An allosteric disulfide bond is involved in enhanced activation of factor XI by protein disulfide isomerase

Essentials Reduction of three disulfide bonds in factor (F) XI enhances chromogenic substrate cleavage. We measured FXI activity upon reduction and identified a bond involved in the enhanced activity. Reduction of FXI augments FIX cleavage, probably by faster conversion of FXI to FXIa. The Cys362-Cys482 disulfide bond is responsible for FXI enhanced activation upon its reduction.

SUMMARY

Background Reduction of factor (F) XI by protein disulfide isomerase (PDI) has been shown to enhance the ability of FXI to cleave its chromogenic substrate. Three disulfide bonds in FXI (Cys118-Cys147, Cys362-Cys482, and Cys321-Cys321) are involved in this augmented activation. Objectives To characterize the mechanisms by which PDI enhances FXI activity. Methods FXI activity was measured following PDI reduction. Thiols that were exposed in FXI after PDI reduction were labeled with 3-(N-maleimidopropionyl)-biocytin (MPB) and detected with avidin. The rate of conversion of FXI to activated FXI (FXIa) following thrombin activation was assessed with western blotting. FXI molecules harboring mutations that disrupt the three disulfide bonds (C147S, C321S, and C482S) were expressed in cells. The antigenicity of secreted FXI was measured with ELISA, and its activity was assessed by the use of a chromogenic substrate. The effect of disulfide bond reduction was analyzed by the use of molecular dynamics. Results Reduction of FXI by PDI enhanced cleavage of both its chromogenic substrate, S2366, and its physiologic substrate, FIX, and resulted in opening of the Cys362-Cys482 bond. The rate of conversion of FXI to FXIa was increased following its reduction by PDI. C482S-FXI showed enhanced activity as compared with both wild-type FXI and C321S-FXI. MD showed that disruption of the Cys362-Cys482 bond leads to a broader thrombin-binding site in FXI. Conclusions Reduction of FXI by PDI enhances its ability to cleave FIX, probably by causing faster conversion of FXI to FXIa. The Cys362-Cys482 disulfide bond is involved in enhancing FXI activation following its reduction, possibly by increasing thrombin accessibility to FXI.

Founder mutation for Huntington disease in Caucasus Jews

Huntington disease (HD), an autosomal dominant disorder involving HTT, is characterized by chorea, psychiatric illness and cognitive decline. Diagnosis and age of onset depend on the degree of expansion of the trinucleotide CAG repeat within the gene. The prevalence of HD is known for Europeans but has not been studied in the Israeli population. Between 2006 and 2011 we diagnosed in our adult genetics clinic ten HD probands, nine of whom were Caucasus Jews (CJ) (Azerbaijani), and one Ashkenazi Jewish. We performed haplotype analysis to look for evidence of a founder mutation, and found that of the nine CJ, eight shared the same haplotype that was compatible with the A1 haplogroup. We calculated the coalescence age of the mutation to be between 80 and 150 years. Ninety percent of our HD patients are CJ, as are 27% of the HD patients in Israel, although the CJ comprise only 1.4% of the Israeli population. Our findings suggest a higher prevalence of HD among CJ compared to the general Israeli population and are consistent with a recent founder mutation. We recommend a higher degree of suspicion for HD in CJ with subtle clinical findings.

Type I mutation in the F11 gene is a third ancestral mutation which causes factor XI deficiency in Ashkenazi Jews

BACKGROUND

Factor XI (FXI) deficiency is one of the most frequent inherited disorders in Ashkenazi Jews (AJ). Two predominant founder mutations termed type II (p.Glu117Stop) and type III (p.Phe283Leu) account for most cases.

OBJECTIVES

To present clinical aspects of a third FXI mutation, type I (c.1716 + 1G>A), which is also prevalent in AJ and to discern a possible founder effect.

METHODS

Bleeding manifestations, FXI levels and origin of members of 13 unrelated families harboring the type I mutation were determined. In addition, eight intragenic and five extragenic polymorphisms were analyzed in patients with a type I mutation, in 16 unrelated type II homozygotes, in 23 unrelated type III homozygotes and in Ashkenazi Jewish controls. Analysis of these polymorphisms enabled haplotype analysis and estimation of the age of the type I mutation.

RESULTS

Four of 16 type I heterozygotes (25%) and 6 of 12 (50%) compound heterozygotes for type I mutation (I/II and I/III), or a type I homozygote had bleeding manifestations. Haplotype analysis disclosed that like type II and type III mutations, the type I is also an ancestral mutation. An age estimate revealed that the type I mutation occurred approximately 600 years ago. The geographic distribution of affected families suggested that there was a distinct origin of the type I mutation in Eastern Europe.

CONCLUSIONS

The rather rare type I mutation in the FXI gene is a third founder mutation in AJ.

A mutation in the β3 cytoplasmic tail causes variant Glanzmann thrombasthenia by abrogating transition of αIIb β3 to an active state

BACKGROUND

The cytoplasmic tails of α(IIb) and β(3) regulate essential α(IIb) β(3) functions. We previously described a variant Glanzmann thrombasthenia mutation in the β(3) cytoplasmic tail, IVS14: -3C>G, which causes a frameshift with an extension of β(3) by 40 residues.

OBJECTIVES

The aim of this study was to characterize the mechanism by which the mutation abrogates transition of α(IIb) β(3) from a resting state to an active state.

METHODS

 We expressed the natural mutation, termed 742ins, and three artificial mutations in baby hamster kidney (BHK) cells along with wild-type (WT) α(IIb) as follows: β(3) -742stop, a truncated mutant to evaluate the effect of deleted residues; β(3) -749stop, a truncated mutant that preserves the NPLY conserved sequence; and β(3) -749ins, in which the aberrant tail begins after the conserved sequence. Flow cytometry was used to determine ligand binding to BHK cells.

RESULTS AND CONCLUSIONS

Surface expression of α(IIb) β(3) of all four mutants was at least 60% of WT expression, but there was almost no binding of soluble fibrinogen following activation with activating antibodies (anti-ligand-induced-binding-site 6 [antiLIBS6] or PT25-2). Activation of the α(IIb) β(3) mutants was only achieved when both PT25-2 and antiLIBS6 were used together or following treatment with dithiothreitol. These data suggest that the ectodomain of the four mutants is tightly locked in a resting conformation but can be forced to become active by strong stimuli. These data and those of others indicate that the middle part of the β(3) tail is important for maintaining α(IIb) β(3) in a resting conformation.

Identification and functional analysis of two novel mutations in the multidrug resistance protein 2 gene in Israeli patients with Dubin-Johnson syndrome

Dubin-Johnson syndrome (DJS) is an inherited disorder characterized by conjugated hyperbilirubinemia and is caused by a deficiency of the multidrug resistance protein 2 (MRP2) located in the apical membrane of hepatocytes. The aim of this study was to identify the mutations in two previously characterized clusters of patients with Dubin-Johnson syndrome among Iranian and Moroccan Jews and determine the consequence of the mutations on MRP2 expression and function by expression studies. All 32 exons and adjacent regions of the MRP2 gene were screened by polymerase chain reaction and DNA sequencing. Two novel mutations were identified in exon 25. One mutation, 3517A-->T, predicting a I1173F substitution, was found in 22 homozygous Iranian Jewish DJS patients from 13 unrelated families and a second mutation, 3449G-->A, predicting a R1150H substitution, was found in 5 homozygous Moroccan Jewish DJS patients from 4 unrelated families. Use of four intragenic dimorphisms and haplotype analyses disclosed a specific founder effect for each mutation. The mutations were introduced into an MRP2 expression vector by site-directed mutagenesis, transfected into HEK-293 cells, and analyzed by a fluorescence transport assay, immunoblot, and immunocytochemistry. Continuous measurement of probenecid-sensitive carboxyfluorescein efflux revealed that both mutations impaired the transport activity of MRP2. Immunoblot analysis and immunocytochemistry showed that MRP2 (R1150H) matured properly and localized at the plasma membrane of transfected cells. In contrast, expression of MRP2 (I1173F) was low and mislocated to the endoplasmic reticulum of the transfected cells. These findings provide an explanation for the DJS phenotype in these two patient groups. Furthermore, the close localization of the two mutations identify this region of MRP2 as important for both activity and processing of the protein.

Correlation between the incidence of myotonic dystrophy in different groups in Israel and the number of CTG trinucleotide repeats in the myotonin gene

Myotonic dystrophy (DM) is associated with an increased number of CTG repeats in the 3' untranslated region of the myotonin gene. Because DM has been observed less frequently in Ashkenazic Jews and non-Jews than in North African and Yemenite Jews in Israel, a study of the CTG repeat polymorphism was undertaken in these four groups. Alleles from 126 unrelated healthy North African Jews, 103 Yemenite Jews, 103 Ashkenazic Jews, and 106 Israeli Moslem Arabs were studied by PCR analysis of the trinucleotide repeat in the DM gene, and the size distribution of the CTG repeat was determined. The alleles ranged in length from 5-28 repeats in the Yemenite Jews, 5-26 in Muslim Arabs and North African Jews, and 5-23 in the Ashkenazic Jews. North African and Yemenite Jews were found to have significantly more large repeats in the normal range than Ashkenazic Jews and Muslim Arabs (for over 18 repeats: 9.1% and 13%, respectively, compared to 2.4% and 3.3%, respectively; P < 0.0001). It is suggested that the more frequent occurrence of large CTG repeats in the normal range may represent a greater predisposition to DM.

The lower incidence of myotonic dystrophy in Ashkenazic Jews compared to North African Jews is associated with a significantly lower number of CTG trinucleotide repeats

Myotonic dystrophy (DM) is associated with an increased number of CTG repeats in the 3' untranslated region of the myotonin gene. Because DM has been observed more frequently in North African Jews than in Ashkenazic Jews in Israel, a study of the CTG repeat polymorphism was undertaken in these 2 groups. Alleles from 70 unrelated North African subjects and 70 unrelated Ashkenazic subjects were studied by PCR analysis of the trinucleotide repeat in the DM gene to determine the ethnic distribution of the number of CTG repeats. The alleles ranged in length from 5 to 26 repeats in the North Africans and 5 to 23 in the Ashkenazim. As has been seen in other populations, none of the chromosomes had a 9-repeat length. North African Jews were found to have significantly more repeats in the normal range than Ashkenazim (for over 14 repeats: 34/140 compared to 7/140; p < 0.0001). It is suggested that this more frequent occurrence of a large number of CTG repeats in the normal range may represent a greater predisposition to DM.