Processing of human factor I in COS-1 cells co-transfected with factor I and paired basic amino acid cleaving enzyme (PACE) cDNA

Complement factor I: Regulatory nexus, driver of immunopathology, and therapeutic

Complement factor I (FI) is the nexus for classical, lectin and alternative pathway complement regulation. FI is an 88 kDa plasma protein that circulates in an inactive configuration until it forms a trimolecular complex with its cofactor and substrate whereupon a structural reorganization allows the catalytic triad to cleave its substrates, C3b and C4b. In keeping with its role as the master complement regulatory enzyme, deficiency has been linked to immunopathology. In the setting of complete FI deficiency, a consumptive C3 deficiency results in recurrent infections with encapsulated microorganisms. Aseptic cerebral inflammation and vasculitic presentations are also less commonly observed. Heterozygous mutations in the factor I gene (CFI) have been demonstrated to be enriched in atypical haemolytic uraemic syndrome, albeit with a very low penetrance. Haploinsufficiency of CFI has also been associated with decreased retinal thickness and is a strong risk factor for the development of age-related macular degeneration. Supplementation of FI using plasma purified or recombinant protein has long been postulated, however, technical difficulties prevented progression into clinical trials. It is only using gene therapy that CFI supplementation has reached the clinic with GT005 in phase I/II clinical trials for geographic atrophy.

A novel method for real-time analysis of the complement C3b:FH:FI complex reveals dominant negative CFI variants in age-related macular degeneration

Age-related macular degeneration (AMD) is linked to 2 main disparate genetic pathways: a chromosome 10 risk locus and the alternative pathway (AP) of complement. Rare genetic variants in complement factor H (CFH; FH) and factor I (CFI; FI) are associated with AMD. FH acts as a soluble cofactor to facilitate FI's cleavage and inactivation of the central molecule of the AP, C3b. For personalised treatment, sensitive assays are required to define the functional significance of individual AP genetic variants. Generation of recombinant FI for functional analysis has thus far been constrained by incomplete processing resulting in a preparation of active and inactive protein. Using an internal ribosomal entry site (IRES)-Furin-CFI expression vector, fully processed FI was generated with activity equivalent to serum purified FI. By generating FI with an inactivated serine protease domain (S525A FI), a real-time surface plasmon resonance assay of C3b:FH:FI complex formation for characterising variants in CFH and CFI was developed and correlated well with standard assays. Using these methods, we further demonstrate that patient-associated rare genetic variants lacking enzymatic activity (e.g. CFI I340T) may competitively inhibit the wild-type FI protein. The dominant negative effect identified in inactive factor I variants could impact on the pharmacological replacement of FI currently being investigated for the treatment of dry AMD.

Proprotein Convertases and the Complement System

Proteins destined for secretion - after removal of the signal sequence - often undergo further proteolytic processing by proprotein convertases (PCs). Prohormones are typically processed in the regulated secretory pathway, while most plasma proteins travel though the constitutive pathway. The complement system is a major proteolytic cascade in the blood, serving as a first line of defense against microbes and also contributing to the immune homeostasis. Several complement components, namely C3, C4, C5 and factor I (FI), are multi-chain proteins that are apparently processed by PCs intracellularly. Cleavage occurs at consecutive basic residues and probably also involves the action of carboxypeptidases. The most likely candidate for the intracellular processing of complement proteins is furin, however, because of the overlapping specificities of basic amino acid residue-specific proprotein convertases, other PCs might be involved. To our surprise, we have recently discovered that processing of another complement protein, mannan-binding lectin-associated serine protease-3 (MASP-3) occurs in the blood by PCSK6 (PACE4). A similar mechanism had been described for the membrane protease corin, which is also activated extracellularly by PCSK6. In this review we intend to point out that the proper functioning of the complement system intimately depends on the action of proprotein convertases. In addition to the non-enzymatic components (C3, C4, C5), two constitutively active complement proteases are directly activated by PCs either intracellularly (FI), or extracellularly (MASP-3), moreover indirectly, through the constitutive activation of pro-factor D by MASP-3, the activity of the alternative pathway also depends on a PC present in the blood.

Functional Analysis of Variants in Complement Factor I Identified in Age-Related Macular Degeneration and Atypical Hemolytic Uremic Syndrome

Complement factor I (FI) is a central inhibitor of the complement system, and impaired FI function increases complement activation, contributing to diseases such as age-related macular degeneration (AMD) and atypical hemolytic uremic syndrome (aHUS). Genetic variation in complement factor I (CFI) has been identified in both AMD and aHUS, with more than half of these variants leading to reduced FI secretion levels. For many of the variants with normal FI secretion, however, functional implications are not yet known. Here we studied 11 rare missense variants, with FI secretion levels comparable to wildtype, but a predicted damaging effects based on the Combined Annotation Dependent Depletion (CADD) score. Three variants (p.Pro50Ala, p.Arg339Gln, and p.Ser570Thr) were analyzed in plasma and serum samples of carriers affected by AMD. All 11 variants (nine for the first time in this study) were recombinantly expressed and the ability to degrade C3b was studied with the C3b degradation assay. The amount of degradation was determined by measuring the degradation product iC3b with ELISA. Eight of 11 (73%) mutant proteins (p.Pro50Ala, p.Arg339Gln, p.Ile340Thr, p.Gly342Glu, p.Gly349Arg, p.Arg474Gln, p.Gly487Cys, and p.Gly512Ser) showed significantly impaired C3b degradation, and were therefore classified as likely pathogenic. Our data indicate that genetic variants in CFI with a CADD score >20 are likely to affect FI function, and that monitoring iC3b in a degradation assay is a useful tool to establish the pathogenicity of CFI variants in functional studies.

Proprotein Convertase Is the Highest-Level Activator of the Alternative Complement Pathway in the Blood

Factor D (FD) is an essential element of the alternative pathway of the complement system, and it circulates predominantly in cleaved, activated form in the blood. In resting blood, mannose-binding lectin-associated serine protease 3 (MASP-3) is the exclusive activator of pro-FD. Similarly to FD, MASP-3 also circulates mainly in the active form. It was not clear, however, how zymogen MASP-3 is activated. To decipher its activation mechanism, we followed the cleavage of MASP-3 in human hirudin plasma. Our data suggest that neither lectin pathway proteases nor any protease controlled by C1-inhibitor are required for MASP-3 activation. However, EDTA and the general proprotein convertase inhibitor decanoyl-RVKR-chloromethylketone completely prevented activation of exogenous MASP-3 added to blood samples. In this study, we show that proprotein convertase subtilisin/kexin (PCSK) 5 and PCSK6 are able to activate MASP-3 in vitro. Unlike PCSK5, PCSK6 was detected in human serum and plasma, and previously PCSK6 had also been shown to activate corin in the circulation. In all, PCSK6 emerges as the MASP-3 activator in human blood. These findings clarify the very first step of the activation of the alternative pathway and also connect the complement and the proprotein convertase systems in the blood.

Functional expression of complement factor I following AAV-mediated gene delivery in the retina of mice and human cells

Dry age-related macular degeneration (AMD) is characterised by loss of central vision and currently has no approved medical treatment. Dysregulation of the complement system is thought to play an important role in disease pathology and supplementation of Complement Factor I (CFI), a key regulator of the complement system, has the potential to provide a treatment option for AMD. In this study, we demonstrate the generation of AAV constructs carrying the human CFI sequence and expression of CFI in cell lines and in the retina of C57BL/6 J mice. Four codon optimised constructs were compared to the most common human CFI sequence. All constructs expressed CFI protein; however, most codon optimised sequences resulted in significantly reduced CFI secretion compared to the non-optimised CFI sequence. In vivo expression analysis showed that CFI was predominantly expressed in the RPE and photoreceptors. Secreted protein in vitreous humour was demonstrated to be functionally active. The findings presented here have led to the formulation of an AAV-vectored gene therapy product currently being tested in a first-in-human clinical trial in subjects with geographic atrophy secondary to dry AMD (NCT03846193).

Classical and Non-classical Presentations of Complement Factor I Deficiency: Two Contrasting Cases Diagnosed via Genetic and Genomic Methods

Deficiency of complement factor I is a rare immunodeficiency that typically presents with increased susceptibility to encapsulated bacterial infections. However, non-infectious presentations including rheumatological, dermatological and neurological disease are increasingly recognized and require a high-index of suspicion to reach a timely diagnosis. Herein, we present two contrasting cases of complement factor I deficiency: one presenting in childhood with invasive pneumococcal disease, diagnosed using conventional immunoassays and genetics and the second presenting in adolescence with recurrent sterile neuroinflammation, diagnosed via a genomic approach. Our report and review of the literature highlight the wide spectrum of clinical presentations associated with CFI deficiency and the power of genomic medicine to inform rare disease diagnoses.

Mutations in complement factor I as found in atypical hemolytic uremic syndrome lead to either altered secretion or altered function of factor I

The complement system is regulated by inhibitors such as factor I (FI), a serine protease that degrades activated complement factors C4b and C3b in the presence of specific cofactors. Mutations and polymorphisms in FI and its cofactors are associated with atypical hemolytic uremic syndrome (aHUS). All 14 complement factor I mutations associated with aHUS analyzed in this study were heterozygous and generated premature stop codons (six) or amino acid substitutions (eight). Almost all of the mutants were expressed by human embryonic kidney 293 cells but only six mutants were secreted into the medium, three of which were at lower levels than WT. The remaining eight mutants were not secreted but sensitive to deglycosylation with endoglycosidase H, indicating that they were retained early in the secretory pathway. Six secreted mutants were purified and five of them were functionally altered in degradation of C4b/C3b in the fluid-phase in the presence of various cofactors and on endothelial cells. Three mutants cleaved surface-bound C3b less efficiently than WT. The D501N mutant was severely impaired both in solution and on surface irrespective of the cofactor used. In conclusion, mutations in complement factor I affect both secretion and function of FI, which leads to impaired regulation of the complement system in aHUS.

Transcriptional profiling of gene expression changes in a PACE-transfected CHO DUKX cell line secreting high levels of rhBMP-2

Chinese hamster ovary (CHO) cells are widely used in the biopharmaceutical industry for the production of recombinant human proteins including complex polypeptides such as recombinant human bone morphogenic protein 2 (rhBMP-2). Large-scale manufacture of rhBMP-2 has associated production difficulties resulting from incomplete processing of the recombinant human protein due to insufficient endogenous levels of the paired basic amino acid cleaving enzyme (PACE) in CHO. In order to resolve this issue, CHO DUKX cells expressing rhBMP-2 were transfected with the soluble version of human PACE (PACEsol) resulting in improved amino-terminal homogeneity and a fourfold increase in rhBMP-2 productivity. In this article, we present a microarray expression profile analysis comparing the parental lineage to the higher producing subclone co-expressing PACEsol using a proprietary CHO-specific microarray. Using this technology we observed 1,076 significantly different genes in the high-productivity cells co-expressing PACEsol. Following further analysis of the differentially expressed genes, the Unfolded Protein Response (UPR) component of the endoplasmic reticulum stress response pathway was identified as a key candidate for effecting increased productivity in this cell system. Several additional ER- and Golgi-localised proteins were identified which may also contribute to this effect. The results presented here support the use of large-scale microarray expression profiling as a viable and valuable route towards understanding the behaviour of bioprocess cultures in vitro.

Characterization of mutations in complement factor I (CFI) associated with hemolytic uremic syndrome

Recent studies have identified mutations in the complement regulatory gene factor I (CFI) that predispose to atypical hemolytic uremic syndrome (aHUS). CFI is a two-chain serine protease in which the light chain carries the catalytic domain while the heavy chain's function is unclear. It downregulates the alternative and classical complement pathways by cleaving the alpha' chains of C3b and C4b in the presence of cofactor proteins (known as cofactor activity). Many CFI mutations in aHUS result in low CFI levels with a consequent quantitative defect in complement regulation. In others, the mutant protein is present in normal amounts but the presumed functional deficiency has not yet been defined. In this report we examine the nature of the functional defect in aHUS-associated CFI mutations. The I322T, D501N and D506V mutations reside in the serine protease domain of CFI and result in secreted proteins that lack C3b and C4b cofactor activity. The delTTCAC (1446-1450) mutant leads to a protein that is not secreted. The R299W mutant lies in a region of the CFI heavy chain of no known function. Our assessments demonstrate decreased C3b and C4b cofactor activity, providing evidence that this region is important for cofactor activity. In two other heavy chain mutants and one probable polymorphic variant, no functional deficiency was identified. These defective mutant proteins will result in an inability to appropriately control the complement cascade at sites of endothelial cell injury. The excessive complement activation for a given degree of damage may result in generation of a procoagulant state and aHUS.

A mutation in factor I that is associated with atypical hemolytic uremic syndrome does not affect the function of factor I in complement regulation

Factor I (FI) is the major complement inhibitor that degrades C3b and C4b in the presence of cofactors such as factor H (FH) and membrane cofactor protein (MCP). Recently, mutations and polymorphisms in complement regulator molecules FH and MCP but also in FI have been associated with atypical hemolytic uremic syndrome (aHUS). HUS is a disorder characterized by hemolytic anemia, thrombocytopenia and acute renal failure. In this study, we report three unrelated patients with an identical heterozygous mutation, G261D, in the FI heavy chain who developed severe aHUS at different time points in their lives. Two of the patients also have polymorphisms in FH previously associated with risk of developing aHUS. Testing in particular one patient and control serum samples we did not observe major differences in complement hemolytic activity, FI plasma levels or the capability to degrade C4b or C3b. A recombinant protein was produced in order to analyze the functional consequences of the mutation. Mutant FI had a slightly different migration pattern during electrophoresis under reducing conditions. An alteration due to alternative splicing or glycosylation was ruled out, thus the altered migration may be due to proximity of the mutation to a cysteine residue. The recombinant mutant FI degraded C3b and C4b in a manner comparable to wild-type protein. In conclusion, despite the association between the heterozygous mutation in FI and aHUS we did not observe any abnormalities in the function of FI regarding complement regulation.

Human complement factor I glycosylation: structural and functional characterisation of the N-linked oligosaccharides

Factor I (fI) is a key serine protease that modulates the complement cascade by regulating the levels of C3 convertases. Human fI circulates in plasma as a heavily N-glycosylated (25-27% w/w) heterodimer composed of two disulphide linked chains, each carrying three N-linked oligosaccharide chains. It had been suggested that the oligosaccharides may have both structural and functional roles in the interactions with the natural substrate and the cofactor during a catalysis. The N-linked glycans of each fI chain were characterised in detail and the analysis revealed a similar composition of the glycan pools with both chains heavily sialylated. Disialylated structures were in excess over monosialylated ones: 55% over 40% for the heavy chain and 62% over 35% for the light chain. The dominant type of glycan identified on both chains was A(2)G(2)S(2), a biantennary structure with chains terminating in sialic acid linked to galactose. The glycan characterisation facilitated a strategy for the partial deglycosylation of the enzyme. Assessment of the proteolytic activities of the native and partially deglycosylated forms of fI showed that both forms of the enzyme have very similar proteolytic activities against C3(NH(3)) indicating that the charged glycans of fI do not influence the fI-cofactor-substrate interactions.

Function of human factor H and I on xenosurface

The cell membrane-bound forms of mini-factor H with 1-4 short consensus repeats (fH-PI) and factor I (fI-PI) were constructed. Swine endothelial cell (SEC) lines and Chinese hamster ovary (CHO) cell expressing fH-PI or fI-PI were established and confirmed by flow cytometry. The cell lysate of the SEC line expressing fH-PI showed strong cofactor activity for the cleavage of C3b, and fI-PI demonstrated the protease activity for C4b and C3b not only in the fluid phase but also on the cell membrane. In addition, fH-PI blocked human complement-mediated cell lysis by approximately 30-40%. An SEC line with a low expression of fI-PI showed a weak inhibition of cell lysis in human serum, whereas a CHO cell transfectant with a high expression of fI-PI showed over a 60% inhibition of cell lysis. The results suggest that fH-PI and fI-PI have potential for use in clinical xenotransplantation.

Transcriptional modulation of the human complement factor I gene in Hep G2 cells by protein kinase C activation

This study examined the role of the protein kinase C (PKC) signalling pathway in the regulation of expression of human complement factor I (CFI) gene. The production of CFI by Hep G2 cells was enhanced in a dose- and time-dependent fashion by 12-O-tetradecanoyl-1,2-phorbol 13-acetate (TPA), a potent PKC activator. 4Alpha-phorbol didecanoate, an inactive phorbol ester, had no effect on CFI synthesis. The TPA-dependent increase in CFI secretion was correlated with an increase in CFI mRNA levels. Forskolin, a cAMP-inducing agent, augmented the TPA response. W7, an inhibitor of protein kinase A and genistein, an inhibitor of protein tyrosine kinase(s) both did not prevent the increase in CFI expression mediated by TPA. However, calphostin C, a specific inhibitor of PKC, abolished the TPA-induced increase in CFI mRNA levels. Down regulation of intracellular PKC levels by prior exposure of Hep G2 cells to a high concentration of TPA also blocked the increase in CFI mRNA levels induced by TPA suggesting that the TPA effects were mediated via activation of PKC. mRNA decay studies indicated that the half-life of CFI mRNA in TPA-induced cells was not significantly different from control. Nuclear run-on transcriptional assays on the other hand demonstrated that whereas the CFI gene is transcribed under basal conditions in Hep G2 cells, TPA induced a 3-4 fold increase in the transcription rate of CFI gene in 24 h. The transcription rate of GAPDH gene did not change, indicating that the effects were not general on gene transcription. Transient transfections of Hep G2 cells with chloramphenicol acetyltransferase reporter gene (CAT) constructs containing a series of sequential 5' deletions of the CFI promoter and CAT assays showed that the sequence between -136 and -130, containing an AP-1 consensus sequence (TGAGTCA) was required for the TPA response. This observation was substantiated by the finding that mutation of this AP-1 site to TttaTCA or TtAtcCA abolished the TPA responsiveness. The enhancement of the activity of transfected chimeric CAT constructs by TPA was abrogated by calphostin C and by pyrrolidine dithiocarbamate (an inhibitor of NF-kappaB and AP-1 transactivation). These results indicate that TPA regulation of CFI gene requires PKC signalling and is mediated by via a TPA response element (TRE) in the CFI promoter region located at -136/-130 and involves the transactivation of AP-1 and NF-kappaB transcription factors. We suggest that PKC may be one of the intracellular pathways that control CFI gene expression and that cellular processes (involving growth factors, hormones, cytokines etc.) that activate PKC may upregulate the expression of the CFI gene.

Rat complement factor I: molecular cloning, sequencing and expression in tissues and isolated cells

Factor I (FI) is a regulatory serine protease of the complement system which cleaves three peptide bonds in the alpha-chain of C3b and two bonds in the alpha-chain of C4b thereby inactivating these proteins. The human protein and the recently characterized mouse factor I are heterodimers of about 88,000 MW which consist of a non-catalytic heavy chain of 50,000 MW which is linked to a catalytic light chain of 38,000 MW by a disulphide bond. For the screening of a rat liver cDNA library we used a hybridization probe produced by polymerase chain reaction (PCR) using degenerated primers which corresponded to conserved parts of the human and the murine factor I nucleotide sequences. One of the identified sequences, which had a length of 2243 base pairs (bp), contained the complete coding region and the whole 3' untranslated region. The length of the coding region in rat consisted of 1812 bp followed by a 3' untranslated region of 207 bp including the polyadenylation signal and the beginning of the poly A tail. Comparison of the rat cDNA-derived coding sequence revealed identities of 87% to the mouse and of 78% to the human FI nucleotide sequence. The translation product of rat FI mRNA was 604 amino acid residues (aa) in length with an identity of 85% to the mouse (603 aa) and 69% to the human protein (583 aa). The comparison of the molecular mass predicted by the primary structure and derived from rat FI isolated from rat serum as detected in immunoblot analyses suggested a glycosylation of more than 20% of the total mass of the FI protein. Expression studies using reverse transcription (RT)-PCR assays indicated that FI-specific mRNA could neither be identified in B cells, nor in T cells, monocytes or granulocytes from rat and human peripheral blood nor in rat peritoneal macrophages. These data were in agreement with the results of RT-PCR obtained with several human lymphoma cell lines (Jurkat, MOLT-4, HUT102, Wil 2-NS, Ramos, Raji, U937) all of which were devoid of FI-specific mRNA. In accord with our data from two rat hepatoma cell lines (FAO and H4IIE) and one from man (HepG2) only isolated rat hepatocytes (HC) but neither Kupffer cells (KC), hepatic stellate cells (HSC; Ito cells) nor sinusoidal endothelial cells (SEC) expressed FI-specific mRNA. FI mRNA was also detected in human umbilical vein endothelial cells (HUVEC) and in the uterus and small intestine of the rat. Spleen and lymph nodes did not contain any detectable FI-specific mRNA.

Human complement factor I: its expression by insect cells and its biochemical and structural characterisation

Factor I is a five-domain plasma serine protease which is essential for the regulation of the complement system. In order to express this, the factor I coding sequence was cloned into a recombinant baculovirus system, which was used to infect Trichoplusia ni cells. Using the native factor I leader sequence, recombinant factor I (rFI) was secreted into the culture medium. Purified rFI was recognised by polyclonal antisera and by the factor I-specific monoclonal antibody MRC-OX21. SDS PAGE showed that rFI was processed into two chains with molecular weights of 48,000 and 36,000. Amino acid sequence analysis showed that the N-terminal sequences of the rFI chains were the same as those of serum-derived factor I (sFI), confirming that processing was correct. Since both molecular weights were less than those observed for sFI, this is attributed to the replacement of complex-type oligosaccharides by high mannose ones in rFI. C3(NH,) cleavage assays showed that rFI had 55% the activity of sFI. Circular dichroism and Fourier transform infrared spectroscopy showed that the protein folding of rFI and sFI were very similar. Both had a secondary structure low in alpha-helix and high in beta-sheet, as expected from crystal structure and multiple sequence alignment analyses. It is inferred that the reduced activity of rFI is attributable to its changed glycosylation. The availability of rFI and structures for the domains in factor I makes possible new approaches to determine the molecular basis of its interactions with factor H and C3b.

Cloning and characterization of the promoter for the human complement factor I (C3b/C4b inactivator) gene

Complement factor I is a serine proteinase that regulates the classical and alternative pathways of complement by cleaving C3b and C4b and preventing the assembly of C3 and C5 convertase enzymes. In order to understand the regulation of factor I gene expression in liver cells, 4kb of the 5' flanking region of the gene was cloned, and the 1474-bp 3'-end was sequenced and shown to contain a number of transcription factor consensus sequences. A major and two minor transcription start sites were identified, respectively, at 152, 178, and 198bp upstream of the translation start site by primer extension analysis. The transcriptional activity of the 1474-bp fragment was analyzed by fusion of 5' deletion constructs to a cat-encoding gene expression vector and transient transfections into Hep G2 cells. A 273-bp fragment located at -112 to +161 relative to the major transcription start site was sufficient for promoter activity. The 3' fragment spanning +3 to +161 and containing a TATA-like element did not demonstrate promoter activity, suggesting that the core promoter resides in a 115-bp sequence located between -112 and +3. This region contains an Inr-like element overlapping the major cap site and a CTF-NF1 element, two potential CCAAT boxes and an AP-2 element partially overlapping an Sp-1 site. Thus, factor I promoter may belong to the TATA-less Inr-driven class II promoters whose transcription is regulated by Sp-1. The transcriptional activity of the 1474-bp 5' flanking fragment was upregulated by PMA, IL-6 and TNF-alpha, suggesting that factor I may be an acute phase reactant.

cDNA cloning, sequencing and chromosomal assignment of the gene for mouse complement factor I (C3b/C4b inactivator): identification of a species specific divergent segment in factor I

Factor I is an essential regulatory serine proteinase of the complement cascade. It cleaves and inactivates the C3b and C4b constituents of the C3 and C5 convertases and thereby regulates many complement-mediated activities. The human protein is a heterodimer composed of a 50 kDa non-catalytic subunit (which contains several domains, i.e. FIM, CD5, LDLr type A) disulfide linked to a 38 kDa catalytic subunit. Recent characterization of Xenopus factor I cDNA revealed a 29 residue negatively charged region in its heavy chain which is absent in the human protein (Kunnath-Muglia et al., Molec. Immun. 30, 1249-1256, 1993). We report the complete cDNA sequence of mouse factor I as well as a partial chicken factor I cDNA sequence. Alignment of these two sequences with the published sequences for human and Xenopus proteins (a) demonstrates an overall conservation of primary structure and domain organization of mouse factor I, and (b) defines a divergent segment (D segment) in each species. In Xenopus protein, the D segment includes the 29 residue negatively charged region. In each of the four species examined, the D segment differed in length, sequence, organization, and number of repeated subregions. These differences reflect a considerable evolution of D segment. The significance of the diversity of the D segment is at present unclear. We also report the chromosomal localization of the mouse factor I gene (Cfi) to distal chromosome 3 near Egf.