Haemophilia A is a classic X-linked disease which affects 1 in 5-10,000 males in all populations and is caused by defects in coagulation factor VIII. Roughly 60% of patients have severe disease with factor VIII activity < 1% of normal; they have frequent spontaneous bleeding into joints, soft tissues, muscles and internal organs. These patients usually require regular injections of plasma-derived or recombinant human factor VIII. Because this is expensive and can potentially lead to life-threatening complications, other forms of therapy, including gene therapy, have been proposed. Natural canine models of factor VIII and factor IX deficiency have been available for many years, and gene therapy attempts on these dogs have met with partial success. However, a small animal model of the disease is desirable for studies of factor VIII function and gene therapy. Using gene targeting, we have made a mouse with severe factor VIII deficiency.

Phosphatidylinositol 3,4,5-trisphosphate is a phospholipid signaling molecule involved in many cellular functions including growth factor receptor signaling, cytoskeletal organization, chemotaxis, apoptosis, and protein trafficking. Phosphorylation at the 3 position of the inositol ring is catalyzed by many different 3-kinases (classified as types I A , I B , II, and III), but the physiological roles played by each of the different 3-kinase isozymes during embryonic development and in homeostasis in animals is incompletely understood. Mammalian type I A kinase isozymes are heterodimers that are active at 37°C when the catalytic 110-kDa subunit interacts through an amino-terminal binding domain with a regulatory 85-or 55-kDa subunit. Using gene targeting in embryonic stem cells, we deleted this binding domain in the gene encoding the ␣ isoform of the 110-kDa catalytic subunit (Pik3ca) of the ␣ isozyme of the type I A kinases, leading to loss of expression of the p110 catalytic subunit. We show that Pik3ca del/del embryos are developmentally delayed at embryonic day (E) 9.5 and die between E9.5 and E10.5. E9.5 Pik3ca del/del embryos have a profound proliferative defect but no increase in apoptosis. A proliferative defect is supported by the observation that fibroblasts from Pik3ca del/del embryos fail to replicate in Dulbecco's modified Eagle's medium and fetal calf serum, even with supplemental growth factors.Phosphorylated phosphoinositides generated by phosphoinositide (PI) 1 3-kinases play crucial roles in signaling through receptors at the cell surface as well as in membrane trafficking mechanisms in Golgi and endosomal compartments, cytoskeletal organization, and regulation of apoptosis (1-9). The genes for many PI 3-kinases have been isolated from a wide range of tissues and organisms. They are divided into three major classes based on their amino acid sequence, the homology among their lipid-kinase domains, and substrate specificities (6, 9). Class I PI 3-kinases phosphorylate the 3Ј sites of PtdIns, PtdIns 4-phosphate, and PtdIns 4,5-diphosphate to form PtdIns 3-phosphate, PtdIns 3,4-diphosphate, and PtdIns 3,4,5-trisphosphate. Class I PI 3-kinases can be further divided into two subclasses. The class I A PI 3-kinases, which include at least three isozymes, ␣, ␤, and ␦, are heterodimers of a catalytic 110-kDa subunit (p110) and a regulatory subunit of 85 or 55 kDa (p85/p55) (10 -14). The interaction of p85/p55 and p110 via the inter-Src homology 2 domain on p85/p55 and the aminoterminal 123 amino acids of p110 is critical for achieving maximal activity of this class of PI 3-kinases in mammalian cells at 37°C (11,15,16).Despite a wealth of knowledge of the biochemistry and cell biological effects of phosphoinositides, very little information is available on the particular physiological roles played by these different enzymes and isozymes during mammalian development. Inhibitors of PI 3-kinases such as wortmannin (17) and LY294002 (18) are very useful for probing the functions of these enzymes but as with a...

SummaryIn order to understand better the mechanism of inhibitor formation in hemophilia A patients, we have characterized the immune response to human factor VIII in a murine model of hemophilia A. Mice with severe factor VIII deficiency caused by targeted gene disruptions in exons 16 and 17 were injected intravenously with human factor VIII. Anti-factor VIII was absent or was detected at only very low levels in hemophilic mice of both strains after a single injection of 0.2 μg factor VIII, but it was present in most mice after a second exposure. Subsequent exposures led to high titer anti-factor VIII antibodies in both ELISA and inhibitor assays. A human factor VIII-specific T cell proliferative response was detected with spleen cells obtained three days after a single injection with human factor VIII, before mice had detectable anti-factor VIII antibodies. Subsequent exposures to factor VIII were followed by an increased T cell proliferative response. These studies indicate that murine hemophilia A is a good model for the study of the immune response to human factor VIII, especially the role of the T cell in the early steps in inhibitor antibody formation.

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