(Hypertension. 1999;34:638-642.)
© 1999 American Heart Association, Inc.
Scientific Contributions |
From the Department of Pediatrics, University of Texas Southwestern Medical Center (B.S.N., F.M.R., P.C.W.), Dallas, Tex; Prince Henry's Institute of Medical Research (F.M.R.), Clayton, Victoria, Australia; Third Department of Internal Medicine, Gifu University School of Medicine (T.M.), Gifu, Japan; Department of Pediatrics, Hamamatsu University School of Medicine (Y.I., Y.N.), Hamamatsu, Japan; Queen Elizabeth Hospital (G.P.), Woodville, South Australia, Australia; Division of Pediatric Nephrology, Cedars-Sinai Medical Center (A.M.), Los Angeles, Calif; Department of Pediatrics, University of Texas Medical Branch (L.B.T.), Galveston, Tex; and the Children's Hospital Oakland Research Institute (M.P., C.S.), Oakland, Calif.
Correspondence to Perrin C. White, MD, Department of Pediatrics, University of Texas Southwestern Medical Center, 5323 Harry Hines Boulevard, Dallas, TX 75235-9063. E-mail pwhit2{at}mednet.swmed.edu
| Abstract |
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Key Words: hydroxysteroid dehydrogenases hypertension hydrocortisone metabolism mutation mineralocorticoids
| Introduction |
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The HSD11B2 (HSD11K) gene encoding this enzyme is located on chromosome 16q22,2 and mutations in this gene have been identified in many patients with AME. Most mutations characterized thus far severely impair enzymatic activity. In the current study, we identified several novel 11-HSD2 mutations in 4 patients with AME; 3 of the mutants retain significant enzymatic activity in whole cells, permitting more robust correlations between genotype and phenotype than previously possible.
| Methods |
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Expression of 11-HSD2 Mutants in Mammalian Cells
Mutations were inserted into a pCMV-5/11-HSD2 construct using 2
rounds of PCR as previously described.6 Mutant constructs
were sequenced completely to ensure that no undesired mutations were
introduced into the 11-HSD2 coding sequence. Cells were transfected as
described,7 and transfection efficiencies were determined
by reverse transcription (RT)PCR within the exponential phase of the
reaction. Metabolism of cortisol and corticosterone to
their respective 11-keto derivatives was determined in whole cells and
in cell lysates using thin-layer chromatography as
previously described.7 For determination of kinetic
parameters, CHOP cells transfected with either pCMV5/11-HSD
wild-type, S180F, or G237A constructs were grown for 24 hours,
collected in 1.5 mL of buffer A (50 mmol/L Tris, 2 mmol/L
EDTA, 1 mmol/L MgCl2, 20% glycerol, 2 mg/mL
aprotinin, 0.5 mg/mL leupeptin, 100 mg/mL PMSF, and 1 mg/mL pepstatin;
pH 8.0) and briefly sonicated. Aliquots (100 µL) of the resulting
cell lysate were incubated with an equal volume of buffer A plus 2
mmol/L NAD+ and 20 to 2560 nmol/L tritiated
cortisol. Cells were incubated for times that resulted in <20%
conversion of substrate to product to ensure first-order kinetics.
Western blot analysis was performed as described
previously.5
| Results |
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Enzymatic Activity in Whole Cells
CHOP cells transfected with wild-type 11-HSD2 were able to convert
both cortisol and corticosterone into cortisone and
11-dehydrocorticosterone, respectively
(Figure 1). Neither sham-transfected CHOP
cells nor CHOP cells transfected with an empty pCMV-5 vector
metabolized cortisol or corticosterone to an appreciable extent (data
not shown). Metabolism of cortisol and corticosterone by
cells transfected with mutants L179R and R208H was not significantly
different from sham-transfected cells or cells transfected with pCMV5
only. Mutants S180F, A237V, and A328V retained significant activity,
having 19%, 72%, and 25% of wild-type activity, respectively, when
cortisol was used as the substrate and 80%, 140%, and 55% of
wild-type activity, respectively when corticosterone was used as the
substrate. CHOP cells cotransfected with equal amounts of A237V and
A328V metabolized both cortisol and corticosterone at rates
intermediate of those transfected with either A237V or A328V alone (not
shown).
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Enzymatic Activity in Cell Lysates
The enzymatic activities of wild-type 11-HSD2 and mutants were
also assessed in lysed CHOP cells (Figure 1). The wild-type
protein retained high activity after lysis. Because mutants L179R and
R208H were inactive in whole cells, their activity in lysed cells was
not assayed. Mutants S180F, A237V, and A328V were 1.8%, 3.1%, and
0.6% as active as the wild-type enzyme, respectively, when cortisol
was used as the substrate and 5.7%, 5.3%, and 3.9% as active,
respectively, when corticosterone was used as the substrate.
Cotransfection of the A237V mutant with A328V did not significantly
alter the stability of either enzyme in cell lysates (not shown).
The wild-type protein was found to have characteristics consistent with those reported for the same enzyme in other studies7 10 (ie, Km=59 nmol/L and Vmax=175 pmol/min · mg-1 protein with cortisol as the substrate). The mutations S180F and A237V generated proteins with decreased affinity for cortisol (Km=631 and 868 nmol/L, respectively) and decreased maximum velocity (Vmax=8 and 16 pmol/min · mg-1 protein, respectively).
Western blot analysis of lysates from transfected CHOP cells indicated that all pCMV5/11-HSD2 constructs were translated into protein, but less immunoreactivity was observed in lysates from cells transfected with mutant constructs as compared with wild-type constructs (not shown). RT-PCR analysis of total RNA from transfected cells indicated comparable production of 11-HSD2 RNA from all constructs (not shown).
| Discussion |
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Indeed, THE/(THF+aTHF) is highly correlated (R2=0.648, P<0.0001) with the percentage of wild-type enzymatic activity when cortisol is used as the substrate (Figure 2). This ratio is also correlated with enzymatic activity in whole cells when corticosterone is the substrate (R2=0.413, P<0.0005) and weakly correlated with enzymatic activity in cell lysates when cortisol is the substrate (R2=0.275, P=0.07).
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The free urinary cortisone/free urinary cortisol ratio (E/F)5 was also correlated with enzymatic activity when cortisol was used as the substrate (R2=0.597, P<0.0001). Although the slightly lower correlation, as compared with THE/(THF+aTHF), was apparently due to the smaller number of subjects for whom free cortisone and cortisol excretion data were available, the data suggest that these 2 ratios have nearly equivalent validity for evaluation of AME.
Several assumptions were made in this analysis. In the entire analysis, patient 4 was the only compound heterozygote with 2 missense mutations associated with different levels of activity. The analysis assumed the patient's actual enzymatic activity to be the mean of the enzymatic activities of each of the 2 mutations, which is consistent with the results obtained when the 2 corresponding expression constructs were transfected together into CHOP cells (not shown). However, the results of the correlation between THE/(THF+aTHF) and whole-cell metabolism of cortisol changed minimally when the activity of either the less (R2=0.627) or more (R2=0.651) severely affected mutant was assumed instead. For mutations R337C and A328V, we observed greater enzymatic activity of the recombinant enzyme than has been reported by others, and we used our own measurements in the analysis. In the case of R337C, the discrepancy applied only to activity in cell lysates,7 14 but A328V9 was previously reported as being inactive in whole cells. Possible explanations for this difference include higher overall levels of expression in our study or perhaps an additional unsuspected mutation in the expression construct used by other investigators. For R279C, activity in whole cells was not reported, and we assumed it to be 70% of wild-type activity, the percentage observed in cell lysates,12 with cortisol as the substrate. Because most mutants have less activity relative to the wild-type enzyme in cell lysates than in whole cells, this is probably an underestimate. The correlation was slightly improved (R2=0.719) when 100% of wild-type activity was assumed. We did not include an average "normal" individual (ratio of 0.9 at 100% activity) in the analysis. The correlation became extremely strong (R2=0.935, P<0.0001) when the regression line was forced through this point.
Finally, patient 3 wasn't included in the analysis because it is difficult to estimate the expression of 11-HSD2 due to normally spliced mRNA in the patient. However, both this patient and a previously reported patient who was homozygous for the same mutation6 have only moderate elevations in (THF+aTHF)/THE ratio (14.6 and 7.9, respectively), suggesting that at least a small proportion of mRNA transcripts of the mutant gene are properly spliced.
It should be noted that the intercept of the regression line (0.045±0.015) differed significantly (P=0.0053) from 0. In other words, an average of 5% of the normal amount of cortisone metabolites is produced in patients who are predicted to completely lack 11-HSD2 activity. In such patients, cortisone must presumably be produced by an enzyme other than 11-HSD2. The most likely candidate is the liver isozyme of 11-HSD, 11-HSD1, which functions predominantly as a reductase in vivo but catalyzes both oxidation and reduction when expressed in cultured cells.15
Correlations Between Genotype and Clinical
Phenotype
The increased number of patients with relatively mild disease
prompted us to attempt to correlate various parameters of
clinical severity with whole-cell enzymatic activity using cortisol as
the substrate (Figure 2). Birth weight
(R2=0.576, P=0.0004) and
age at diagnosis (R2=0.614,
P<0.0001) were both strongly correlated with enzymatic
activity, and serum potassium levels were weakly but significantly
correlated (R2=0.133,
P<0.05). Because blood pressure varies with age, Z scores
(number of standard deviations above the mean for that age) were
calculated for blood pressures at presentation by using
published normative data.16 No correlation with
either systolic or diastolic blood pressure could
be established regardless of whether all patients or only the first
diagnosed patient in each kindred was included. This may reflect the
inherent "noisiness" of random blood pressure measurements compared
with many other clinical parameters, or the pathophysiology
of AME may result in maximal stimulation of mineralocorticoid receptors
with even moderate compromise of 11-HSD2 enzymatic activity.
Mutations Cluster in Exons 3 to 5
It is noteworthy that all reported mutations cluster in exons 3-5,
a region representing 60% of the coding
sequence.2 The likelihood of this occurring by chance is 1
in 7.1x10-5. The explanation for this is not
immediately obvious; 50% of AME mutations are CpG to TpG changes, but
exon 1 has a higher proportion of CpG dinucleotides (20%
versus 4%) than the remainder of the gene. However, CpG to TpG
mutations occur by deamination of methylcytosine, so
hypomethylation of the 5' end of this gene in germ cells is a possible
partial explanation for the observed distribution of mutations. It
cannot explain the distribution of other mutations. Moreover, several
codons, including R208, L250, and R337, have undergone several
independent mutations, as evidenced by both occurrence in unrelated
ethnic groups and the presence of different mutations in each codon;
the latter 2 codons have undergone both CpG to TpG and other types of
mutations in different patients. Half of all kindreds carry mutations
in at least 1 of these 3 codons, suggesting that these codons are
actual mutational hot spots.
Functional Effects of Mutations
The missense mutations identified in the present study are all
in rather highly conserved positions in the enzyme, although none
affects residues known to be involved in binding the
nucleotide cofactor or in catalysis. These mutations all
decrease enzymatic activity in cell lysates more than activity in whole
cells, suggesting that they affect stability of the enzyme.
Consistent with this, decreased amounts of the mutant proteins
are detected on Western blots. This has been previously noted with
other mutations.7
Although at least 1 mutant has been previously reported to have full activity in whole cells,13 it is not surprising that altered stability of such mutant enzymes might nevertheless have adverse effects on activity in vivo. Presumably, 11-HSD2 is synthesized at lower levels in renal tubular epithelial cells than in cultured cells transfected with a high level expression vector, and steady-state levels of the enzyme may be more sensitive to its rate of degradation when levels of synthesis are relatively low.
How Common Is AME?
It is unlikely that mild forms of AME could be a significant cause
of essential hypertension as previously suggested,12 13
except perhaps in the relatively inbred populations in which several of
these mutations have been reported. AME clearly behaves as an autosomal
recessive disease in the great majority of kindreds. Thus, it will be
rare unless there are frequent unsuspected mutations in the population.
This is also unlikely; most patients, including those who have mild
disease, are homozygous for a single mutation and come from relatively
inbred populations, suggesting that matings between 2 carriers are rare
in the general population. Moreover, searches for coding sequence
polymorphisms in normal individuals have been unrewarding
(Smolenicka et al17 and unpublished observations). Thus,
we cannot recommend routine genetic characterization of this locus in
patients with low-renin essential hypertension, although it may be
worthwhile in individuals with a suggestive history and altered
cortisol/cortisone metabolite ratios. However, polymorphisms
outside the coding sequence that affect gene expression have not been
ruled out and could be a more important risk factor for hypertension,
particularly that of the salt-sensitive type.
| Acknowledgments |
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| Footnotes |
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Received May 13, 1999; first decision June 2, 1999; accepted June 7, 1999.
| References |
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