized to telomere ends in association with TRF proteins [6, 7]. We show that blood cells from NBS patients have shortened telomere DNA ends. Likewise, cultured NBS fibroblasts that exhibit a premature growth cessation were observed with correspondingly shortened telomeres. Introduction of the catalytic subunit of telomerase, TERT, was alone sufficient to increase the proliferative capacity of NBS fibroblasts. However, NBS, but not TERT, restores the capacity of NBS cells to survive gamma irradiation damage. Strikingly, NBS promotes telomere elongation in conjunction with TERT in NBS fibroblasts. These results suggest that NBS is a required accessory protein for telomere extension. Since NBS patients have shortened telomeres, these defects may contribute to the chromosome instability and disease associated with NBS patients.

Characterization of seven low incidence blood group antigens carried by erythrocyte band 3 protein

Recent studies have demonstrated that band 3 carries antigens of the Diego blood group system and have elucidated the molecular basis of several previously unassigned low incidence and high incidence antigens. Because the available serological data suggested that band 3 may carry additional low incidence blood group antigens, we screened band 3 genomic DNA encoding the membrane domain of band 3 for single-strand conformational polymorphisms. We found that the putative first ectoplasmic loop of band 3 carries blood group antigen ELO, 432 Arg-->Trp; the third putative loop harbors antigens Vga (Van Vugt), 555 Tyr-->His, BOW 561 Pro-->Ser, Wu (Wulfsberg), 565 Gly-->Ala, and Bpa (Bishop), 569 Asn-->Lys; and the putative fourth ectoplasmic loop carries antigens Hga (Hughes), 656 Arg-->Cys, and Moa (Moen), 656 Arg-->His. We studied erythrocytes from carriers of five of these blood group antigens. We found similar levels of reticulocyte mRNA corresponding to the two band 3 gene alleles, normal content and glycosylation of band 3 in the red blood cell membrane, and normal band 3-mediated sulfate influx into red blood cells, suggesting that the mutations do not have major effect on band 3 structure and function. In addition to elucidating the molecular basis of seven low incidence blood group antigens, these results help to create a more accurate structural model of band 3.

Autosomal dominant distal renal tubular acidosis is associated in three families with heterozygosity for the R589H mutation in the AE1 (band 3) Cl-/HCO3- exchanger

Distal renal tubular acidosis (dRTA) is characterized by defective urinary acidification by the distal nephron. Cl-/HCO3- exchange mediated by the AE1 anion exchanger in the basolateral membrane of type A intercalated cells is thought to be an essential component of lumenal H+ secretion by collecting duct intercalated cells. We evaluated the AE1 gene as a possible candidate gene for familial dRTA. We found in three unrelated families with autosomal dominant dRTA that all clinically affected individuals were heterozygous for a single missense mutation encoding the mutant AE1 polypeptide R589H. Patient red cells showed approximately 20% reduction in sulfate influx of normal 4,4'-diisothiocyanostilbene-2,2'-disulfonic acid sensitivity and pH dependence. Recombinant kidney AE1 R589H expressed in Xenopus oocytes showed 20-50% reduction in Cl-/Cl- and Cl-/HCO3- exchange, but did not display a dominant negative phenotype for anion transport when coexpressed with wild-type AE1. One apparently unaffected individual for whom acid-loading data were unavailable also was heterozygous for the mutation. Thus, in contrast to previously described heterozygous loss-of-function mutations in AE1 associated with red cell abnormalities and apparently normal renal acidification, the heterozygous hypomorphic AE1 mutation R589H is associated with dominant dRTA and normal red cells.

Characterization of the underlying molecular defect in hereditary spherocytosis associated with spectrin deficiency

Several subsets of patients with hereditary spherocytosis (HS) have been defined based on the specific red blood cell membrane protein deficiencies involving spectrin, ankyrin, band 3, and protein 4.2. Mutations of the genes encoding these proteins are currently being uncovered. Regarding spectrin, only three isolated cases of beta-spectrin gene mutations were recently reported in association with HS and spectrin deficiency. We have screened the coding region of the beta-spectrin gene using the SSCP technique, in 40 families with HS associated with spectrin deficiency or combined spectrin and ankyrin deficiencies. In this report we describe six frameshift and nonsense mutations and four missense mutations of the beta-spectrin gene in 11 unrelated families. Taking advantage of modifications in the restriction enzyme recognition sequences introduced by the mutations, we show, in all cases of frameshift and nonsense mutations, the loss of heterozygosity at the cDNA level when compared to genomic DNA, reflecting the absence of the mutant mRNA transcripts. In one family with a large pedigree including six generations and 112 members, we firmly establish the autosomal dominant inheritance of one of the beta-spectrin null mutations. Most of the mutations described are responsible for a phenotype of mild to moderate autosomal dominant form of HS associated with a conspicuous spherocytosis with frequent spiculated cells (8% to 15% acanthocytes). One missense mutation appears to be associated with a recessive form of the disease. Five common restriction enzyme polymorphisms of the coding region of the beta-spectrin gene are also described. Overall, these findings underscore the importance of the beta-spectrin gene mutations in the pathogenesis of HS and reemphasizes the extreme heterogeneity of the underlying molecular basis of this condition.

Blood group antigens Rb(a), Tr(a), and Wd(a) are located in the third ectoplasmic loop of erythroid band 3

BACKGROUND

Rb(a), Tr(a), and Wd(a) are three low-incidence blood group antigens that have not been assigned to a particular structure of the red cell membrane. Recent genetic and serologic data suggested erythroid band 3 as a possible carrier of these three antigens.

STUDY DESIGN AND METHODS

Ten band 3 gene exons that encode the membrane domain of band 3 were screened for single strand conformation polymorphism (SSCP). Exons displaying SSCP were cloned and sequenced, and the presence of the mutations was verified by restriction digestion.

RESULTS

Substitutions 548 Pro-->Leu, 551 Lys-->Asn, and 557 Val-->Met, all located in the third ectoplasmic loop of band 3, were detected in the subjects with Rb(a+), Tr(a+), and Wd(a+) red cells, respectively. The presence of the Rb(a) and Wd(a) mutations was confirmed in additional carriers of these blood group antigens. Chymotryptic cleavage at Tyr 553 and Tyr 555 abolished the agglutinability of Tr(a+) and Wd(a+) cells with the corresponding antisera, further demonstrating that the epitopes are located in the third ectoplasmic loop of band 3. Similar quantities of mRNA corresponding of the two band 3 alleles, a normal pattern of red cell membrane proteins, and normal DIDS (4,4'-diisothiocyanatostilbene-2,2'-disulphonic acid, disodium salt)-inhibitable sulfate flux were detected, which suggests that the mutations do not affect band 3 mRNA stability or band 3 protein expression and transport function.

CONCLUSION

Wd(a) and Rb(a), and tentatively Tr(a), can be assigned to the Diego blood group system.

A Thr552 -->Ile substitution in erythroid band 3 gives rise to the Warrior blood group antigen

BACKGROUND

Recent family studies established that the low-incidence red cell antigen WARR is not part of the MNS, Lutheran, Lewis, Duffy, Kidd, Xg, Chido/ Rodgers, Kx, or Gerbich blood group systems. Continued serologic and genetic studies of WARR suggest that it is carried on erythroid band 3.

STUDY DESIGN AND METHODS

To test the hypothesis that expression of WARR is controlled by the anion exchanger 1 gene (AE1), AE1 intronic primers that flank the exons encoding the membrane domain of band 3 were prepared. Polymerase chain reaction-amplified products corresponding to exons 11-20 of AE1 were analyzed for single-strand conformational polymorphism (SSCP) in DNA from WARR-positive and WARR-negative individuals.

RESULTS

An SSCP was detected in exon 14. Subsequent sequencing revealed a C-->T mutation in codon 552 that leads to a Thr-->Ile substitution. Because the C-->T mutation eliminates a Bbs I restriction site, it was possible to confirm the phenotypes of all family members. To study the possible effect of the Thr552-->Ile substitution on the expression and function of band 3, polymerase chain reaction-amplified reverse-transcribed reticulocyte mRNA was digested with Bbs I. Both alleles of band 3 mRNA were detected in similar quantities, which suggests that the substitution underlying WARR did not interfere with mRNA stability. Comparison of sodium dodecyl sulfate-polyacrylamide gel electrophoretic mobility and size patterns revealed no difference between proteins isolated from WARR-positive and WARR-negative red cells. Further, the presence of WARR did not alter the di-isothiocyano-dihydrostilbene disulfonate (DIDS)-inhibitable influx of radiolabeled sulfate.

CONCLUSION

Although it appears inconsequential to the function of band 3, the red cell polymorphism known as WARR is controlled by AE1. WARR should be therefore included in the Diego blood group system.

Characterization of 13 novel band 3 gene defects in hereditary spherocytosis with band 3 deficiency

Hereditary spherocytosis (HS) is a common hemolytic anemia of variable clinical expression. Pathogenesis of HS has been associated with defects of several red cell membrane proteins including erythroid band 3. We have studied erythrocyte membrane proteins in 166 families with autosomal dominant HS. We have detected relative deficiency of band 3 in 38 kindred (23%). Band 3 deficiency was invariably associated with mild autosomal dominant spherocytosis and with the presence of pincered red cells in the peripheral blood smears of unsplenectomized patients. We hypothesized that this phenotype is caused by band 3 gene defects. Therefore, we screened band 3 DNA from these 38 kindred for single strand conformational polymorphisms (SSCP). In addition to five mutations detected previously by SSCP screening of cDNA, we detected 13 new band 3 gene mutations in 14 kindred coinherited with HS. These novel mutations consisted of two distinct subsets. The first subset included seven nonsense and frameshift mutations that were all associated with the absence of the mutant mRNA allele from reticulocyte RNA, implicating decreased production and/or stability of mutant mRNA as the cause of decreased band 3 synthesis. The second group included five substitutions of highly conserved amino acids and one in-frame deletion. These six mutations were associated with the presence of comparable levels of normal and mutant band 3 mRNA. We suggest that these mutations interfere with band 3 biosynthesis leading thus to the decreased accumulation of the mutant band 3 allele in the plasma membrane.

Combination of two mutant alpha spectrin alleles underlies a severe spherocytic hemolytic anemia

We studied a patient with a severe spherocytic hemolytic anemia without family history of spherocytosis. Analysis of patient's erythrocyte membrane proteins revealed spectrin deficiency and a truncated alpha spectrin protein. We determined that the patient is a compound heterozygote with two mutations in alpha spectrin gene. Mutation in the paternal allele, designated alpha spectrin(PRAGUE), is a transition A to G in the penultimate position of intron 36 that leads to skipping of exon 37, frameshift, and production of the truncated alpha spectrin protein. The maternal allele, designated alpha spectrin(LEPRA), contains transition C-->T in position -99 of intron 30. This mutation enhances an alternative acceptor splice site 70 nucleotides upstream from the regular site. The alternative splicing causes a frameshift and premature termination of translation leading to a significant decrease in alpha spectrin production. The alpha(LEPRA) mutation is linked to a spectrin alphaIIa marker that was found to be associated with recessive or nondominant spectrin-deficient hereditary spherocytosis in approximately 50% of studied families. We conclude that the alpha(LEPRA) mutation combined in trans with the alpha(PRAGUE) mutation underlie the severe hemolytic anemia in the proband. We suggest that allele alpha spectrin(LEPRA) may be frequently involved in pathogenesis of recessive or nondominant spectrin-deficient hereditary spherocytosis.

Overexpression of AE1 Prague, but not of AE1 SAO, inhibits wild-type AE1 trafficking in Xenopus oocytes

Mutations in the AE1 (band 3) anion exchanger of human erythrocytes have been associated with altered red cell shape and heritable disease. The Southeast Asian Ovalocytosis (SAO) AE1 mutation, a 27 nt deletion producing the delta 400-408 form of AE1, and the AE1 Prague mutation, a 10 nt insertion producing a frameshift after AE1 aa 821 leading to premature termination, are found only in the heterozygous state. We therefore examined accumulation and function of wt AE1 polypeptide in Xenopus oocytes when coexpressed with AE1 SAO and with AE1 Prague. Our SAO construct lacked the K56E (AE1 Memphis) polymorphism present in the endogenous AE1 SAO protein. Neither mutant AE1 mediated Cl- uptake into cRNA-injected Xenopus oocytes. Coinjection of mutant and wt cRNAs led to dose-dependent inhibition of wt function by AE1 Prague, but not by SAO. Though in vitro translation of the two mutants revealed little difference in their insertion into microsomal membranes, AE1 Prague accumulated in Xenopus oocytes to lower levels than did AE1 SAO or wt. Unlike AE1 SAO polypeptide, AE1 Prague polypeptide was not detectable at the oocyte surface. Moreover, overexpression of AE1 Prague, in contrast to AE1 SAO, reduced the accumulation of wt AE1, at the oocyte surface. This inhibition occurred in the absence of detectable heteromer formation between the AE1 Prague and wt AE1 polypeptides.

Beta spectrin PRAGUE: a truncated beta spectrin producing spectrin deficiency, defective spectrin heterodimer self-association and a phenotype of spherocytic elliptocytosis

Spherocytic elliptocytosis is a phenotypic hybrid between hereditary spherocytosis (HS) and hereditary elliptocytosis (HE) characterized by the presence of spheroovalocytes and spherocytes which exhibit increased osmotic fragility, indicating a deficiency of surface area. Both the spherocytic red cell morphology and the increased osmotic fragility distinguish this clinical entity from common HE. In contrast to common HE, the molecular basis of spherocytic elliptocytosis is unknown. Here we describe two members of a family who both have the characteristic features of spherocytic HE. We show that the underlying defect involves a G to C transversion at the -1 position of the acceptor splice site upstream of exon X of beta spectrin leading to skipping of exon X from the mutant beta spectrin mRNA allele. The mutant mRNA is present in reticulocytes in similar amounts as the normal mRNA. Pulse-labelling of erythroblasts prepared from peripheral blood in a two-phase liquid-culture system reveals a decreased synthesis of the truncated beta spectrin, a finding which is likely to underlie the moderately severe spectrin deficiency in the two patients. In addition, this mutant spectrin, similar to the previously reported spectrins, is defective in spectrin heterodimer self-association. The spectrin deficiency, which represents a common finding in the majority of patients with HS, together with weakened spectrin heterodimer self-association, as found in the majority of patients with common HE, provides a molecular explanation for the phenotype of spherocytic elliptocytosis in this kindred and, most likely, in other patients carrying similar beta spectrin mutations.

Comparison of the ankyrin (AC)n microsatellites in genomic DNA and mRNA reveals absence of one ankyrin mRNA allele in 20% of patients with hereditary spherocytosis

Combined deficiency of ankyrin and spectrin represents the most common biochemical abnormality in hereditary spherocytosis (HS). To examine whether a decrease in ankyrin mRNA represents a frequent cause of this type of HS, we took advantage of the reported (AC)n microsatellite polymorphism in the 3' untranslated region of ankyrin cDNA. We first measured the number of AC repeats in genomic DNA encoding erythrocyte ankyrin in 36 unrelated Czech HS patients with combined ankyrin and spectrin deficiency and found 21 of these subjects (58%) to be heterozygotes for the (AC)n microsatellite size. Further analysis of reticulocyte RNA showed that ankyrin cDNA from 7 of these 21 heterozygotes (33%) contained only one of the two ankyrin alleles. We conclude that approximately 1/3 of ankyrin-deficient autosomal dominant HS is caused by reduced expression of one ankyrin allele which, in turn, is caused by either a reduced transcription of one allele of the mutated ankyrin gene or abnormal processing or decreased stability of the mutant ankyrin mRNA. Because ankyrin deficiency is detected in approximately 60% of HS subjects, this result suggests that approximately 20% of all HS is caused by a decreased expression of one ankyrin mRNA allele.

A nonsense mutation 1669Glu-->Ter within the regulatory domain of human erythroid ankyrin leads to a selective deficiency of the major ankyrin isoform (band 2.1) and a phenotype of autosomal dominant hereditary spherocytosis

We describe a nonsense mutation in the regulatory domain of erythroid ankyrin associated with autosomal dominant hereditary spherocytosis with a selective deficiency of the ankyrin isoform 2.1 (55% of normal), a deficiency of spectrin (58% of normal) proportional to the decrease in ankyrin 2.1, and a normal content of the other main ankyrin isoform, protein 2.2. PCR amplification of cDNA encoding the regulatory domain of ankyrin revealed a marked decreased in the ratio of ankyrin 2.1 mRNA to the ankyrin 2.2 mRNA. Sequencing of ankyrin gene in the region where the 2.1 and 2.2 mRNA differ detected a nonsense mutation 1669Glu-->Ter (GAA-->TAA) in one ankyrin allele. Only normal ankyrin 2.1 mRNA was detected in the reticulocyte RNA. Since the alternative splicing within the regulatory domain of ankyrin retains codon 1669 in ankyrin 2.1 mRNA and removes it from ankyrin 2.2 mRNA, we propose that the 1669Glu-->Ter mutation decreases the stability of the abnormal ankyrin 2.1 mRNA allele leading to a decreased synthesis of ankyrin 2.1 and a secondary deficiency of spectrin.

Mutations of conserved arginines in the membrane domain of erythroid band 3 lead to a decrease in membrane-associated band 3 and to the phenotype of hereditary spherocytosis

To elucidate the molecular basis of band 3 deficiency in a recently defined subset of patients with autosomal dominant hereditary spherocytosis (HS), we screened band 3 cDNA for single-strand conformation polymorphism (SSCP). In 5 of 17 (29%) unrelated HS subjects with band 3 deficiency, we detected substitutions R760W, R760Q, R808C, and R870W that were all coinherited with the HS phenotype. The involved arginines are highly conserved throughout evolution. To examine whether or not the product of the mutant allele is inserted into the membrane, we studied one HS subject who was doubly heterozygous for the R760Q mutation and the K56E (band 3sMEMPHIS) polymorphism that results in altered electrophoretic mobility of the band 3 Memphis proteolytic fragments. We detected only the band 3MEMPHIS in the erythrocyte membrane indicating that the protein product of the mutant, R760Q, band 3 allele is absent from the red blood cell membrane. These findings suggest that the R760Q substitution, and probably the other arginine subsitutions, produce band 3 deficiency either by precluding incorporation of the mutant protein into the red blood cell membrane or by leading to loss of mutant protein from differentiating erythroid precursors.

The structure and organization of the human erythroid anion exchanger (AE1) gene

The AE1 (anion exchanger, band 3) protein is expressed in erythrocytes and in the A-type intercalated cells of the kidney distal collecting tubule. In both cell types it mediates the electroneutral transport of chloride and bicarbonate ions across the lipid bilayer, and, in erythrocytes, it also serves as the critical attachment site of the peripheral membrane skeleton. We have characterized the human AE1 gene using overlapping clones isolated from a phage library of human genomic DNA. The gene spans approximately 20 kb and consists of 20 exons separated by 19 introns. The structure of the human AE1 gene corresponds closely with that of the previously characterized mouse AE1 gene, with a high degree of conservation of exon/intron junctions, as well as exon and intron nucleotide sequences. The putative upstream and internal promoter sequences of the human AE1 gene used in erythroid and kidney cells, respectively, are described. We also report the nucleotide sequence of the entire 3' noncoding region of exon 20, which was lacking in the published cDNA sequences. In addition, we have characterized 9 Alu repeat elements found within the body of the human AE1 gene that are members of 4 related subfamilies that appear to have entered the genome at different times during primate evolution.

Duplication of 10 nucleotides in the erythroid band 3 (AE1) gene in a kindred with hereditary spherocytosis and band 3 protein deficiency (band 3PRAGUE)

We describe a duplication of 10 nucleotides (2,455-2,464) in the band 3 gene in a kindred with autosomal dominant hereditary spherocytosis and a partial deficiency of the band 3 protein that is reflected by decreased rate of transmembrane sulfate flux and decreased density of intramembrane particles. The mutant allele potentially encodes an abnormal band 3 protein with a 3.5-kD COOH-terminal truncation; however, we did not detect the mutant protein in the membrane of mature red blood cells. Since the mRNA levels for the mutant and normal alleles are similar and since the band 3 content is the same in the light and dense red cell fractions, we conclude that the mutant band 3 is either not inserted into the plasma membrane or lost from the membrane prior to the release of red blood cells into circulation. We further show that the decrease in band 3 content principally involves the dimeric laterally and rotationally mobile fraction of the band 3 protein, while the laterally immobile and rotationally restricted band 3 fraction is left essentially intact. We propose that the decreased density of intramembrane particles decreases the stability of the membrane lipid bilayer and causes release of lipid microvesicles that leads to surface area deficiency and spherocytosis.

Spectrin cagliari. an Ala-->Gly substitution in helix 1 of beta spectrin repeat 17 that severely disrupts the structure and self-association of the erythrocyte spectrin heterodimer

The spectrin tetramer, the principal structural element of the red cell membrane skeleton, is formed by stable head-to-head self-association of two spectrin heterodimers. The self-association site appears to be formed by interactions between helices 1 and 2 of beta spectrin repeat 17 of one dimer with helix 3 of alpha spectrin repeat 1 of the other dimer to form two combined alpha-beta triple-helical segments. The head of the heterodimer appears to involve similar intradimer interactions. We describe the first example of an amino acid substitution in helix 1 of this combined alpha-beta triple-helical segment, which, although relatively minor, profoundly impairs tetramer formation. Strikingly, low angle rotary shadowing electron microscopy of isolated spectrin dimers reveals that this mutation also severely disrupts the head of the heterodimer causing it to be open. Following linkage studies which were most consistent with a beta spectrin gene mutation, a nucleotide change was identified in codon 2018, resulting in an Ala-->Gly substitution in the first helical domain of beta spectrin repeat 17. Because glycine is a strong helix breaker, this change is predicted to disrupt the conformation of this helical domain. Our results indicate that this helical domain must play direct roles in the alpha-beta interdimer interactions that form the self-association site of the tetramer and in the alpha-beta intradimer interactions at the head of the heterodimer.

Band 3 Memphis: a widespread polymorphism with abnormal electrophoretic mobility of erythrocyte band 3 protein caused by substitution AAG----GAG (Lys----Glu) in codon 56

Band 3 Memphis (b3M) is a variant of the erythrocyte band 3 protein detected in individuals of virtually all ethnic groups and characterized by a reduced mobility of proteolytic fragments derived from the N-terminus of the cytoplasmic domain of band 3 (cdb3). We have sequenced band 3 cDNA corresponding to cdb3 in 12 heterozygotes for the b3M polymorphism including one white, one black, one Chinese, one Philippino, one Malay, and seven Melanesian subjects. In all individuals, we found a single-base substitution in codon 56 of one band 3 allele changing lysine to glutamic acid (AAG----GAG) which, in some of them, was linked with an additional mutation in cdb3. Since the change of codon 56 from AAG to GAG was the only mutation in the studied individuals found within the cDNA segment coding for the abnormally migrating fragment of cdb3, we conclude that it represents the underlying molecular basis of the b3M polymorphism. We further support this conclusion by showing that electrophoresis in the presence of 4 mol/L urea abolished the difference in migration between proteolytic products of b3M and normal band 3, and that a fusion protein prepared from cDNA coding for the b3M allele again exhibits reduced electrophoretic mobility compared with the normal fusion protein. Finally, since most of the previously cloned mouse, rat, and chicken band 3 and band 3-related proteins contain glutamic acid in the position corresponding to amino acid 56 in the human band 3, we propose that the Memphis variant is the evolutionarily older form of band 3.

Band 3 Tuscaloosa: Pro327----Arg327 substitution in the cytoplasmic domain of erythrocyte band 3 protein associated with spherocytic hemolytic anemia and partial deficiency of protein 4.2

Protein 4.2 is a major red blood cell (RBC) protein that interacts with the band 3 protein and with ankyrin. Inherited deficiencies of this protein are associated with spherocytic hemolytic anemia, but the molecular basis of this defect is unknown. We have studied the underlying defect in a patient with spherocytic hemolytic anemia whose RBCs had a partial (29% +/- 5%) deficiency of protein 4.2. We have first studied the binding of normal ankyrin and protein 4.2 to patient inside-out vesicles (IOVs) stripped of peripheral proteins. While the binding of ankyrin was normal, the predicted maximal binding capacity of patient IOVs for band 4.2 was 20% to 33% lower than that of control IOVs, suggesting a defect in the cytoplasmic domain of band 3 (cdb3). An additional line of evidence pointing to a possible abnormality of band 3 was an abnormal proteolytic digest of cdb3. To elucidate the underlying molecular defect, we have cloned and sequenced the cDNA coding for cdb3 from the patient. One band 3 allele was found to be normal, while clones corresponding to the other allele contained two mutations: substitution A----G in nucleotide 166, changing codon 56 from AAG to GAG (Lys----Glu), and substitution C----G in nucleotide 980, changing codon 327 from CCC to CGC (Pro----Arg). Since the Lys56----Glu56 substitution is found in a common asymptomatic variant of the band 3 protein designated band 3 Memphis, we conclude that either the Pro327----Arg327 substitution itself, or in combination with the band 3 Memphis polymorphism, underlies the abnormal binding of protein 4.2 to cdb3 and results in the spherocytic phenotype.

Deletion in erythrocyte band 3 gene in malaria-resistant Southeast Asian ovalocytosis

Southeast Asian ovalocytosis (SAO) is a hereditary condition that is widespread in parts of Southeast Asia. The ovalocytic erythrocytes are rigid and resistant to invasion by various malarial parasites. We have previously found that the underlying defect in SAO involves band 3 protein, the major transmembrane protein, which has abnormal structure and function. We now report two linked mutations in the erythrocyte band 3 gene in SAO: (i) a deletion of codons 400-408 and (ii) a substitution, A----G, in the first base of codon 56 leading to substitution of Lys-56 by Glu-56. The first defect leads to a deletion of nine amino acids in the boundary of cytoplasmic and membrane domains of band 3. This defect has been detected in all 30 ovalocytic subjects from Malaysia, the Philippines, and two unrelated coastal regions of Papua New Guinea, whereas it was absent in all 30 controls from Southeast Asia and 20 subjects of different ethnic origin from the United States. The Lys-56----Glu substitution has likewise been found in all SAO subjects. However, it has also been detected in 5 of the 50 control subjects, suggesting that it represents a linked polymorphism. We conclude that the deletion of codons 400-408 in the band 3 gene constitutes the underlying molecular defect in SAO.

Molecular defect of the band 3 protein in southeast Asian ovalocytosis

BACKGROUND

Southeast Asian ovalocytosis is a form of hereditary elliptocytosis in which the red cells are rigid and resistant to malaria invasion. The underlying molecular defect is unknown.

METHODS AND RESULTS

We studied the red cells of 54 patients with ovalocytosis and 122 normal controls. We found that ovalocytes contain a structurally and functionally abnormal band 3 protein, the principal transmembrane protein of red cells. The structural lesion of ovalocyte band 3 was revealed by limited proteolytic cleavage of the protein, which produced fragments of abnormal size that were derived from the cytoplasmic domain of the protein. The structural lesion was present in all the subjects with ovalocytosis but none of the controls. This region of band 3 serves as the principal binding site for the membrane skeleton, a submembrane protein network composed of ankyrin, spectrin, actin, and protein 4.1. The structural defect is dominantly inherited, being tightly linked with the inheritance of ovalocytosis (the probability of linkage is in excess of 10 million to 1). Ovalocyte band 3 bound considerably more tightly than normal band 3 to ankyrin, which connects the membrane skeleton to the band 3 protein. This tight binding of ovalocyte band 3 to the underlying skeleton containing ankyrin was directly confirmed in intact cells by the finding that ovalocyte band 3 had markedly reduced lateral mobility in the membrane.

CONCLUSIONS

The red cells in Southeast Asian ovalocytosis carry a structurally and functionally abnormal band 3 protein. This molecular defect may underlie the increased rigidity of the red cells and their resistance to invasion by malaria parasites.

Effect of hemoglobin oxidation products on the stability of red cell membrane skeletons and the associations of skeletal proteins: correlation with a release of hemin

Oxidative injury to hemoglobin (Hb) leads to formation of methemoglobin (MetHb), reversible hemichromes (rHCRs), and irreversible hemichromes (iHCRs). iHCRs precipitate and form Heinz bodies that attach to the red cell membrane causing injury that leads to hemolysis. The molecular mechanisms of this membrane damage have not been fully elucidated. We have studied the effect of Hb oxidation products on the mechanical stability of red cell membrane skeletons and the associations of membrane skeletal proteins. Hb and MetHb stabilized the isolated membrane skeletons, whereas further oxidation to rHCRs abolished this stabilizing effect. Crude iHCRs prepared by phenylhydrazine oxidation of Hb destabilized membrane skeletons by decreasing formation of the spectrin-protein 4.1-actin complex, the effect similar to that of pure hemin. Whereas virtually no hemin was released from Hb and MetHb, high concentrations of hemin were released from crude iHCR preparations. After removal of this hemin fraction by Dowex resin, the iHCRs lost their destabilizing effect. We conclude that as the oxidation of Hb proceeds, the stabilizing effect of Hb on the membrane skeleton is gradually lost and the deleterious effect increases. The destabilization of the red cell membrane skeleton in the presence of crude iHCR is caused by release of hemin, which lowers the stability of membrane skeleton by weakening the spectrin-protein 4.1-actin interaction.

Structural and functional heterogeneity of alpha spectrin mutations involving the spectrin heterodimer self-association site: relationships to hematologic expression of homozygous hereditary elliptocytosis and hereditary pyropoikilocytosis

Defects involving alpha spectrin (Sp) are found in patients with hereditary elliptocytosis and a related disorder, hereditary pyropoikilocytosis (HPP). We have previously found that the severity of hemolysis was related to the total spectrin content of the cells and the percentage of unassembled dimeric Sp (SpD) in the membranes, which, in turn, reflected the amount of mutant Sp in the cell. However, no data are available comparing differences in the function of various alpha Sp mutations to clinical severity. We now report studies of nine homozygotes or double heterozygotes for four alpha Sp mutations: alpha 1/74, alpha 1/46, alpha 1/65, and alpha 1/61, whose red blood cells (RBCs) contained only the mutant Sp and no normal Sp. Sp alpha 1/74, Sp alpha 1/46, and alpha 1/65 homozygotes differed strikingly in the severity of hemolysis that correlated with the severity of mutant Sp dysfunction, as reflected by the fraction of unassembled SpD in the membranes and the self-association of mutant Sp on inside-out vesicles. Homozygotes for Sp alpha 1/74 had a very severe hemolytic anemia and their SpD were virtually incapable of self-association, whereas SpD alpha 1/46 were not as severely affected. The Sp alpha 1/65 homozygotes had a relatively mild hemolytic anemia and their SpD showed the least impairment of function. Ultrastructural examination of membrane skeletons from subjects whose SpD self-association was severely impaired showed gross skeletal disruption and loss of hexagonal structure. In striking contrast, the homozygote for the mildly dysfunctional Sp alpha 1/65 had only a moderate disruption of the skeleton. Some of the homozygous or doubly heterozygous subjects also exhibited a partial deficiency of Sp that correlated with a RBC morphology characteristic of HPP, namely, marked microspherocytosis with virtual absence of elliptocytes. These data demonstrate striking differences in the function and structure of various alpha Sp mutants that underlie differences in clinical expression.

Severe hemolysis and red cell fragmentation caused by the combination of a spectrin mutation with a thrombotic microangiopathy

Two patients are described who presented with severe hemolysis and erythrocyte fragmentation. One patient had renal allograft rejection and disseminated intravascular coagulation, and the other had thrombotic thrombocytopenia purpura. The severity of hemolysis and the red cell abnormalities were considerably more profound than usually seen in patients with thrombotic microangiopathies. After evaluation of blood smears prepared before the onset of the disease and biochemical characterization of proteins of the red blood cell skeleton, a mutation of the skeletal protein spectrin, designated Sp alpha l/65, was identified. In the heterozygous form, this mutation manifests as mild, often asymptomatic, hereditary elliptocytosis. We conclude that in these two patients with thrombotic microangiopathy, the intrinsic red cell membrane instability resulting from the underlying skeletal defect aggravated the mechanical red cell fragmentation, producing morphological features similar to the severe hemolytic form of hereditary elliptocytosis or hereditary pyropoikilocytosis.