Characterization of mutations causing factor VII deficiency in 61 unrelated Israeli patients

Genetic Landscape of Factor VII Deficiency: Insights from a Comprehensive Analysis of Pathogenic Variants and Their Impact on Coagulation Activity

Congenital factor VII (FVII) deficiency is a rare genetic bleeding disorder characterized by deficient or reduced activity of coagulation FVII. It is caused by genetic variants in the F7 gene. We aimed to evaluate the rate of detection of pathogenic variants in the F7 gene in a large group of patients with FVII deficiency and investigate the correlations between the F7 genotype and FVII activity (FVII:C). Moreover, the influence of the common genetic variant rs6046: c.1238G>A; p.(Arg413Gln), designated as the M2 allele, on FVII:C was investigated. Genetic analysis of the F7 gene was performed on 704 index patients (IPs) using either direct Sanger- or next-generation sequencing. Genetic variants were detected in 390 IPs, yielding a variant detection rate (VDR) of 55%. Notably, the VDR exhibited a linear decline with increasing FVII:C levels. We identified 124 genetic variants, of which 48 were not previously reported. Overall, the frequency of the M2 allele was considerably higher in patients with mild deficiency (FVII:C > 20 IU/dl). Furthermore, IPs lacking an identified pathogenic variant exhibited a significantly higher prevalence of the M2 allele (69%) compared to IPs with a disease-causing variant (47%). These results strongly support the association of the M2 allele with decreased FVII:C levels. This study shows the utility of FVII:C as a predictive marker for identifying pathogenic variants in patients with FVII deficiency. The M2 allele contributes to the reduction of FVII:C levels, particularly in cases of mild deficiency.

Molecular Genetic Analysis of Russian Patients with Coagulation Factor FVII Deficiency

Coagulation factor VII (proconvertin) is one of the proteins starting the blood coagulation cascade. Plasma FVII concentration is regulated by different factors. A low level of FVII could also be a result of FVII deficiency (MIM# 227500), the rare autosomal recessive inherited disease caused by pathogenic variants in the F7 gene. The aim of this study was to describe a mutation spectrum of the F7 gene and genotype-phenotype relationship in patients with FVII deficiency in Russia for the first time. We studied the primary structure of the F7 gene of 54 unrelated patients with FVII deficiency by direct Sanger sequencing. Pathogenic variants in the F7 gene were detected in 37 (68.5%) of them. We identified 24 different mutations located mostly in the serine protease domain. Five pathogenic variants had never been reported before. A major mutation in the Russian population was c.1391delC (p. Pro464Hisfs*32), linked with rs36209567 and rs6046 functional polymorphisms, that is widely distributed in East Europe. As in other countries, the F7 genotypes poorly correlated with the severity of clinical manifestations but were quite well associated with FVII levels. Minor alleles of functional polymorphisms rs510335, rs5742910, rs561241, rs36209567, and rs6046 could also participate in the F7 genotype and influence FVII levels.

Utility of ACMG classification to support interpretation of molecular genetic test results in patients with factor VII deficiency

Background

The American College of Medical Genetics and Genomics (ACMG) and the Association for Molecular Pathology (AMP) have introduced an internationally shared framework for variant classification in genetic disorders. FVII deficiency is a rare inherited autosomal recessive bleeding disorder with sparse data concerning ACMG classification.

Methods

To develop an approach which may improve the utility of molecular genetic test results, 129 patients with FVII deficiency were retrospectively assigned to six subgroups for exploratory analysis: F7 gene wildtype (group 1), ACMG 1 (benign variant) or ACMG 2 (likely benign variant), only (group 2), ACMG 3 (variant of uncertain significance) ± ACMG 1-2 heterozygous or not classified variant (group 3), ACMG 4 (likely pathogenic variant), or ACMG 5 (pathogenic variant) single heterozygous ± ACMG 1-3 single heterozygous (group 4), ACMG 4-5 homozygous or ≥2 ACMG 4-5 heterozygous or ≥1 ACMG 4-5 heterozygous plus either ACMG 1 c.1238G>A modifying variant homozygous or ≥2 ACMG 1-3 (group 5), FVII deficiency and another bleeding disorder (group 6).

Results

Eleven of 31 patients (35.5%) in group 5 had abnormal ISTH-BS (n = 7) and/or history of substitution with recombinant factor VIIa (n = 5) versus 4 of 80 patients (5.0%, n = 1 abnormal ISTH-BS, n = 3 substitution) in groups 1 (n = 2/22), 2 (n = 1/29), 3 (n = 0/9), and 4 (n = 1/20). Four of 18 patients (22.2%) with FVII deficiency and another bleeding disorder (group 6) had an abnormal ISTH-BS (n = 2) and/or history of substitution with recombinant factor VIIa (n = 3).

Conclusion

Patients with a homozygous ACMG 4-5 variant or with specific combinations of heterozygous ACMG 4-5 ± ACMG 1-3 variants exhibited a high-risk bleeding phenotype in contrast to the remaining patients without another bleeding disorder. This result may serve as a basis to develop a genotype/phenotype prediction model in future studies.

Molecular spectrum of inherited FVII deficiency in North India revealed a recurrent variant with a founder effect

INTRODUCTION

Inherited Factor VII (FVII) deficiency is commonest among the rare bleeding disorders. A small number of patients present in infancy with severe bleeding, and many may remain asymptomatic but detected before surgery/invasive procedures. Genetic testing may be helpful in predictive testing/prenatal diagnosis in severe cases.

AIM

Characterisation of clinical and genotypic spectrum of patients with inherited FVII deficiency.

METHODS

Retro-prospectively, 35 cases with prolonged prothrombin time and FVII activity (FVII:C) <50 IU/dl were subjected to targeted resequencing. After in-silico analysis, variant/s were validated by Sanger sequencing in index cases and family members. Haplotype analysis was done for F7 polymorphisms.

RESULTS

Severe FVII deficiency was found in 50% of patients (FVII:C ≤1 IU/dl), and 42.9% were asymptomatic. Clinical severity assessment revealed 17% severe, 17% moderate and 22.9% patients with mild bleeds. FVII levels ranged from .3 to 38 IU/dl. Molecular analysis revealed variants in 30/35 cases, of which 17 were homozygous, 10 were compound heterozygous and 3 were heterozygous. Twelve genetic variants were identified, one promoter variant c.-30A>C; seven missense (c.215C>G, c.244T>C, c.253G>C, c.904G>A, c.961C>T, c.1109G>T, c.1211G>A), two deletions (c.21delG, c.868_870delATC), and one each of nonsense c.634C>T and splice-site variant c.316+1G>A. Recurrent variants c.1109G>T and c.215C>G were found in 17 and 8 cases, 12 of the former cases were homozygous. They had the same haplotype, indicating the founder effect in North Indians.

CONCLUSION

This is the largest cohort of FVII genotyping from India, confirming heterogeneity in terms of clinical manifestations, FVII activity and zygosity of the variants with a limited genotypic phenotypic correlation.

Extensive genetic screening of Iranian Factor FVII-deficient individuals unraveled several novel mutations and postulated founder effects in some cases

Background

As the most frequent congenital rare bleeding disorder that transmits in an autosomal recessive manner, factor VII (FVII) deficiency is a serious bleeding complication in populations with high rate of in-marriages. While diagnosis mainly relies on clinical and laboratory phenotypes, plasma FVII antigen and activity levels do not often correlate with symptoms' severity.

Objectives

Genetic profiling of the affected individuals potentially improves our biological understanding of this complicated rare disorder.

Methods

Conventional polymerase chain reaction-Sanger sequencing and whole-exome sequencing were applied for genetic profiling of F7 gene in 66 symptomatic FVII-deficient individuals from 62 independent pedigrees. Thirty-nine asymptomatic relatives of the patients were also studied.

Results

Thirty different F7 pathogenic variations were identified in the studied cases of which 11 have not been reported before. The novel mutations include 5 missenses (c.715G>A, c.794T>C, c.1090C>G, c.1222C>A, c.1265T>C), 3 splicing (c.316+1G>T, c.682-2A>G, c.572-16C>G), 2 nonsenses (c.790delC, c.1248G>A), and 1 frameshift (c.1346delA). A founder effect is proposed for c.790delC that was detected in 8 independent pedigrees who were all from similar geographical regions and ethnic backgrounds. Homozygous c.790delC reduces plasma FVII activity to <1% and causes spontaneous intracranial hemorrhage in early infancy.

Conclusion

From the 66 studied symptomatic FVII-deficient individuals, 58 were homozygous carriers of the identified variations. Identification of homozygotes clarifies the potential role of nucleotide variations in reducing FVII activity and their contributions to a certain phenotype. Some of those variations, such as c.1A>G, c.509G>A, c.634C>T, and c.1285G>A have only been previously reported as heterozygous.

Novel mutation in coagulation factor VII (Carmel mutation): Identification and characterization

BACKGROUND

Measurement of factor VII (FVII) activity does not enable prediction of bleeding tendency in individuals with inherited FVII deficiency.

OBJECTIVE

To characterize the molecular and functional features of FVII in a family with FVII deficiency and correlate them with the bleeding tendency.

PATIENTS/METHODS

We studied 7 family members with very low FVII activity using prothrombin time (PT), activated factor VII (FVIIa), FVII activity level, and thrombin generation. The factor 7 gene was sequenced and the mutation was analyzed by prediction software.

RESULTS

The proband has very low FVII activity (0%-4%), with PT ranging between 5% to 18% depending on the tissue factor (TF) origin. Direct sequencing demonstrated a single homozygous nucleotide substitution G > A in exon 6, predicting a novel missense mutation Cys164Tyr. Three members of the family were found to be heterozygous carriers of this mutation. One of them was a compound heterozygote, carrying both the Cys164Tyr and Ala244Val mutation (linked to Arg353Gln polymorphism). Her FVII activity and antigen levels were 3%-7% and 8%, respectively. The other heterozygous carriers demonstrated FVII activity of 41%-54%, FVII antigen of 46%-66%, and FVIIa activity of 30%. FVIIa was undetectable in the homozygous and compound heterozygous subjects. Thrombin generation was normal in the presence of calcium, but no response to TF addition was observed in the homozygous proband, and a reduced response was observed in the compound heterozygous subject.

CONCLUSION

The patient homozygous for the "Carmel" mutation has mild clinical manifestations despite very low FVII activity, which correlates with thrombin generation results.

Thrombotic Events in Homozygotes with a Proven or Highly Probable Arg304Gln Factor VII Mutation (FVII Padua)1): Only Limited Replacement Therapy is Needed in Case of Surgery

OBJECTIVE

To investigate the prevalence of thrombotic events among patients with proven or highly probable homozygosis for the Arg304Gln (Factor VII Padua) defect or compound heterozygosis containing the Arg304Gln mutation.

METHODS

Homozygotes and compound heterozygotes proven by molecular studies to have the Arg304Gln mutation were gathered from personal files and from two PubMed searches. In addition, patients with probable homozygosis on the basis of clotting tests (discrepancies among Factor VII activity levels according to the tissue thromboplastin used) were also gathered.

RESULTS

30 proven homozygotes and 17 probable ones were gathered together with 8 compound heterozygotes. In the latter use, the associated mutation was Cys135Arg (twice), Gly180Arg, Arg304Trp, Arg315Trp, His348Gln, Gly365Cys. The prevalence of venous thrombotic events was 16.6, 11.8 and 11.1 percent, respectively for the three groups of patients. Heterozygotes showed no thrombotic event. The difference for proven homozygotes was statistically significant, while for the other groups only a trend was present.

CONCLUSION

proven homozygous or compound heterozygous patients with the Arg304Gln mutation showed a higher than expected incidence of thrombotic events. The same is true for probable cases gathered only on the basis of clotting tests. These patients, because of their frequent lack of bleeding and for their relatively high prevalence of thrombosis should probably receive only limited replacement therapy in case of surgical procedures.

Functional and Molecular Characterization of C91S Mutation in the Second Epidermal Growth Factor-Like Domain of Factor VII

Background

Coagulation Factor VII is a vitamin K-dependent serine protease which has a pivotal role in the initiation of the coagulation cascade. The congenital Factor VII deficiency is a recessive hemorrhagic disorder that occurs due to mutations of F7 gene. In the present study C91S (p.C91S) substitution was detected in a patient with FVII deficiency. This mutation has not been characterized by a functional study.

Objectives

In this study, we aimed to evaluate the impact of C91S substitution on factor VII expression and function.

Materials and Methods

The F7 complete cDNA was isolated from HepG2 cell line and inserted into the pcDNA3.1 mammalian expression vector. The desired mutation was generated by the site-directed mutagenesis and the wild-type and mutated constructs were transfected into CHO-K1 cells. The protein activity and antigen level (antigen concentration) were validated in the culture medium and cell lysate of the transiently transformed cells. An immunocytochemistry procedure was also performed to evaluate the intracellular localization of the mutated and the wild-type FVII, as well.

Results

The present in vitro study has demonstrated that C91S antigen expression was increased in the transfected CHO-K1 cells compared to the wild-type (WT) protein. Despite an increased protein secretion, the factor VII coagulant activity was diminished following C91S substitution when it was assessed by a standard one-stage analysis. In addition, the immunocytochemistry procedure revealed that there was no difference in the intracellular localization of the C91S mutated FVII compared to the WT protein.

Conclusions

Our results present that C91S mutation has an effect on the coagulation activity, secretion, biosynthesis, and probably folding of the FVII leading to the FVII deficiency.

Factor VII deficiency: a novel missense variant and genotype-phenotype correlation in patients from Southern Italy

This study aimed at attempting to correlate genotype and phenotype in factor VII deficiency. Here, we present molecular and clinical findings of 10 patients with factor VII deficiency. From 2013 to 2016, 10 subjects were referred to our center because of a prolonged prothrombin time identified during routine or presurgery examinations or after a laboratory assessment of a bleeding episode. Mutation characterization was performed using the bioinformatics applications PROMO, SIFT, and Polyphen-2. Structural changes in the factor VII protein were analyzed using the SPDB viewer tool. Of the 10 variants we identified, 1 was responsible for a novel missense change (c.1199G>C, p.Cys400Ser); in 2 cases we identified the c.-54G>A and c.509G>A (p.Arg170His) polymorphic variants in the 5′-upstream region of the factor VII gene and exon 6, respectively. To our knowledge, neither of these polymorphic variants has been described previously in factor VII-deficient patients. In silico predictions showed differences in binding sites for transcription factors caused by the c.-54G>A variant and a probable damaging effect of the p.Cys400Ser missense change on factor VII active conformation, leading to breaking of the Cys400-Cys428 disulfide bridge. Our findings further suggest that, independently of factor VII levels and of variants potentially affecting factor VII levels, environmental factors, e.g., trauma, could heavily influence the clinical phenotype of factor VII-deficient patients.

F7 gene variants modulate protein levels in a large cohort of patients with factor VII deficiency. Results from a genotype-phenotype study

Congenital factor VII (FVII) deficiency is a rare bleeding disorder caused by mutations in F7 gene with autosomal recessive inheritance. A clinical heterogeneity with poor correlation with FVII:C levels has been described. It was the objective of this study to identify genetic defects and to evaluate their relationships with phenotype in a large cohort of patients with FVII:C<50 %. One hundred twenty-three probands were genotyped for F7 mutations and three polymorphic variants and classified according to recently published clinical scores. Forty out of 123 patients (33 %) were symptomatic (43 bleedings). A severe bleeding tendency was observed only in patients with FVII:C<0.10 %. Epistaxis (11 %) and menorrhagia (32 % of females in fertile age) were the most frequent bleedings. Molecular analysis detected 48 mutations, 20 not reported in the F7 international databases. Most mutations (62 %) were missense, large deletions were 6.2 %. Compound heterozygotes/homozygotes for mutations presented lower FVII:C levels compared to the other classes (Chi2=43.709, p<0,001). The polymorphisms distribution was significantly different among the three F7 genotypic groups (Chi2=72.289, p<0,001). The presence of truncating mutations was associated with lowest FVII:C levels (Chi2=21.351, p=0.002). This study confirms the clinical and molecular variability of the disease and the type of symptoms. It shows a good correlation between the type of F7 mutation and/or polymorphisms and FVII:C levels, without a direct link between FVII:C and bleeding tendency. The results suggest that large deletions are underestimated and that they represent a common mechanism of F7 gene inactivation which should always be investigated in the diagnostic testing for FVII deficiency.

Factor VII Tokushima: the first case of factor VII Cys22Gly with the development of myocardial infarction in the proband receiving recombinant factor VIIa replacement therapy

An 81-year-old man was referred to our department because of suspected factor VII (FVII) deficiency. His FVII activity was under 1%, whereas the FVII activity levels of his son and granddaughter were 65 and 109%, respectively. The nucleotide at position 3886 of his FVII gene was homozygous for G. A single T to G substitution results in the replacement of wild-type Cys at residue 22 by Gly. His son was heterozygous for G and T at position 3886, whereas his granddaughter was homozygous for wild-type T. These results suggest that he was homozygous for FVII Cys22Gly. He underwent radiofrequency ablation (RFA) for hepatocellular carcinoma, receiving 20 μg/kg of recombinant FVIIa prior to RFA and 10 μg/kg of recombinant FVIIa twice after RFA. He showed no bleeding tendency; however, a myocardial infarction was diagnosed and percutaneous coronary intervention was performed.

Recombinant factor VIIa treatment for asymptomatic factor VII deficient patients going through major surgery

Factor VII deficiency is the most common among the rare autosomal recessive coagulation disorders worldwide. In factor VII deficient patients, the severity and clinical manifestations cannot be reliably determined by factor VII levels. Severe bleeding tends to occur in individuals with factor VII activity levels of 2% or less of normal. Patients with 2-10% factor VII vary between asymptomatic to severe life threatening haemorrhages behaviour. Recombinant factor VIIa (rFVIIa) is the most common replacement therapy for congenital factor VII deficiency. However, unlike haemophilia patients for whom treatment protocols are straight forward, in asymptomatic factor VII deficiency patients it is still debatable. In this study, we demonstrate that a single and very low dose of recombinant factor VIIa enabled asymptomatic patients with factor VII deficiency to go through major surgery safely. This suggestion was also supported by thrombin generation, as well as by thromboelastometry.

Recurrent mutations and genotype-phenotype correlations in hereditary factor VII deficiency in Korea

Coagulation factor VII (FVII) deficiency is a rare hereditary coagulopathy caused by mutations in the F7 gene. The aims of this study were to characterize the molecular defect of F7 in Korean patients with FVII deficiency and to find genotype-phenotype correlations. Study individuals consisted of 14 unrelated Korean patients with FVII deficiency with residual FVII activities ranging from 1 to 34%. To identify causative mutations, we performed PCR amplification and direct sequencing of all exons and flanking sequences of F7 gene. In all 14 patients, one (N = 4) or two (N = 10) mutant alleles were identified. A total of 11 unique mutations were detected, of which four were novel (c.-1C>T, p.V54RfsX53, p.R59_R60dupRR, and p.L314 V). Four recurrent mutations were observed in 86% of patients (12 of 14) (C389G, C115X, G343S, and c.572-1G>A) and accounted for 71% of all mutant alleles (17 of 24). The residual FVII activity was more than 5% in all six asymptomatic patients (21%, range 6-34%), whereas it was 5% or less in all eight symptomatic patients (2%, range 1-5%). In addition, the mean residual FVII activity in four patients with a single mutant allele was 23.6% (range 3-34%), which was significantly higher than that in 10 patients with two mutant alleles that was 4.6% (range 1-19%) (P = 0.023). In summary, the data from this study on the largest number of FVII deficiency in Korea showed recurrent mutations in this population and suggested genotype-phenotype correlations.

Factor VII Deficiency due to Compound Heterozygosity for Leu-48Pro Mutation and a Novel Pro260Leu Mutation

We investigated the mechanisms responsible for factor VII (FVII) deficiency in a compound heterozygous Japanese patient with mutations both in the signal peptide and in the catalytic domain. FVII activity (FVII:C) and antigen (FVII:Ag) levels of the patient were 14.5% and 12.5% of those of the normal controls, respectively. In all, 2 heterozygous point mutations were identified in the patient: one was the mutation substituting Pro for Leu-48 in the prepeptide domain of FVII; the other one was a novel mutation substituting Leu for Pro260 in the catalytic domain. FVII activity and FVII:Ag levels in the condition medium that transiently coexpressed the 2 different FVII mutants in baby hamster kidney (BHK) cells were 4.81% and 5.18% of the wild-type FVII. Factor VII defect of the patient may be combined with both impairing endoplasmic reticulum (ER) targeting and altering FVII folding/biosynthesis, but cotransfection of 2 different FVII mutants may interfere with their expression in BHK cells.

Familial factor VII deficiency with foetal and neonatal fatal cerebral haemorrhage associated with homozygosis to Gly180Arg mutation

Inherited factor VII (FVII) deficiency is a rare autosomal recessive disorder with a wide heterogeneous clinical pattern. Intracranial haemorrhage in infants has been previously reported in the severe form of the FVII deficiency and it has a high fatality rate. We report a family with high consanguineous relations, who experienced death of two baby girls, the first with prenatal manifestation of foetal hydrocephalus secondary to intracranial bleeding and the second with postnatal intracranial bleeding, both with less than 1% activity of FVII. Genetic analysis revealed that both parents are heterozygous and both daughters homozygous for a point mutation gG9639A in exon 7, predicting Gly180Arg substitution. This mutation was described previously in a compound heterozygous patient with mild bleeding manifestation. It seems that in this family, the mutation in its homozygous state is fatal and the lethal clinical expression can appear in utero at an early stage of gestation.

Age estimates of ancestral mutations causing factor VII deficiency and Dubin–Johnson syndrome in Iranian and Moroccan Jews are consistent with ancient Jewish migrations

Factor VII (FVII) deficiency and Dubin-Johnson syndrome (DJS) are rare autosomal recessive disorders caused by mutations in F7 and MRP2 genes, respectively. Both disorders are relatively frequent among Iranian and Moroccan Jews. FVII deficiency in both populations is caused by a founder A244V mutation in the F7 gene and DJS is caused by two founder mutations, I1173F and R1150H in the MRP2 gene that are specific for Iranian and Moroccan Jewish patients, respectively. We estimated the age of FVII A244V and MRP2 I1173F by analysis of microsatellite markers flanking F7 and MRP2 genes, respectively, in 13 Iranian Jewish homozygotes for the I1173F mutation and 21 Iranian and Moroccan Jewish homozygotes for the A244V mutation. Dating of the mutations was estimated by the DMLE+2.0 program employing observed linkage disequilibria of multiple genetic markers. The estimated age of the I1173F mutation was approximately 1500 years, and the age of the A244V mutation was approximately 2600 years. These estimates suggest that I1173F causing DJS in Iranian Jews occurred after the separation of Iranian Jews from Moroccan Jews 2000-2600 years ago, while A244V causing FVII deficiency in Iranian and Moroccan Jews occurred prior to the divergence of these two populations.

Characterization of a Cys329Gly mutation causing hereditary factor VII deficiency

We have previously reported a homozygous Cys329Gly mutation in a Chinese patient with factor VII (FVII) deficiency. Others have found a heterozygous Cys329Gly mutation in the F7 gene from patients of three different pedigrees. However, none of the reports included the expression and characterization of the mutant FVII in vitro. To investigate the effect of Cys329Gly on FVII function, we carried out transient transfections of baby hamster kidney cells (BHK-21) with a mutant FVII construct and compared the results to those obtained using a wild-type FVII construct and vector control. The results demonstrate that the level of FVII:Ag secreted into the medium by transfected BHK-21 cells with mutant construct was not affected, but the coagulation activity of the mutant FVII was undetectable. We conclude that Cys329 is critical to FVII coagulation, and the replacement of cysteine 329 by glycine leads to the loss of coagulation activity in the patients, possibly the molecular basis for FVII deficiency in the patients.

Of four mutations in the factor VII gene in Tunisian patients, one novel mutation (Ser339Phe) in three unrelated families abrogates factor X activation

Hereditary factor VII (FVII) deficiency is a rare bleeding disorder. Dysfunctional FVII variants characterized by normal or reduced levels of FVII antigen and discordantly low FVII activity have been described. In this study, seven unrelated Tunisian patients with FVII deficiency were examined. Molecular analysis revealed that three probands harbored a novel Ser339Phe mutation, one proband was inferred to have a novel splice site mutation in intron 2, c.226-2 A>G and three probands had two previously described mutations, Arg304Gln and Cys310Phe. Expression of Ser339Phe in baby hamster kidney cells yielded secretion of FVII antigen at a concentration of 225+/-50 ng/ml, compared with 181+/-47 ng/ml in cells transfected with wild-type FVII but with no demonstrable FVII activity. FVII Ser339Phe bound to tissue factor similarly to the binding of commercial recombinant activated FVII or recombinant wild-type FVII and was normally activated by activated factor X. The major defect of FVII Ser339Phe was its inability to activate factor X in the presence of tissue factor. Modeling predicted that the substitution of Ser339 by Phe abrogated substrate docking.