Filamins are large actin-binding proteins that stabilize delicate three-dimensional actin webs and link them to cellular membranes. They integrate cellular architectural and signalling functions and are essential for fetal development and cell locomotion. Here, we describe the history, structure and function of this group of proteins.

A group of hematologists, involved with hemophilia research and care in the U.S.A., met under the sponsorship of the Division of Blood Diseases and Resources of the National Heart and Lung Institute. In order to improve future communication among ourselves, we decided to alter our individual methods of measurement of inhibitors to the extent necessary to permit a uniform, although arbitrary, description of inhibitor units. We agreed to the following standards: (1) The incubation mixture consists of one part citratecl patient plasma, undiluted or diluted, plus an equal part of citrated pooled normal human plasma. (2) A control incubation mixture consists of equal parts of normal pooled plasma and imidazole buffer, as formulated by Dr. Biggs. (3) The mixtures are incubated at 37° C for two hours. (4) Assays specific for Factor VIII are then performed and the Factor VIII activity in the patient mixture is divided by the Factor VIII activity in the control mixture to determine the percent residual Factor VIII activity. (5) A patient plasma giving a residual Factor VIII activity of 50 percent in this test is said to contain one “Bethesda unit” of inhibitor per ml. (6) On a graph, the log percent residual Factor VIII activity is plotted against inhibitor units. If the residual Factor VIII activity of the incubation mixture is between 75 and 25 percent, the inhibitor units are read from the graph. Plasmas containing strong inhibitors are diluted with imidazole buffer before being placed in the incubation mixture. A dilution is sought which will result in a residual Factor VIII activity between 75 and 25 percent. The units of inhibitor read from the graph are then multiplied by the dilution factor to determine the number of Bethesda units of inhibitor per ml of undiluted patient plasma.We invite interested colleagues to join us in the use of this method, and we invite discussion of better methods of describing inhibitor potency.

The filamins are cytoplasmic proteins that regulate the structure and activity of the cytoskeleton by cross-linking actin into three-dimensional networks, linking the cell membrane to the cytoskeleton and serving as scaffolds on which intracellular signaling and protein trafficking pathways are organized (reviewed in refs. 1,2). We identified mutations in the gene encoding filamin B in four human skeletal disorders. We found homozygosity or compound heterozygosity with respect to stop-codon mutations in autosomal recessive spondylocarpotarsal syndrome (SCT, OMIM 272460) and missense mutations in individuals with autosomal dominant Larsen syndrome (OMIM 150250) and the perinatal lethal atelosteogenesis I and III phenotypes (AOI, OMIM 108720; AOIII, OMIM 108721). We found that filamin B is expressed in human growth plate chondrocytes and in the developing vertebral bodies in the mouse. These data indicate an unexpected role in vertebral segmentation, joint formation and endochondral ossification for this ubiquitously expressed cytoskeletal protein.Morphogenesis in vertebrate organisms requires the integration of extracellular signals with alterations in the cellular cytoskeleton. Filamins regulate the organization of cytoskeletal F-actin into either parallel bundles or orthogonal gel networks 3 and also mediate interactions between subcortical actin networks and transmembrane receptors to modulate cell-cell, cell-matrix and intracytoplasmic signal transduction 1,2,4 . Mammals have three filamin genes, FLNA, FLNB and FLNC. FLNA and FLNB seem to be ubiquitously expressed 5,6 ; FLNC is predominantly expressed in muscle. Human filamin genes are highly similar with conserved exon-intron structure, and there is ∼70% homology at the protein level 2,7 . The filamin monomer comprises an N-terminal actin binding domain (ABD) followed by a series of 24 β-sheet repeats that collectively bind many cytoplasmic and transmembrane proteins 1,2 . Filamins exist in vivo as dimers. Dimerization, leading to homo-and possibly heterodimer formation, is mediated by interactions between C-terminal sequences 5,8,9 . Mutations in FLNA produce a spectrum of X-linked malformation and osteochondrodysplasia syndromes. FLNA loss-of-function mutations are usually embryonically lethal in males and underlie a neuronal migration disorder in females 10 . Mutations producing structural changes in the protein lead to numerous developmental anomalies in the brain, skeleton and viscera 11 .Recently the gene associated with SCT, an autosomal recessive disorder characterized by short stature and vertebral, carpal and tarsal fusions 12,13 , was localized on chromosome 3p14 (ref. 14). These studies and further recombination mapping (data not shown) identified a 4.7-cM candidate region, which included a 1.4-Mb region of homozygosity containing 14 genes. Mutations were not found in the candidate genes WNT5A 14 , ASB14 and IL17RD (also known as SEF) in affected individuals from the linked families. The gene FLNB localizes to this interval and, considering the r...

Approximately one fifth of patients undergoing cardiopulmonary bypass surgery have heparin-induced platelet antibodies detectable before the procedure as a result of prior heparin exposure, and many more develop antibodies after surgery. The absence of an association between these antibodies and thromboembolic complications in this study may be, in part, attributable to careful avoidance of heparin after surgery. The high prevalence of heparin-induced antibodies in this setting suggests that these patients may be at risk of developing thrombotic complications with additional heparin exposure.

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We describe here a test for lupus anticoagulants based on a modified Russell viper venom time (RVVT), using limiting amounts of phospholipid and venom. We have studied 29 patients with a prolonged dilute RVVT. Five of the 29 had a normal activated partial thromboplastin time and three of 14 tested by the tissue thromboplastin inhibition test were normal. In 17 of 19 patients tested, the dilute RVVT was completely normal when ionophore-treated platelets were substituted for phospholipid; the remaining two patients, both with very long phospholipid-dependent dilute RVVT's, were nearly completely normalized. The dilute RVVT is not prolonged in the presence of antibodies to factors VIII, IX, or XI. Thus, the dilute RVVT appears to be a simple, reproducible, sensitive, and relatively specific method for the detection of lupus anticoagulants.

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The therapeutic efficacy of prothrombin-complex concentrates in patients with hemophilia and inhibitors (antibodies) to factor VIII has been increasingly debated. We therefore entered 51 hemophiliacs with factor VIII inhibitors into a double-blind randomized crossover study to compare two commercial prothrombin-complex concentrates (Konyne and Proplex) and an albumin placebo. Acute hemarthrosis of the elbow, knee, or ankle was treated with a single dose of a test preparation and assessed six hours later with objective and subjective criteria. In all measurements the concentrates were significantly more effective than the placebo. The data indicate that although prothrombin-complex concentrates, when used in a single dose, are only partially effective in the treatment of joint hemorrhage in hemophiliacs with inhibitors, their continued use for acute hemarthrosis is justified in the absence of any other effective and readily available therapy for this disorder.