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Original Studies |
Research Laboratory of Nephrology and Hypertension (C.S., E.B.P.), the Department of Pediatrics, Skejby Sygehus (S.R.), and Research Unit for Molecular Medicine (P.H.A., B.S.A., N.G.), Aarhus University Hospital, and the Institute of Human Genetics, Aarhus University (T.J.C., T.G.J., L.B.), Aarhus, Denmark; and the Department of Medicine, Northwestern University Medical School (G.L.R.), Chicago, Illinois 60611
Address all correspondence and requests for reprints to: Charlotte Siggaard, M.D., Department of Pediatrics, Skejby Sygehus, Aarhus University Hospital, DK-8200 Aarhus N, Denmark. E-mail: rittig{at}iekf.au.dk
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Abstract |
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The aims of the present study were to relate the clinical phenotype to the specific genotype and to the molecular genetic effects of the most frequently reported adFNDI mutation located at the cleavage site of the signal peptide of AVP-NPII [Ala(-1)Thr]. Genetic analysis and clinical studies of AVP secretion, urinary AVP, and urine output were performed in 16 affected and 16 unaffected family members and 11 spouses of a Danish adFNDI kindred carrying the Ala(-1)Thr mutation. Mutant complementary DNA carrying the same mutation was expressed in a neurogenic cell line (Neuro2A), and the cellular effects were studied by Western blotting, immunocytochemistry, and AVP measurements.
The clinical studies showed a severe progressive deficiency of plasma and urinary AVP that manifested during childhood. The expression studies demonstrated that the Ala(-1)Thr mutant cells produced 8-fold less AVP than wild-type cells and accumulated excessive amounts of 23-kDa NPII protein corresponding to uncleaved prepro-AVP-NPII. Furthermore, a substantial portion of the intracellular AVP-NPII precursor appeared to be colocalized with an endoplasmic reticulum antigen (Grp78).
These results provide independent confirmation that this Ala(-1)Thr mutation produces adFNDI by directing the production of a mutant preprohormone that accumulates in the endoplasmic reticulum, because it cannot be cleaved from the signal peptide and transported to neurosecretory vesicles for further processing and secretion.
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Introduction |
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TopAbstractIntroductionSubjects and MethodsResultsDiscussionReferences |
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Since 1991, adFNDI has been linked to 31 different mutations in 1 allele of the gene that codes for the AVP-neurophysin II precursor protein, AVP-NPII (6, 11, 12, 13, 14, 15, 16, 17, 18, 19, 20, 21, 22, 23, 24, 25, 26, 27, 28, 29, 30). This gene is located in chromosome 20 and consists of 3 exons that encode, respectively, 1) the 19-amino acid signal peptide (SP), vasopressin (AVP), and the amino-terminal region of the transport protein NPII; 2) the highly conserved central part of NPII; and 3) the carboxyl-terminal region of NPII and a glycoprotein, copeptin (31). Of the 31 different mutations identified in adFNDI, 4 are predicted to alter the signal peptide (6, 13, 17, 20, 21, 22, 24, 25), 1 is predicted to alter the AVP moiety (26), and the other 26 are predicted to alter the NP moiety (11, 12, 14, 15, 16, 17, 18, 19, 20, 23, 24, 25, 27, 28, 29, 30). Of the 4 SP mutations, 3 are located near the cleavage site. One that substitutes threonine for alanine at position -1 is the most common mutation described in adFNDI, as it has now been found in 7 apparently unrelated families in America, Japan, Denmark, and Spain.
Based on the location and type of mutations identified in adFNDI and the lack of any major differences in the clinical phenotypes, we and others have postulated that all of the mutations act by replacing or deleting one or more amino acids important for proper folding and processing of the preprohormone (17). As a result of this defect, the mutant precursor cannot be folded and dimerized and is retained in the endoplasmic reticulum, where it accumulates and eventually kills the cell by interfering with the orderly processing of other essential proteins. This hypothesis is supported by recent in vitro expression studies that demonstrate accumulation of mutant AVP-NPII precursor within the endoplasmic reticulum (ER) (32, 33). In the resent study, we have investigated both the clinical phenotype and the cellular and biochemical effects of the AVP-NPII gene mutation most often identified in adFNDI.
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Subjects and Methods |
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TopAbstractIntroductionSubjects and MethodsResultsDiscussionReferences |
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Studies were performed on 32 living members (16 men and 16
women; mean age, 34 yr; range, 2–69 yr) from 3 generations of a Danish
kindred previously shown to have FNDI (4) in association with a
missense mutation predicted to alter the C-terminal residue of the
signal peptide (17) (Fig. 1
). Also
studied were 11 spouses of the affected kindred members (aged 47.1
± 11 yr). The local ethical committee approved the study, and
appropriate informed consent was obtained from all human subjects.
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Members of the family were questioned about the signs and symptoms of diabetes insipidus. All but two of those with a history of polyuria and polydipsia had one or more tests to confirm the diagnosis of diabetes insipidus. The tests included 1) measurements of the 24-h urine volume, urine osmolality, plasma osmolality, and plasma AVP when the subjects were untreated and on ad libitum fluid intake; 2) measurements of plasma osmolality, plasma vasopressin, and urine osmolality during a fluid deprivation test or infusion of hypertonic (3%) saline (34); and 3) measurement of spot urinary osmolality during treatment with therapeutic doses of 1-des-amino-D-arginine-8 AVP (DDAVP). In some affected and unaffected family members, vasopressin was also measured in spot urines collected randomly under conditions of ad libitum fluid intake.
Laboratory
Plasma AVP was measured by RIA, which was a modification of the method described previously (35). Before the assay procedure, C18 Sep-Pak extraction was performed, a rabbit anti-AVP was used (ICM Immunochemicals, Tumba, Sweden). The minimum level of detectability was 0.5 pg/mL. The coefficients of variation were 13% (interassay) and 9% (intraassay). Urinary AVP was determined after lyophilization and reconstitution to isotonicity with distilled water, and the values are expressed per mg creatinine as previously described (6). Plasma and urine samples were measured for osmolality by freezing point depression (Advanced Cryometric Osmometer, 3C2, Advanced Instruments, Needham, MA). Plasma creatinine was measured by routine methods.
Amplification and sequencing of genomic DNA
Genomic DNA was extracted from the buffy coat of peripheral leukocytes as described previously (6). All exons of the VPNPII gene were amplified separately by PCR using 30-bp primers flanking each exon. The primer sequences and locations as well as the PCR cycling conditions have been described previously (17). For restriction enzyme digestion analysis, the PCR product was digested with the endonuclease BstUI following the manufacturer’s instructions.
Construction of expression vectors
A pc-DNA 1 plasmid (Invitrogen, San Diego, CA) containing a 700-bp human wild-type complementary DNA (cDNA) fragment encoding the entire human AVP-NPII precursor protein, pcDNA-HV2 (36), was provided by Prof. D. Richter, Eppendorf University (Hamburg, Germany). Sequencing of the plasmid revealed a deviation from the normal genomic sequence, namely a G to T substitution at position 2120 in the genomic sequence (position 579 in the cDNA sequence (36), predicting an amino acid change in the NPII moiety (Gly88Val). This discrepancy was corrected by replacing the distal part of the cDNA with a PCR-generated fragment from the genomic DNA of a control subject.
The signal peptide mutation Ala(-1)Thr was introduced by site-directed mutagenesis according to the PCR-based method described by Kuipers et al. (37). In the first PCR, a DNA fragment was amplified with a flanking primer in the central portion of exon 2 and a mutagenesis primer containing a G to A substitution at position 279 in the genomic sequence. The purified PCR product from the first PCR was then used as a megaprimer together with the T7 primer in a second PCR with the wild-type plasmid as a template. After digestion with HindIII and SmaI, the purified product from the second PCR was ligated back into the wild-type plasmid. To confirm that no PCR-derived errors were present, the constructed plasmid and the wild-type plasmid were checked by sequencing.
Cell culture and transfection
Mouse Neuro2A cells were obtained from the American Type Culture Collection (Manassas, VA) and were grown in DMEM (Life Technologies, Inc., Gaithersburg, MD) supplemented with 10% FCS. The cells were maintained in a 5% CO2 atmosphere at 37 C. One day before transfection, the cells were recultivated in culture flasks at 50% confluence. Transfection of the cells with recombinant plasmids or pcDNA 1 vector as a control was performed using a calcium phosphate coprecipitation method (38, 39).
Northern blotting and measurement of secreted immunoreactive AVP
Sixteen hours after transfection, the cells were plated in either six-well plates for measurement of AVP immunoreactivity in the cell medium or in culture bottles for Northern blot analysis. The following day the cells from the six-well plates were washed in PBS and supplied with 2 mL fresh culture medium. After 24 h, the culture medium was harvested. The cells were lysed, and the protein concentration of the lysates was determined by Bradford protein assay (Bio-Rad Laboratories, Inc., Hercules, CA). For Northern blotting, the cells were grown for 48 h in culture medium. Total ribonucleic acid (RNA) was isolated from the transfected cells using a RNA extraction kit (RNAzol, WAK Chemical Co., Bad-Sodem, Germany). Total RNA (5 µg) was subjected to agarose gel electrophoresis and transferred to a Zeta Probe membrane (Bio-Rad Laboratories, Inc.) by capillary blotting. A 600-bp human AVP-NPII cDNA was radiolabeled (Prime It, Stratagene, La Jolla, CA) with 32P and used as a probe. Hybridization and autoradiography were performed using standard methods.
Western blot analysis
Forty-eight hours after transfection, cells for Western blotting were lysed with Ndet buffer (1% Nonidet P-40, 0.4% desoxycholate, 66 mmol/L ethylenediamine tetraacetate, and 10 mmol/L Tris-HCl, pH 7.4) supplemented with protease inhibitors (Trasylol, 200 U/mL; leupeptin, 10 µg/mL; bacitracin, 200 µg/mL; phenylmethylsulfonylfluoride, 250 µmol/L; Sigma Chemical Co., St. Louis, MO) (33). When differentiated cells were studied, cells were grown in serum-free medium for 24 h before cell lysis. Cell extracts were subjected to 16.5% Tris-tricine gel electrophoresis, and separated proteins were electrotransferred to a polyvinylidene difluoride membrane using a transfer buffer containing 50 mmol/L boric acid (pH 9.0) and 20% methanol. For the detection of NPII proteins, the membranes were blocked and incubated with anti-NPII antibody (DAKO Corp., Glostrup, Denmark), and the proteins were detected with a chemiluminescence detection system (Western-Light Plus Kit, Tropix, Bedford, MA).
Immunostaining and confocal laser scanning microscopy
Immediately after transfection, cells were grown in chamber slides (Nunc, Copenhagen, Denmark) for 24 h in culture medium containing 10% serum followed by 24 h in serum-free medium to induce differentiation to neuronal cells. The cells were then fixed with 4% paraformaldehyde and immunostained essentially as described by Jensen et al. (40). The primary antibodies were directed specifically against NPII (anti-NPII, rabbit anti human-NPII, ICN Biochemicals, Inc. Costa Mesa, CA). The secondary antibody was fluorescein isothiocyanate-conjugated porcine rabbit antibody (Dakopatts, Copenhagen, Denmark). For confocal laser scanning microscopy (Leica Corp., Heidelberg, Germany), we used a two-layer immunostaining procedure separated by a washing period in PBS. The cells were incubated for 60 min at room temperature with the NPII antibody diluted (1:1000) in blocking reagent (Boehringer Mannheim, Mannheim, Germany) followed by incubation with the secondary antibody (1:200) for another 60 min.
To look for colocalization of NPII and endoplasmatic reticulum or Golgi apparatus, cells were also incubated with either a mouse antibody directed against an ER antigen (Grp78; Stress Gen, Biotechnologies, Victoria, BC, Canada; diluted 1:50) (33) or a mouse antibody directed against a Golgi antigen (microtubule-binding Golgi membrane protein 58K, Sigma; 1:50), followed by incubation with rhodamine-conjugated goat antimouse antibody (Sigma; diluted 1:200). After washing, coverslips were mounted with one droplet of anti-fade solution (41).
The Grp78 antibody is monoclonal and identifies glucose-regulated protein 78 (Grp78) also known as Ig heavy chain binding protein and additional proteins containing the KDEL retention signal sequence. The KDEL retention signal is a carboxyl-terminal sequence (Lys-Asp-Glu-Leu) shared by luminal ER proteins and is shown to be attached to proteins retained in the ER (42). The FITC-conjugated antibody and rhodamine-conjugated antibodies are visualized by confocal laser scanning microscopy as red and green fluorescent colors, respectively. Colocalization of the NPII protein and Grp78 protein results in a merge between these two colors that is visualized as an orange/yellow fluorescent color.
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Results |
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Sixteen members (7 women and 9 men) from 3 generations of the
kindred had a history of polyuria and polydipsia since childhood. The
clinical results are listed in Table 1.
In summary, the age of onset in this kindred was rather high and
averaged 3.2 yr (range, 2.5–9 yr). All but one patient (FIII-11)
showed complete diabetes insipidus with an inability to concentrate
urine and a severely deficient secretion of AVP during osmotic
stimulation. There was no significant gender difference in the severity
of polyuria even when values were corrected for body weight (data not
shown). All 15 patients responded well to treatment with
DDAVP and, except for 1 who chose not to continue
treatment, currently maintain normal urine volumes and osmolalities on
intranasal doses ranging from 10–40 µg (mean, 22 µg/day). As in
other adFNDI kindreds, 1 diseased affected family member (FI-1) had
experienced a gradual decline in urine volume after the age of 50 yr to
a level at which treatment was no longer necessary.
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, mean urinary
vasopressin measured in a random sample and expressed as a function of
the concurrent urinary creatinine was significantly lower in 14
affected subjects than in 11 unaffected family members and 11 spouses
(15 ± 8, 39 ± 15, and 35 ± 13 pg/mg creatinine,
respectively). Although the individual values in the 3 groups
overlapped slightly, at a cut-off level of 25 pg/mg creatinine the
positive predictive value for detecting adFNDI by this method was
81.3%, and the negative predictive value was 95.2%. As shown in the
lower panel of Fig. 2, urinary vasopressin excretion
correlated negatively with age in both affected and unaffected family
members (r = -0.78; P < 0.001 and r =
-0.71; P < 0.05, respectively).
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Sequencing the coding region of AVP-NPII gene in 12 of the
16 affected family members revealed that all of them have a single base
substitution (G
A) at position 279 in exon 1 of 1 allele. This
mutation predicts the exchange of Ala with Thr at the -1 position of
the signal peptide moiety (Fig. 3). No
abnormalities were found in the rest of the coding region of the gene.
As the mutation eliminates a restriction site for the endonuclease
BstUI, digestion of exon 1 with this enzyme resulted in an
abnormal 269-bp fragment in affected subjects (Fig. 4). This abnormal product was found in
all 16 affected family members, but in none of the 16 who were
unaffected or any of the 11 spouses (Figs. 1
and 4). Two children (FV-7
and FV-10) who had no symptoms but were too young to assign a phenotype
both showed a normal digestion pattern.
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As shown in Fig. 5, Neuro2A cells
transiently transfected with the wild-type cDNA secreted significantly
more immunoreactive AVP into the medium (41.2 ± 5.6 pg/µg cell
protein·24 h) than the cells transfected with Ala(-1)Thr mutant cDNA
(5.8 ± 0.9) or the unmodified vector (0.1 ± 0.01). Northern
blotting showed similar levels of messenger RNA expression (data not
shown).
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, Western blotting of
lysates from cells transiently transfected with wild-type cDNA revealed
one band of protein that reacted with the NPII antibody. It
corresponded in size to the 21-kDa glycosylated pro-AVP precursor. In
contrast, cells transfected with the mutant cDNA produced a slightly
larger band of protein that had a molecular size of approximately 23
kDa, which is similar to uncleaved, glycosylated prepro-AVP.
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, cells transfected
with wild-type cDNA contained small aggregates of NPII immunoreactivity
(green color) that was distributed throughout the
cytoplasma, especially in the tips of the cellular processes where the
secretory vesicles normally are located (Fig. 7A). However, in cells
expressing the mutant cDNA, the immunofluorescence was localized to the
perinuclear network and was observed to a much lesser extent in the
tips of the cells (Fig. 7B). Labeling with the Grp78 antibody
(red color) showed a uniform perinuclear staining in both
cells expressing the wild-type (Fig. 7C) and those expressing the
mutant cDNA (Fig. 7D). When the cells were double labeled with the NPII
antibody (green) and the Grp78 antibody (red),
the wild-type cells showed no colocalization (Fig. 7E). However, double
labeling of the cells expressing the mutant cDNA showed that a
substantial portion of the NPII protein appeared to be colocalized with
the ER antigen Grp78 (orange/yellow color, Fig. 7F). Double
labeling of the cells with the NPII antibody and the Golgi antibody
revealed no colocalization in either wild-type or mutant cultures (not
shown).
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Discussion |
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TopAbstractIntroductionSubjects and MethodsResultsDiscussionReferences |
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The present study also supports a previous report (6) that the AVP deficiency produced by this Ala(-1)Thr mutation is not present at birth, but develops early in childhood and worsens progressively with time. Thus, we found that the DI in our kindred reportedly began at ages ranging from 2–9 yr, and the urinary AVP excretion in these patients correlated negatively with age. However, as in other adFNDI families, the debut of symptoms (2–9 yr) as well as the severity of polyuria (6.6–28.8 L/24 h) and the AVP deficiency (assessed by both plasma and urine content) varied considerably within the family. Because of this variability and the limited number of patients usually available for careful evaluation, it is difficult to determine with certainty whether there are significant differences in the severity of the disease produced by the various AVP-NPII gene mutations. However, there is some evidence suggesting that the age of onset is lower in several kindreds with mutations in the NPII moiety (Leu50Pro, Gly57Arg, and Arg66Pro) than in those with the Ala(-1)Thr mutation in the SP (27, 29). This difference is consistent with theoretical expectations, because mutations affecting the SP cleavage site would be expected to allow the formation of some normal prohormone from the mutant alleles, whereas the NP mutations would not (17, 27).
The wide variability in the age of onset and severity of the AVP deficiency among patients with the same Ala(-1)Thr mutation is unexplained but is probably due to individual differences in other genetic or environmental influences on the neurohypophysis. For example, the rate of production of the mutant precursor could vary due to individual differences in the control of gene expression and/or the intensity of neurohypophyseal stimulation. Susceptibility to the postulated toxic effect of the mutant precursor could also vary due to individual differences in the capacity to degrade or otherwise dispose of the mutant precursor. Finally, the secretory reserve of the neurohypophysis could also vary due to individual differences in the development of the gland.
Our finding that the basal urinary AVP/creatinine ratio declines with age in unaffected as well as affected family members was unexpected because the plasma AVP response to osmotic stimulation increases with age in healthy adults (43). This discrepancy may be due to age-related differences in urinary AVP clearance or other variables, such as the rate of basal water intake and insensible loss. Either way, it is a reminder that even when adjusted to the rate of creatinine excretion, urinary AVP excretion is not always a reliable index of AVP secretion (44) or AVP secretory capacity. Thus, it raises questions about the significance of the age-related decline in urinary AVP that we and others (6) have observed in patients with adFNDI. Nevertheless, the measurement of the AVP/creatinine ratio in spot urine samples appears to be a relatively good predictor of AVP deficiency, at least in severely affected patients. As such, it seems worthy of further investigation as an alternative to measurement of plasma AVP during osmotic stimulation.
Expression of the Ala(-1)Thr mutation in a neurogenic cell line with a secretory apparatus capable of processing the wild-type AVP-NPII gene through the regulated pathway (45, 46) provides further evidence that the mutation impairs the production of AVP by interfering with cleavage of the signal peptide from the preprohormone and its trafficking from the ER to the secretory vesicles. Thus, compared to Neuro2A cells transiently transfected with the wild-type AVP-NPII cDNA, cells transfected with the Ala(-1)Thr mutant cDNA produced about 8-fold less immunoreactive AVP, accumulated a NPII-containing protein that had electrophoretic properties indistinguishable from those of uncleaved prepro-AVP-NPII, and remained associated with the endoplasmic reticulum.
In the only strictly comparable study published to date, Ito et
al. (32) also found that the Ala(-1)Thr mutation impaired AVP
secretion by Neuro2A cells. The degree of impairment was about the same
as that with two other mutations (Gly57Ser and
Cys67stop), but appeared to be less than that with another
(
Glu47) that was also associated with adFNDI. Robertson
et al. also reported that Neuro2A cells transiently or
stably transfected with the Ala(-1)Thr mutant produced 10- to 100-fold
less immunoreactive AVP and died sooner after differentiation to
postmitotic neurons than cells similarly transfected with cDNA lacking
this mutation (47, 48). Although the human AVP-NPII cDNA used in these
studies was found later to deviate from the standard sequence at a
locus in exon 3 (see Materials and Methods), the relative
deficiencies in AVP production and viability associated with the
Ala(-1)Thr mutation are also relevant here because the deviation in
exon 3 was also present in the wild-type cDNA, was located in an
unconserved coding region of the gene, and has not been reported to
affect AVP production in any system.
There are several possible mechanisms by which the Ala(-1)Thr mutation could cause the observed abnormalities in vasopressin production, precursor processing, and cellular trafficking. Theoretically, the Ala(-1)Thr mutation could either impair cleavage of the signal peptide or misdirect cleavage to an alternate site one or more residues upstream (17, 49, 50). Misdirected cleavage of the signal peptide was not observed in this or another study (32) that used pulse-chase techniques, and Western blotting in the present study also revealed only one abnormal band of approximately 23 kDa, consistent with uncleaved glycosylated prepro-AVP-NPII. Whether the lack of the 21-kDa band should be taken as evidence for a complete inhibition of SP cleavage in the mutant is not clear, as it can be argued that any correctly cleaved fraction of the prohormone would be further processed into the regulated pathway and thus would not show up on a Western blot of cell lysate. In any case, failure to remove all of the signal peptide from the N-terminus of the AVP moiety is likely to hinder its insertion in the AVP-binding pocket in neurophysin, a process that is necessary for normal folding and dimerization of the prohormone (51).
The present study also confirms that, in contrast to the wild-type prepro-AVP-NPII, a substantial portion of the mutant precursor appears to be colocalized with the ER. Immunostaining using confocal laser scanning microscopy clearly demonstrates that antibodies against the mutant protein colocalize with those against the Grp78 protein that resides in the ER. In contrast, no colocalization was found using an antibody against a Golgi protein (data not shown). In contrast, antibodies against the wild-type protein did not colocalize to either the ER or Golgi, but were concentrated elsewhere in the cell body, especially in the cell processes. This finding agrees with the results of other studies (32, 33), which showed that this and other mutations associated with adFNDI seem to result in retention of the mutant precursor in the ER. In the study by Ito et al. (32), however, NPII and ER protein were stained separately, and colocalization was judged by the localization of fluorescent material in the cell.
Although the results of this in vitro study help to clarify some of the cellular effects of this mutation, they do not demonstrate the mechanism of its dominant negative effect in vivo. Autopsy studies in a few patients with adFNDI have consistently shown atrophy of the posterior pituitary, gliosis, and a paucity of magnocellular neurons in the supraoptic nucleus (7, 8, 9, 10). These observations as well as the delayed onset of the AVP deficiency (6) suggest that the disease is due to postnatal degeneration of vasopressin-producing neurons in the neurohypophysis. This degeneration might occur because the mutant precursor forms misfolded aggregates that accumulate in the ER and interfere with processing of other proteins essential for cell survival. Preliminary evidence for such a toxic effect has been obtained from other in vitro expression studies (32, 48). However, it is also possible that degeneration of neurosecretory neurons is an inconstant, late, or incidental event and that AVP production by the normal allele is impaired primarily because normal and mutant precursors interact to form heterodimers that also cannot be folded or processed properly and are, therefore, rapidly degraded. Moreover, neither mechanism for the dominant negative effect is completely sufficient to explain why the capacity to produce AVP appears to be normal for the first few years of life (6). Further investigation of these patients as well as of in vitro cell systems in which both normal and mutant cDNA are stably coexpressed may help to clarify these issues.
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Acknowledgments |
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Footnotes |
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Received August 31, 1998.
Revised April 12, 1999.
Accepted April 19, 1999.
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References |
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Leu) is associated with onset and transmission of
autosomal dominant neurohypophyseal diabetes insipidus. J Clin
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Val in neurophysin II. J Clin Endocrinol Metab. 82:3643–3646.This article has been cited by other articles:
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, unaffected members; 
, spouses.
