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Short Communication |
Ser Causes Hereditary Cardiac Amyloidosis, and the Amyloid Fibrils Are Constituted by the 93-Residue N-Terminal Polypeptide



From the Biotechnology Research Laboratories,*
Institute
of Human Pathology
and Division of
Cardiology,
IRCCS Policlinico San Matteo,
Pavia; the Departments of Internal
Medicine§
and
Biochemistry¶
and the Division and Chair of
Cardiac Surgery,||
University of Pavia, Pavia; and the
Department of Organic Chemistry,**
University of
Padova, Padova, Italy
| Abstract |
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Ser, in a patient
affected by familial systemic nonneuropathic amyloidosis. The amyloid
deposits mostly affected the heart of the proband, who
underwent transplantation for end-stage congestive heart failure.
Amyloid fibrils of myocardial and periumbilical fat samples
immunoreacted exclusively with anti-ApoA-I antibodies. Amyloid fibrils
extracted from the heart were constituted, according to amino
acid sequencing and mass spectrometry analysis, by an
amino-terminal polypeptide ending at Val93 of
apolipoprotein A-I (apoA-I); no other significant fragments were
detected. The mutation segregates with the disease; it was demonstrated
in the proband and in an affected uncle and excluded in three healthy
siblings. The plasma levels of high-density lipoprotein and apoA-I were
significantly lower in the patient than in unaffected individuals. This
represents the first case of familial apoA-I amyloidosis in which the
mutation is outside the polypeptide fragment deposited as fibrils.
Visualization of the mutation in the three-dimensional structure of
lipid-free apoA-I, composed of four identical polypeptide
chains, indicates that position 174 of one chain is located
near position 93 of an adjacent chain and suggests that the amino acid
replacement in position 174 is permissive for a proteolytic split at
the C-terminal of Val93.
| Introduction |
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So far, six different mutations in the apoA-I gene have been correlated
with various forms of hereditary amyloidosis. Three of these are
missense mutations affecting the N-terminal domain of the protein and
conferring an extra positive charge on the molecule.7-12
A further mutation, Leu90
Pro, apparently does not
change the isoelectric point but introduces a more polar residue able
to break the helical structure.13,14
One is a
deletion,11
and one is a deletion/insertion
mutation12
; both increase the isoelectric point of apoA-I.
All amyloidoses associated with known apoA-I mutations as well as all
cases in which wild-type apoA-I is deposited in either localized or
systemic amyloidosis, share the origin and size of the fibrillar
polypeptide. When the primary structure has been studied, it has been
shown that the polypeptide isolated from the fibrils starts at the
N-terminal of mature apoA-I, and its length ranges from 70 to 100
residues. A further feature shared by all amyloidogenic mutants that
cause systemic amyloidosis is the presence of the amino acid
replacement inside the polypeptide that accumulates in the amyloid
deposits. We have identified a new apoA-I variant associated with
hereditary systemic amyloidosis predominantly involving the heart in
which the mutation is localized at position 174, outside the 93-residue
N-terminal polypeptide that represents the main protein component of
the extracted fibrils.
| Materials and Methods |
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The family, originating in central Italy, included two affected members, the proband and his maternal uncle, who died at the age of 52. Biopsies of the heart, skin, and gingiva were taken from the uncle and stained with Congo red. The maternal grandfather died at the age of 50 from cardiac failure; pathological documentation is not available. The proband's parents both died of lung cancer in their fifties. The patient's brother and two sisters are clinically well, with abdominal fat biopsies negative for amyloid deposits. The only affected living family member is the proband. Maternal relatives underwent clinical evaluation, serum lipid and lipoprotein studies, molecular analysis of the apoA-I gene, and periumbilical fat biopsy for detection and characterization of amyloid deposits. Although the amyloid deposition was systemic, the overall clinical phenotype was dominated by problems related to the massive heart involvement, which was evaluated with cardiac M-mode, 2D, and color-Doppler echocardiogram and by radiolabeled 99Tc aprotinin scintigraphy.15
Plasma Lipid Studies
Plasma apoA-I and apoB levels were determined in 12-hour fasting samples by immunoturbidimetric methods; high-density lipoprotein (HDL) cholesterol, total cholesterol, and triglycerides were measured by standard clinical chemistry methods.
DNA Amplification and Analysis
Total genomic DNA was isolated from the peripheral blood of the proband, his brother, and two sisters, as well as from 100 control individuals. Three fragments comprising the entire coding region of the apoA-I gene were amplified by polymerase chain reaction (PCR), using primers described by Soutar et al.10 Amplification conditions were 1 minute at 95°C, 50 seconds at 66°C, and 2 minutes at 72°C for 29 cycles.
PCR products were gel purified (QIAQuick gel extraction kit; Qiagen, Chatworth, CA) and sequenced in both directions, using the same oligonucleotides as the primers. Automated sequencing reactions were carried out with an ABI PRISM dye terminator cycle sequencing kit, and the products were analyzed on an ABI 377 DNA sequencer.
To confirm the presence of the mutation, amplified products of exon 4 from the proband and other family members, as well as from 100 controls, were digested with EagI (New England Biolabs), because the mutation creates a new restriction site. Digested products were electrophoresed in a 2.5% agarose gel, stained with ethidium bromide, and visualized with UV light. Genomic DNA from the maternal uncle was extracted from a paraffin-embedded skin biopsy. DNA extraction was performed according to the method of Coates et al.16 Briefly, 3 x 5 µm slices of the biopsied material were incubated overnight at 55°C in a lysis solution containing proteinase k. Inactivation of proteinase k was achieved by heating at 95°C for 10 minutes, and then the sample was stored at 4°C to facilitate removal of the paraffin layer. PCR amplification of a DNA fragment of 134 bp comprising the mutation site was accomplished by using the same conditions described above and the following primers: 5'GACGCGCTGCGCACGCATCTG3', 5'CTCAGATGCT-CGGTGGCCTTG3'. The same restriction enzyme, EagI, was used on this purified PCR product to detect the mutation.
Immunohistochemical Studies
Detection of amyloid deposits was carried out through Congo red
staining followed by microscopic examination under polarized light.
Immunoelectron microscopy was used for in situ
characterization of amyloid fibrils, as previously
described,17
using a panel of antibodies purchased from
Dako (Copenhagen, Denmark) (anti-k and
light chains,
anti-transthyretin, anti-fibrinogen, anti-lysozyme, anti-amyloid A,
anti-ß2-microglobulin, and anti-SAP); the rabbit
anti-human apoA-I antibody was purchased from Genzyme (San Carlos, CA).
Identification, Isolation, and Characterization of Amyloid Fibrils
Fibrils were isolated from heart tissue by water extraction in the presence of 1.5 mmol/L phenylmethylsulfonyl fluoride (PhMeSO2F) after repeated homogenization in 2 ml of 10 mmol/L Tris/EDTA/140 mmol/L NaCl/0.1% NaN3 (pH 8.0), containing 1.5 mmol/L PhMeSO2F/100 mg of tissue, and centrifugation at 60,000 x g in an ultracentrifuge (Beckman L8704; Beckman Instruments) for 30 minutes. The yield in fibrils was monitored by the Congo red staining procedure and microscopic analysis of the extracted material.
The extracted fibrils were fractionated by fast protein liquid chromatography (FPLC) gel filtration on a Superose G12 column (Pharmacia) equilibrated and eluted with 6 mol/L Gdn/HCl and 10 mmol/L sodium phosphate (pH 7.5). The major peak containing the 10-kd band, identified by reduced sodium dodecyl sulfate-polyacrylamide gel electrophoresis (SDS-PAGE), was dialyzed against distilled water and lyophilized.
The N-terminal sequence of the purified amyloid fibril subunits was determined by adsorptive biphasic column technology, using an HPG1000 A protein sequenator (Hewlett Packard), with the routine 3.0 chemistry procedure and PTH 4 M HPLC method.
The lyophilized amyloid subunit peptide was redissolved in 2% acetic
acid/acetonitrile (50:50 v/v) at a concentration of
20 pmol/µl and
analyzed on an LCQ mass spectrometer (ThermoQuest) by means of
electrospray ionization. The sample was introduced at a solvent flow of
3 µl/minute, and data were acquired in a multiple-channel analysis
mode at 10 seconds per scan over the m/z range of 500-2000.
| Results |
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Molecular Analysis
Sequence analysis performed on the entire coding region as well as
in the promoter of the apoA-I gene revealed the presence in the proband
of a T-to-C transition at nucleotide 2069 of the apoA-1
gene.18
This mutation produces a Ser for Leu substitution
at position 174 of the polypeptide chain. No other point mutations were
found in the genomic region analyzed. The T-to-C mutation introduces a
new restriction site for the enzyme EagI, which allows
confirmation of the point mutation by restriction fragment length
polymorphism (RFLP) analysis. The patient shows the expected
fragment of 370 bp (using primers described by Soutar et
al10
) and two additional fragments of 255 and 115 bp, as
would be expected in a heterozygote for the mutated allele. His healthy
sisters and brother are not carriers of the mutation, because they
possess only the normal fragment. A paraffin-embedded skin biopsy
belonging to the maternal uncle, who died of cardiac amyloidosis, was
used for the DNA extraction and search for the transition T-to-C in
position 2069 of the apoA-1 gene. DNA amplification was accomplished by
two primers that delimit a 134-bp sequence that includes the
restriction site of EagI created by the mutation. We
prepared this short PCR product to avoid the problems created by DNA
fragmentation that were expected from the way in which the tissue
sample was stored. Digestion of this 134-bp PCR product by
EagI confirmed the presence of the mutation in the proband
and demonstrated that the mutation was present in the affected uncle
(Figure 1)
. The healthy sisters and
brother of the proband were all negative.
|
The heart excised at transplantation was grossly enlarged (1.030
kg in weight) and heavily infiltrated by amyloid fibrils (Figure 2)
. The amyloid material isolated from
the heart was analyzed by EM and appeared to be constituted by pure
straight and rigid fibrils with a diameter of approximately 9 nm;
minimal amorphous protein aggregate was associated with the fibrils
(Figure 2c)
. The material was separated by gel filtration as reported
in Figure 3A
, and the content of the
chromatographic fractions was analyzed by SDS-PAGE (Figure 3B)
. The
main component of the fibrils, represented by the polypeptide eluted in
fraction 3 of gel filtration and lane 3 of SDS-PAGE, was analyzed for
N-terminal sequence and molecular mass. The sequence of the first 20
N-terminal residues, the first eight of which are indicated at the top
of the peak of fraction 3, demonstrates its identity with the
N-terminal of mature apoA-I. Determination of the peptide mass showed
the presence in fraction 3 of gel filtration of a single polypeptide
species of 10,724 ± 2 Da, which is consistent with the
93-residue N-terminal (expected mass 10,720) of mature apoA-I (Figure 3C)
.
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| Discussion |
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Chemical analysis of the fibril constituents demonstrated the presence
of one main fragment consisting of sequence 193 of normal apoA-I.
This finding emphasizes the amyloidogenic potential of the N-terminal
domain of wild-type apoA-I and points to future investigation of the
proteolytic event that releases the N-terminal polypeptide from the
full-length protein. The compartment in which this putative proteolytic
event takes place and the enzyme involved are not known, despite the
fact that high susceptibility to cleavage by thrombin near residue 100
has been documented.20
This apoA-I variant
Leu174
Ser appears to be particularly sensitive to a
limited and particularly selective proteolytic event able to release a
93-residue polypeptide. The currently available molecular model of
wild-type apoA-I was determined by single-crystal X-ray diffraction
analysis at 4-Å resolution (Protein Data Bank ID code
1AV1).21
The limited resolution does not at present allow
a detailed analysis of interactions at atomic levels; the following
discussion must be considered preliminary, and a better definition of
the molecular structure must await more thorough interpretation of our
data. However, available 3D structural data offer an important tool for
the interpretation of the peculiar pathological pathway of this apoA-I
variant. The protein is composed of four identical polypeptide chains,
designated A to D, each displaying an
-helical conformation. The
four chains run antiparallel to each other in such a way that chains A
and B display identical interactions with respect to C and D.
Consequently, in our variant the mutated residue 174 in chain A
(hereafter referred to as Leu174A) presents an atomic
environment that is slightly different from that of residue 174 in
chain B. Identical observations can be made for chains C and D. With
the present identification system, chain A is equivalent to C and chain
B to D. In all cases the mutated residue 174 is relatively close to
position 93 (Figure 4)
, which corresponds
to the site of proteolytic cleavage: the C
-C
distance is 12.9 Å
between Leu174A and Val93B, and 12.8 Å between
Leu174B and Val93A. The shorter distance
between side-chain atoms of the two residues is about 10 Å. In
contrast, a different environment is observable around position 174 in
the two cases: the side chain of Leu174A in fact interacts
with C
and S
of MetB86 (3.6 Å and 3.5 Å), C
of
Arg177A (3.9 Å), and O
of Ser52C (3.7 Å).
On the other hand, Leu174B is close to two other leucines,
Leu178B (3.5 Å) and Leu211C (4.3 Å). The
effect of the mutation Leu174
Ser could consequently
have contrasting effects in the two cases: in fact, the replacemenmt of
a Leu by a Ser in a hydrophobic environment, as in chain B, could be
slightly destabilizing for the surrounding residues, and a long-range
effect could be transmitted to residues 9394, making them more
exposed to proteolytic cleavage. However, the same mutation on chain A
could stabilize the interaction with chain C, because a H bond could be
formed between Ser174A and Ser52C.
Furthermore, the formation of this new interaction between chains A
and C could in some way modify or destabilize the relationship between
chains A and B. An important fact nevertheless can be clearly stated
from the protein model: a mutation in chain B (or A) should produce the
peptide 1B-93B (or 1A-93A), respectively.
|
Ser mutation would
imply that the polypeptide 193 could derive from either the wild-type
or the variant. To our knowledge this represents a new mechanism able
to prime the amyloidogenic process; the contact between wild-type
and mutant may also favor the release of amyloidogenic polypeptide from
the wild-type, and in vitro studies are in progress to
verify this hypothesis.
| Acknowledgements |
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| Footnotes |
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Supported by grants from Telethon (grant n.E.793), European Community Biomed 2 Programme no. BMH4-CT 98-3689, the Italian Ministry of Health, University Hospital IRCCS Policlinico San Matteo, Probetto d'Ateneo University of Pavia, and MURST.
Accepted for publication April 30, 1999.
| References |
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