Inhibitor development and mortality in non-severe hemophilia A.

BACKGROUND: The life expectancy of non-severe hemophilia A (HA) patients equals the life expectancy of the non-hemophilic population. However, data on the effect of inhibitor development on mortality and on hemophilia-related causes of death are scarce. The development of neutralizing factor VIII antibodies in non-severe HA patients may dramatically change their clinical outcome due to severe bleeding complications. OBJECTIVES: We assessed the association between the occurrence of inhibitors and mortality in patients with non-severe HA. METHODS: In this retrospective cohort study, clinical data and vital status were collected for 2709 non-severe HA patients (107 with inhibitors) who were treated between 1980 and 2011 in 34 European and Australian centers. Mortality rates for patients with and without inhibitors were compared. RESULTS: During 64,200 patient-years of follow-up, 148 patients died (mortality rate, 2.30 per 1000 person-years; 95% confidence interval (CI), 1.96-2.70) at a median age of 64 years (interquartile range [IQR], 49-76). In 62 patients (42%) the cause of death was hemophilia related. Sixteen inhibitor patients died at a median age of 71 years (IQR, 60-81). In ten patients the inhibitor was present at time of death; seven of them died of severe bleeding complications. The all-cause mortality rate in inhibitor patients was > 5 times increased compared with that for those without inhibitors (age-adjusted mortality rate ratio, 5.6). CONCLUSION: Inhibitor development in non-severe hemophilia is associated with increased mortality. High rates of hemophilia-related mortality in this study indicate that non-severe hemophilia is not mild at all and stress the importance of close follow-up for these patients.

Impact of polymorphisms of the major histocompatibility complex class II, interleukin-10, tumor necrosis factor-alpha and cytotoxic T-lymphocyte antigen-4 genes on inhibitor development in severe hemophilia A.

BACKGROUND: Approximately 25% of severe hemophilia A (HA) patients develop antibodies to factor VIII protein. PATIENTS: In the present case-controlled cohort study, 260 severely affected, mutation-type-matched HA patients were studied for association of human leukocyte antigen (HLA) class II molecules and polymorphisms in the genes encoding interleukin-10 (IL-10), tumor necrosis factor-alpha (TNF-alpha) and cytotoxic T-lymphocyte antigen-4 (CTLA-4) and development of inhibitors. RESULTS: Our results demonstrate a higher frequency of DRB1*15 and DQB1*0602 alleles as well as of the haplotype DRB1*15/DQB1*0602 in inhibitor patients [odds ratio (OR) 1.9; P < 0.05]. In TNF-alp[ha, the A allele of the 308G>A polymorphism was found with higher frequency in the inhibitor cohort (0.22 vs. 0.13, OR 1.80). This finding was more pronounced for the homozygous A/A genotype (OR 4.7). For IL-10, the 1082G allele was observed more frequently in patients with inhibitors (0.55 vs. 0.43; P = 0.008). The functional cytokine phenotype was determined for the first time, on the basis of the genetic background, and this showed that 12% of patients with inhibitors were high-TNF-alpha/high-IL-10 producers, as compared with 3% of non-inhibitor patients (OR 4.4). A trend for a lower frequency of the A allele of the CT60 polymorphism in CTLA-4 was found in inhibitor patients (0.42 vs. 0.50). CONCLUSIONS: In conclusion, the reported data clearly highlighted the participation of HLA molecules in inhibitor formation in a large cohort of patients. The higher frequencies of the 308G>A polymorphism in TNF-alpha and 1082A>G in IL-10 in inhibitor patients confirmed the earlier published data. The CT60 single-nucleotide polymorphism in CTLA-4 is of apparently less importance.

Kinetic parameters of monoclonal antibodies ESH2, ESH4, ESH5, and ESH8 on coagulation factor VIII and their influence on factor VIII activity.

The murine monoclonal antibodies ESH2, ESH4, ESH5, and ESH8 specifically bind and inhibit the procoagulant activity of human coagulation factor VIII (FVIII). They are frequently used as a model of inhibitors which are raised against injected FVIII in about 25% of hemophiliacs as a serious side effect of substitution therapy. However, binding kinetics of the interaction of these antibodies with FVIII and their influence on FVIII activity (inhibition) have not yet been examined systematically. For this, we examined association and dissociation of protein:antibody interaction using surface plasmon resonance (SPR) and determined their ability to inhibit the FVIII activity in a one-stage and a two-stage assay. SPR-analysis revealed that the equilibrium dissociation constants (K(D)) of ESH8 and ESH4 are low and in a similar range (ESH8: K(D(ESH8)) = 0.542 nM; ESH4: K(D(ESH4)) = 0.761 nM). A 5.7 times higher K(D) than for ESH4 was observed for ESH2 (4.33 nM), whereas ESH5 showed the highest K(D) of 28.8 nM. In accordance with the lowest K(D), ESH8, and ESH4 reduced FVIII activity of normal human plasma almost completely in a one-stage clot inhibition assay (ESH8: 91.9%; ESH4: 90.1%). However, ESH8 inhibited FVIII activity more efficiently as only 1.0 microg/ml ESH8 was sufficient to obtain maximum inhibition compared to up to 600 microg/ml of ESH4. Despite its attenuated K(D), ESH2 inhibits FVIII:C still efficiently, reducing 61.3% of FVIII activity at a concentration of 9 microg/ml in the one-stage clotting assay. However, a discrepancy of inhibitory efficiency was found depending on the method used to measure FVIII activity. These effects seem to be mainly caused by differences of activation time of FVIII during both FVIII activity assays. The systematic assessment of these results should support FVIII interaction studies, and can provide data to rationally test peptides/mimotopes to remove or neutralize inhibitors of FVIII activity.

Efficient factor VIII affinity purification using a small synthetic ligand.

BACKGROUND: Hemophilia A is currently treated by infusions of the coagulation factor (F) VIII, of which production and purification remain a challenging task. Current purification procedures using immunoaffinity chromatography are cumbersome, expensive, and suffer from the instability of the applied antibody ligands, which elute along with the product and contaminate it. Recently, FVIII was purified using octapeptide ligands, but their use is limited due to the low resistance to proteases. OBJECTIVE: Our goal was to develop and evaluate a novel ligand for FVIII purification, overcoming the drawbacks of current procedures. METHODS: Peptide ligands were screened for binding of (125)I-plasma-derived-FVIII (pdFVIII) in a microbead assay. A selected ligand-coated Toyopearl resin was then used for pdFVIII purification from cell-conditioned Delbucco's modified Eagle's medium (DMEM) containing fetal bovine serum. The proteolytic stability of ligand was measured by incubating with human serum and proteinase K, and its cytotoxicity towards human OV-MZ-6 cells was assayed. RESULTS: A high-affinity octapeptidic FVIII ligand was modified into the small, highly stable and non-toxic peptidomimetic ligand L4 by rational and combinatorial design without affecting its affinity for FVIII. Using ligand L4-coated Toyopearl resin, pdFVIII was isolated from cell-conditioned medium with high purity and 89% column retention after elution with a mild buffer containing 0.6 m NaCl at pH 6.8. CONCLUSIONS: Ligand L4 offers a valuable alternative to antibody-based procedures for laboratory and industrial production. Its synthesis by established solid-phase procedures is straightforward and considerably cheaper than the biotechnological production of antibodies, and safety concerns associated with the use of biological material are overcome.

Modified expression of coagulation factor VIII by addition of a glycosylation site at the N terminus of the protein.

Recently, it was shown that glycoproteins with N-glycans close to the NH(2) terminus can directly enter the calnexin/calreticulin cycle and bypass BiP binding. This should allow efficient secretion of glycoproteins such as factor VIII (FVIII) whose secretion is negatively affected by BiP interaction. Examination of the glycosylation pattern of the NH(2) terminus of FV and FVIII revealed N-glycans at positions 23 and 27 in FV and at position 41 in FVIII. To improve FVIII secretion, a 14-amino-acid-long polypeptide with (G3) or without (G0; control) three N-linked glycosylation consensus sites was inserted upstream of the NH(2) terminus of a B-domain deleted FVIII protein. Expression of G3- and G0-constructs in three different cell lines resulted in the same or even higher expression rate of protein as found for the B-domain deleted FVIII. However, as demonstrated by Western blot analysis, the G3- as well as the G0-protein variants were mainly retained inside the cells in similar amounts. Thus, glycosylation alone does not automatically lead to higher secretion rates, but must be in context to the normal structure of the FVIII protein.

Analysis of mRNA in hemophilia A patients with undetectable mutations reveals normal splicing in the factor VIII gene.

BACKGROUND: haemophilia A (HA) is characterized by partial or total deficiency of factor VIII (FVIII) protein activity. It is caused by a broad spectrum of mutations in the FVIII gene. Despite tremendous improvements in mutation screening methods, in about 2% of HA patients no DNA change could be found, even after sequencing the whole coding part of the FVIII gene including the flanking splice sites, as well as the promotor and the 3' UTR regions. OBJECTIVES, PATIENTS AND METHODS: In the present study we performed a detailed RNA analysis of three groups of patients. The first included control patients with known splicing defects, the second included two patients with already identified nucleotide changes close to splicing sites, that could potentially alter the normal splicing process, and a third group of 11 unrelated patients whose genomic DNA have already been screened for mutations by DHPLC and direct sequencing with no mutation being identified. RESULTS: Both candidate splice site mutations were shown to result in either skipping or alternative splicing of at least one exon, therefore these DNA changes must be considered as causal for the patients' HA phenotype. In contrast, no abnormalities on the RNA level were observed in any of 11 unrelated patients without mutations in the FVIII gene. CONCLUSIONS: These findings exclude mutations that could be located deep in the introns and affecting either normal splicing or lead to mechanisms causing some unknown rearrangements of the FVIII gene. In fact, our results point to the presence of still unknown factor(s) causing HA, which might be either allelic or in the close proximity of the FVIII gene or non-allelic associated with other genetic loci that are involved in the processing of the FVIII protein.

A novel tetracycline-controlled transactivator-transrepressor system enables external control of oncolytic adenovirus replication.

The use of restricted replication-competent adenoviruses (RRCAs) inducing tumor cell-specific lysis is a promising approach in cancer gene therapy. However, the use of RRCAs in humans carries considerable risk, since after injection into the patient, further regulation or inhibition of virus replication from the outside is impossible. Therefore, we have developed a novel system allowing external pharmacological control of RRCA replication. We show here that a tumor-selective E1B-deleted RRCA can be tightly regulated by use of doxycycline (dox)-controlled adenoviral E1A gene expression, which in turn determines vector replication. RRCA replication is switched on by addition and switched off by withdrawal of dox. The system results in efficient tumor cell killing after induction by dox, whereas cells are unaffected by the uninduced system. It was also employed for efficient external control of transgene expression from cotransfected replication-deficient adenovectors. Furthermore, the use of a liver cell-specific human alpha1-antitrypsin (hAAT)-promoter driving a tetracycline-controlled transcriptional silencer allowed specific protection of cells with hAAT-promoter activity in the absence of dox in vitro and in vivo, delineating a new principle of 'tissue protective' gene therapy. The concept of external control of RRCAs may help to improve the safety of cancer gene therapy.

[2% Haemophilia A patients without mutation in the FVIII gene]. / 2% Hämophilie-A-Patienten ohne Mutation im FVIII-Gen.

In Germany, approximately 6,000 patients are suffering from haemophilia A. Screening methods cover 97% of the mutations. For the other patients the coding sequences of the FVIII gene have to be sequenced in total. Out of 1,350 patients, no mutation was observed in 80 patients. In 5 patients, we observed an inversion in intron 1. Known mutations were detected in 16 patients, and in 19 cases novel mutations were characterized (14 in coding regions and 5 in flanking introns). The mutations are mainly base pair substitutions, small deletions or insertions (max. 4 bp) and predicted to cause amino acid exchanges or frameshifts leading to premature stop codons. Moreover, 5 polymorphisms were identified in exons 14 and 26 as well as in introns 7 and 19. Further studies are necessary to identify their causative effects. Surprisingly, in 23 patients out of this subgroup of 80, no mutation was identified in the FVIII gene. Therefore, mutations in non-coding areas or even in other genes have to be considered responsible for the haemophilia A like phenotype. One of them codes for the von Willebrand factor (vWF). We confirmed in two of our cases mutations in the vWF gene.

[Significance of mutation analysis in patients with haemophilia A]. / Bedeutung der Mutationsdiagnostik bei Patienten mit Hämophilie A.

Haemophilia A represents the most frequent hereditary bleeding disorder in humans. The disease is caused by mutations within the factor VIII gene leading to decreased or absent factor VIII activities with a bleeding tendency depending on the degree of factor VIII deficiency. Nowadays, the causative mutations can be routinely detected and have substantially improved diagnostic and understanding of the pathophysiology of haemophilia A. Identification of the gene defects in haemophilic families have enabled fast and save carrier diagnosis. The correlation of the genetic defects with the clinical course revealed that the type of mutation represents the most important genetic predisposing factor for inhibitor formation, the most severe complication of treatment with factor VIII concentrates. Mitigated clinical courses of haemophilia A were shown to be due to special types of mutations or the presence of concomitant thrombophilic mutations. Molecular models of the factor VIII protein allowed to investigate the effects of specific mutations thus giving new insights in the structure/function relationship of the factor VIII molecule. These findings might promote the development of novel recombinant factor VIII concentrates with higher efficacy, longer half life and reduced immunogenicity.

[Characterization of factor VIII antibody epitopes from haemophilia A patients using cellulose bound FVIII peptide libraries]. / Charakterisierung von Faktor-VIII-Antikörperepitopen mit Faktor-VIII-Peptid-Bibliotheken.

Approximately 30% of patients suffering from severe haemophilia A develop antibodies against factor VIII (FVIII) neutralizing the effect of the pro-coagulant activity of intravenously injected FVIII as a complication of replacement therapy. Generally, various epitopes on the FVIII molecule are bound by these antibodies. The detailed structure of such epitopes is unknown. In this study epitopes on the FVIII molecule are identified using solid phase bound peptide arrays carrying the whole amino acid sequence of FVIII as small oligopeptides. The binding of FVIII antibodies by specific peptide sequences on the array indicates potential epitopes. FVIII antibodies of inhibitor patients and healthy blood donors are currently investigated by this method. Identified epitopes may lead to new concepts in therapy aiming at avoidance of inhibitor formation or improvement of inhibitor eradication. As participant of the 'haemophilia A' consortium dealing with genotype/phenotype correlation in haemophilia A we investigate, if the site or type of the mutation correlates with the epitopes, and if there is any relation between epitopes and clinical course. Furthermore, the influence of epitopes on therapeutical effects and the outcome of immune tolerance induction is under scrutiny.

[The national GTH haemophilia registry as database within the scope of the German human genome project]. / Das bundesweite GTH-Hämophilie-Register als Datenbasis im Rahmen des Deutschen Human-Genom-Projekts.

The Committee of Haemophilia of the GTH has established a central registry for all German centers treating patients with haemophilia. The intention was to establish a suitable system for collecting and analyzing epidemiological data relevant to bleeding disorders. The registry provides the database within the scope of the German Human-Genome-Project. The set goal is the complete molecular characterization of the genetic mutations on chromosome X of haemophilia A patients in Germany and subsequent correlation with the phenotype. An electronic network is applied for communication. A Java-application was developed for online electronic data acquirement by the participating centers. Offline data entry and sending encrypted data carriers is possible, too. A high level of security is assured by personalized access. Data are anonymized and scrambled by secure encoding. The concept was confirmed by the official data security offices. A considerable improvement for the epidemiological sciences and a better basis on therapy for patients with bleeding disorders is expected. Furthermore the registry is available for other scientific projects.

[Gathering and evaluation of phenotype data of haemophilia A patients for correlation with genotype data]. / Erfassung und Bewertung phänotypischer Parameter von Hämophilie-A-Patienten und Korrelation mit dem Genotyp.

Haemophilia A is caused by a genetic defect of the factor VIII gene resulting in complete or considerable functional loss of factor VIII molecule within blood. The high bleeding risk of patients can be prevented by intravenous injections of factor VIII protein. However, 25% of patients affected with severe haemophilia, develop factor VIII antibodies against the concentrate substituted. Within this study we try to comprise the phenotypic parameters (e. g. detailed documentation of disease course, basic laboratory values) and the therapy-associated data (e. g. applicated type and amount of factor VIII, number of substitutions, factor VIII recovery, inhibitor development and inhibitor elimination). We hope to identify differences of variable therapeutic treatments on course of disease as already identified for the factor VIII gene defects. At least we expect that certain mutations and mutation types, respectively, can be referred to typical phenotypes and similar course of treatment protocols.

Quality management and quality assurance in haemophilia care: a model at the Bonn haemophilia centre.

The severe clotting defects associated with the diagnosis of severe haemophilia A and B require a quality management and quality assurance system designed to avoid both bleeding sequelae (such as damaged joints) through early on-demand or prophylactic treatment in a home-care setting, and side-effects such as infectious diseases (hepatitis A-G and human immunodeficiency virus), allergic reactions, haemolysis and if possible inhibitor formation, by using highly purified, virus-inactivated or recombinant products in which the factor VIII and IX proteins are as natural as possible. As the intravenous injection of the required clotting factor is entrusted to the patients in home treatment, the haemophilia centre has to check treatment protocols and, when necessary, joint and muscle status. In addition, it is imperative to ensure the safety of the product, and checks must be carried out to make sure that batch numbers are recalled as soon as possible if side-effects are observed. These are the reasons for several Acts of Parliament in Germany requiring special treatments and regular checks (the Disabled Act, recommendations by the German Medical Council, the Transfusion Act). Thus, at the haemophilia centre in Bonn we have established a special quality management and quality assurance system taking into account the great number of patients (> 800), the often considerable distance between the centre and the patient, and the aforementioned regulations and laws. Quality management involves dealing with daily practicalities such as 24-h availability of a physician, medical technologist and nurse, careful instruction of patient and family in home care, genetic counselling, regular laboratory tests (especially recovery time, half-life, inhibitors and gene defects, clinical chemistry and serology) and clinical investigations (especially joint and muscle status). It also includes co-operation with family doctors and different departments at our university hospital (e.g. orthopaedic, microbiology), daily conferences with staff, information for nursery schools, schools, training institutions and/or the workplace in case of emergency, and cooperation with German haemophilia foundations. For quality assurance, several self-controlling systems are in place, such as distribution of concentrate, laboratory data, treatment protocols, joint and muscle status and bleeding tendencies. All these and more are double-checked and interactive, controlling data and activities with the help of EDP. Exceptional staff motivation and patient compliance are important for this quality system.

Inhibitor development in correlation to factor VIII genotypes.

Alloantibodies (inhibitors) against factor VIII (FVIII) develop in 20-30% of patients with severe haemophilia A and render classical FVIII substitution therapy ineffective. Several studies have shown that genetic factors, the type of FVIII gene mutation and immune response genes (e.g. the Major Histocompatibility Complexes), influence the risk of inhibitor formation. In particular, the type of FVIII gene mutation has proven to be a decisive risk factor. Patients with severe molecular gene defects (e.g. large deletions, nonsense mutations, intron-22 inversion) and no endogenous FVIII synthesis have a 7-10 times higher inhibitor prevalence than patients with milder molecular gene defects (e.g. missense mutations, small deletions, splice site mutations). To date, at least 10 distinct classes of mutations have been shown which have differing risks of associated inhibitor formation. A challenging observation in inhibitor patients is the heterogeneity of the antibody epitopes with respect to their number and their specificity. At least five epitopes in the FVIII molecule have been identified that constitute the targets for antibodies in most inhibitor patients. These epitopes are located in the ar3 region and the A2, A3, C1, C2 domains which correspond to the functional binding sites of the ligands of the FVIII protein. At present, the determinants of the characteristics of these epitopes and the subsequent inhibitor titre are unknown. A relationship of the mutation site and the epitope localization has been shown for some individual patients with mild haemophilia A. However, in severely affected haemophilia A patients, the influence of patient genetics on inhibitor titre and epitope specificity has yet to be elucidated.

Molecular biology of blood coagulation.

A complex network of hemostasis proteins maintains the blood flow and integrity of the vascular system. Molecular biology techniques have led to identification and cloning of the corresponding genes, thereby providing the basis for development of various recombinant clotting factor concentrates. Further analysis of these genes allowed for phenotype and genotype correlations in patients with hemorrhagic or thromboembolic disorders and analysis of structure and function relationships of the involved proteins. All these efforts result in a greatly advanced understanding of the hemostatic network. The aim of this article is to illustrate this progress by reporting on the recent results in representative hereditary hemorrhagic and such thromboembolic conditions as hemophilia, von Willebrand disease, and thrombotic disorders.

Gene therapy of hemophilia.

Hemophilia A and B are X-linked bleeding disorders caused by mutations within the factor VIII and factor IX genes, respectively. Although both disorders can be easily treated by substitution with concentrates of functional factor VIII and factor IX, considerable effort has been undertaken to develop a gene therapy for hemophilia in order to improve patients' life quality and reduce high costs of therapy. The principle of gene therapy is the introduction of an intact copy of the factor VIII/factor IX gene in somatic cells, compensating for the defective gene. To do this, retroviral, adenoviral, and adeno-associated virus (AAV) vector systems, among others, were used. Encouraged by the results of preliminary experiments using preponderant mouse and canine models, three clinical phase I studies on hemophilia A and B patients have been initiated, one of which has been preliminarily reported successful.

Lithuanian haemophilia A and B registry comprising phenotypic and genotypic data.

Haemophilia represents the most common hereditary severe bleeding disorder in humans. About 100 families with this condition live in Lithuania, one of the Baltic states with a population of 3.7 million. Haemophilia care and genetic counselling are still rendered difficult owing to limited availability of clotting factor concentrate and molecular genetic diagnosis. In the present study, a haemophilia registry, comprising phenotypic and genotypic data of the majority of Lithuanian haemophilia A and B patients, was established. The phenotype includes the degree of severity, factor VIII:C, factor VIII:Ag, factor IX:C, von Willebrand factor and antigen (VWF:RiCoF, vWF:Ag) and inhibitor status. Genotyping of the factor VIII and IX genes was performed using mutation screening methods and direct sequencing. In 61 out of 63 patients with haemophilia A (96.8%) and all eight patients with haemophilia B (100%), the causative mutations could be detected. Nineteen of the factor VIII gene defects and two of the factor IX gene mutations are reported for the first time. Identified mutations allowed direct carrier diagnosis in 83 female relatives revealing 44 carriers, 38 non-carriers and one somatic mosaicism. The information provided by this registry will be helpful for monitoring the treatment of Lithuanian haemophilia patients and also for reliable genetic counselling of the affected families in the future.

Expression and characterization of von Willebrand factor dimerization defects in different types of von Willebrand disease.

Dimerization defects of von Willebrand factor (vWF) protomers underlie von Willebrand disease (vWD) type 2A, subtype IID (vWD 2A/IID), and corresponding mutations have been identified at the 3' end of the vWF gene in exon 52. This study identified and expressed 2 additional mutations in this region, a homozygous defect in a patient with vWD type 3 (C2754W) and a heterozygous frameshift mutation (8566delC) in a patient with vWD type 2A, subtype IIE. Both mutations involve cysteine residues that we propose are possibly essential for dimerization. To prove this hypothesis, transient recombinant expression of each of the 2 mutations introduced in the carboxy-terminal vWF fragment II and in the complete vWF complementary DNA, respectively, were carried out in COS-7 cells and compared with expression of vWD 2A/IID mutation C2773R and the wild-type (WT) sequence in COS-7 cells. Recombinant WT vWF fragment II assembled correctly into a dimer, whereas recombinant mutant fragments were monomeric. Homozygous expression of recombinant mutant full-length vWF resulted in additional dimers, probably through disulfide bonding at the amino-terminal multimerization site, whereas recombinant WT vWF correctly assembled into multimers. Coexpression of recombinant mutant and recombinant WT vWF reproduced the multimer patterns observed in heterozygous individuals. Our results suggest that a common defect of vWF biosynthesis--lack of vWF dimerization--may cause diverse types and subtypes of vWD. We also confirmed previous studies that found that disulfide bonding at the vWF amino-terminal is independent of dimerization at the vWF carboxy-terminal. (Blood. 2001;97:2059-2066)

De novo factor VIII gene intron 22 inversion in a female carrier presents as a somatic mosaicism.

The intron 22 inversion represents the most prevalent factor VIII gene defect in severe hemophilia A, accounting for about 40% of all mutations. It is hypothesized that the inversion mutations occur almost exclusively in germ cells during meiotic cell division by intrachromosomal recombination between 1 of 2 telomeric copies of the Int22h region and its intragenic homologue. The majority of inversion mutations originate in male germ cells, where the lack of bivalent formation may facilitate flipping of the telomeric end of the single X chromosome. This is the first intron 22 inversion that presents as a somatic mosaicism in a female, affecting only about 50% of lymphocyte and fibroblast cells of the proposita. Supposing a post-zygotic de novo mutation as the usual cause of somatic mosaicism, the finding would imply that the intron 22 inversion mutation is not restricted to meiotic cell divisions but can also occur during mitotic cell divisions, either in germ cell precursors or in somatic cells. (Blood. 2000;96:2905-2906)