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Clinical Studies |
Departments of Biochemistry (E.O., Y.I.), Endocrinology (N.N.), and Pediatrics (S.O., S.I., Y.I.), and Research Equipment Center (H.M.), Kagawa Medical University, Kita-gun, Kagawa 76107; and the Department of Pediatrics, Osaka City General Hospital (K.F.), Toshima-ku, Osaka 534, Japan
Address all correspondence and requests for reprints to: Dr. Y. Ichikawa, Department of Biochemistry, Kagawa Medical University, Kita-gun, Kagawa 76107, Japan. E-mail: yichikaw{at}kms.ac.jp
| Abstract |
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| Introduction |
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The cholesterol side-chain cleavage enzyme, namely cytochrome P450scc (CYPXIA1), is the terminal enzyme of the cytochrome P-450-linked monooxygenase system (side-chain cleavage enzyme of cholesterol; EC 1.14.15.6), which catalyzes the conversion of cholesterol to pregnenolone on the matrix side of the mitochondrial inner membranes of the adrenal cortexes, ovaries, testes, placenta in a long pregnancy, and brain (3, 4, 5, 6, 7, 8), and is the initial rate-limiting reaction in the biosynthesis of steroid hormones other than renal ones (9). As the activity of the cytochrome P450scc linked monooxygenase system in adrenocortical mitochondria from patients with lipoid CAH was extremely low compared with that in normal subjects, it has been postulated that a deficiency of cytochrome P450scc causes this disease (10, 11, 12, 13). Therefore, many workers have focused on the detection of a mutation(s) in the cytochrome P450scc gene from affected individuals. However, no mutation has been found in this gene to date (14, 15, 16, 17).
LH and ACTH stimulate steroid biosynthesis (18, 19, 20) by increasing the transport of cholesterol from the outer mitochondrial membranes to the inner membranes. This process depends on a mechanism involving a cycloheximide-sensitive agent (21, 22, 23, 24, 25) and is the true regulated, rate-limiting step in the acute response of steroidogenic cells to ACTH stimulation (24).
Recently, a novel LH-induced mitochondrial protein, termed the steroidogenic acute regulatory protein (StAR), was purified from MA-10 mouse Leydig tumor cells (26). Extensive work has been carried out to characterize this protein (27, 28, 29, 30, 31). The details of this work can be summarized as follows. The StAR protein is expressed in steroidogenic organs, including the adrenals and gonads, but not in the placenta or brain. In the acute response of steroidogenesis to tropic hormone stimulation, the StAR protein is synthesized as a 37-kDa preprotein, which stimulates cytochrome P450scc activity, probably by acting on the outer mitochondrial membrane. Then, the StAR preprotein may be imported into the mitochondrial matrix and processed to the 30-kDa mature form, presumably to terminate its steroidogenic action. Recently, a number of mutations were detected in the StAR gene from patients with lipoid CAH (32, 33, 34). In other words, these reports proved an essential role for the StAR protein in steroidogenesis. In this report, we discuss the biological significance of a novel intronic mutation in the StAR gene.
| Subjects and Methods |
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A patient from a Korean family diagnosed as having lipoid CAH
according to the clinical and laboratory data was studied. She was the
3630-g, 52.5-cm product of a 40-week, 2-day gestation and thrived
normally for 1 yr after birth without hormonal replacement therapy. It
should be noted that as she was born without any abnormality, except
for slight swarthiness, and seemed to thrive normally thereafter, a
medical practitioner decided that her swarthiness was not severe enough
for a close medical examination. However, about 1 yr of age her
pigmentation became conspicuous, so she was hospitalized for
inspection. The laboratory data are summarized in Table 1
. Unlike typical laboratory data for lipoid CAH, the
steroid hormone and electrolyte values in this patient were normal (see
Discussion). However, ACTH and PRA were considerably high,
with the result that she had hyperpigmentation of the skin.
Furthermore, no response of the cortisol level to the administration of
ACTH was observed (see Fig. 1
). Taking into account
these results, she was diagnosed as having lipoid CAH. At 3.5 yr of
age, she was 15.6 kg and 97.6 cm, with 138 mEq/L Na, 4.0 mEq/L K, and
108 mEq/L Cl. Her parents are not consanguineous. Informed consent was
obtained from the subject. This study was approved by the Department of
Pediatrics, Kagawa Medical University.
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Genomic DNAs were extracted from leukocytes of this patient, her mother, and healthy controls using a commercially available genomic DNA extraction kit, Sepa Gene (Sanko Junyaku Co., Tokyo, Japan), according to the manufacturers recommendations.
PCR amplification
We amplified the 5'-flanking regions, and all exons of the
cytochrome P450scc and StAR genes by means of PCR amplification (35) to
identify the mutation(s) causing lipoid CAH. For this amplification,
oligonucleotides were originally synthesized based on the nucleotide
sequence of the human cytochrome P450scc gene (36) and that of the StAR
gene (28) (see Table 2
).
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All of these reactions were performed with a Program Temp Control System PC-700 (Astec Co., Fukuoka, Japan).
Molecular cloning and sequencing
The PCR products described above were then cloned into the pT7Blue T-Vector (Novagen, Madison, WI). The sequencing reactions were performed by cycle sequencing, using a commercially available cycle sequencing kit (ABI PRISM Dye Terminator Cycle Sequencing Ready Reaction Kit with AmpliTaq DNA Polymerase, FS, Perkin-Elmer/Cetus, Norwalk, CT). The sequence analyses were performed using the ABI Prism 377 DNA Sequencer. We cloned and sequenced 10 clones/target to determine the genotype and to eliminate the PCR error(s). The nucleotide sequences were determined for both strands of the subcloned DNA for the sake of preciseness.
Analyses of DNA sequences
Analyses of the nucleotide sequences were carried out using the package, GENETYX. The nucleotide sequence of each clone described above was compared with those of the corresponding regions of the cytochrome P450scc and StAR genes previously reported (28, 36).
Construction of expression vectors
To construct expression vectors pcDNA-StAR and pcDNA-StAR(T),
harboring the whole StAR genes from the normal subject and the patient
with lipoid CAH, respectively, the PCR-amplified StAR genes were each
digested with HindIII and then inserted into a pcDNA3
plasmid (Invitrogen Corp., San Diego, CA). The cloning junctions were
verified by DNA sequencing analyses, and the plasmids were purified
using commercially available DNA purification systems (Wizard Minipreps
DNA Purification Systems, Promega Co.) before transfection. The
oligonucleotides used for amplification of the whole StAR genes are
listed in Table 2
. The PCR protocol for this amplification comprised
1-min melting of the strands at 94 C, then 20 min of denaturation at 98
C, and 15 min of annealing and extension at 68 C, using Ex Taq (Takara
Shuzo Co., Shiga, Japan), for 30 cycles. Final extension was then
performed for 10 min at 72 C.
Transient expression of the StAR gene in COS-7 cells
COS-7 cells (simian virus 40-transformed monkey kidney) (37) were obtained from the Riken Gene Bank (Tsukuba Science City, Ibaragi, Japan). FBS was obtained from ICN Biochemicals Japan Co. (Osaka, Japan). Other reagents were obtained from Life Technologies (Gaithersburg, MD). COS-7 cells were cultured in DMEM supplemented with 10% (vol/vol) FBS and antibiotics (penicillin, 100 IU/mL; streptomycin, 100 mg/mL) at 37 C under a 5% (vol/vol) CO2 atmosphere. The cells were subcultured before transfection and then seeded into 35-mm plates at a density of 2 x 105 cells/plate. Transfection was carried out using a commercially available transfection reagent, TransIT (Takara Shuzo Co.), according to the manufacturers recommendations. The cells were transfected with either the pcDNA-STAR or the pcDNA-StAR(T) plasmid.
Ribonucleic acid (RNA) isolation
Total RNA was isolated from the transfected COS-7 cells cultured in 100-mm plates for 3 days using a commercially available nucleic acid and protein purification system, Isogen (Nippongene, Tokyo, Japan), according to the manufacturers recommendations. Briefly, the cells were lysed with Isogen and then scraped from the plates. RNA was extracted into the aqueous phase by treating the homogenate with chloroform and was precipitated with isopropanol. After quantification, the total RNA preparations were subjected to Northern blot analysis and reverse transcriptase-PCR (RT-PCR) analysis for StAR messenger RNA (mRNA; see Results).
Northern blotting
Northern blot analyses were carried out as described previously (38), using exon 2 of the human StAR gene as a probe.
Probe for Northern and Southern blotting
The probe, i.e. exon 2 of the human StAR gene, used for Northern and Southern blot analyses was labeled with digoxigenin-11-deoxy-UTP by means of PCR. The PCR protocol comprised 2-min melting of the strands at 94 C, then 1 min of denaturation at 94 C, 2 min of annealing at 60 C, and 2 min of extension at 72 C, using Taq DNA polymerase (Promega Co.) for 30 cycles. Final extension was performed for 8 min at 72 C. The amplified DNA fragment was purified by electrophoresis on 2.5% (wt/vol) agarose using a commercially available DNA retrieval system, Suprec-01 (Takara Shuzo Co.), according to the manufacturers recommendations.
RT-PCR
Total RNA isolated from transfected COS-7 cells was reverse
transcribed using 20 mmol/L StAR-Rev primer by incubation for 40 min at
42 C with 20 U AMV RT XL (Life Science, Petersburg, FL), 25 U
ribonuclease inhibitor (Toyobo Co., Tokyo, Japan), 1 mmol/L deoxy-NTPs,
25 mmol/L Tris-Cl (pH 8.3), 50 mmol/L KCl, 5 mmol/L MgCl2,
and 2 mmol/L dithiothreitol in a final reaction volume of 30 mL. Then
the complementary DNAs (cDNAs) were subjected to PCR amplification. The
PCR protocol comprised 5-min melting of the strands at 95 C, then 1 min
of denaturation at 95 C, sloping to 55 C over 2 min, 2 min of annealing
at 55 C, and 2 min of extension at 72 C, using Taq DNA
polymerase (Promega Co.), for 35 cycles. The final extension was
performed for 5 min at 72 C. The oligonucleotides used for this
analysis are listed in Table 2
.
Southern blotting
Southern blot analysis was carried out as described previously (39) with the same probe as that used for Northern blot analysis.
| Results |
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Although we investigated the cytochrome P450scc gene as well as
the StAR gene of the patient, no mutation was found in either gene
except for an insertion of T, 3 bp from the splice donor site of exon 2
of the StAR gene (Fig. 2
). On account of this insertion,
the preferred base changed from purine to pyrimidine (40). Furthermore,
the patient and her mother were homozygous and heterozygous for this
insertion, respectively, and none of the control subjects had this
insertion. Taking into account these results, we hypothesized that the
T insertion into intron 2 of the StAR gene disturbed the consensus
splice donor sequence, 5'-GTA/G-3', with the result that intron 2 could
not be removed from the mature StAR mRNA of the patient.
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As adrenal or gonadal RNA of the patient was not available, we
could not perform in vivo analysis on them. Therefore, to
verify the above hypothesis, we first checked whether this T insertion
influenced the processing of mRNA in COS-7 cells by Northern blot
analysis. We constructed the pcDNA-StAR and pcDNA-StAR(T) plasmids,
harboring the whole StAR genes from a normal subject and the patient,
respectively, and transfected them into COS-7 cells. Exon 2 of the StAR
gene was used as a probe in this analysis. If the StAR mRNA from the
patient could not remove intron 2, the length of this mRNA must be
about 300 bp longer than that from the normal subject. The result
clearly indicated that the pcDNA-StAR(T) plasmid yielded a longer StAR
mRNA than the StAR mRNA yielded by the pcDNA-StAR plasmid (see Fig. 3
). It should be noted that the pcDNA-StAR(T) plasmid
only yielded abnormally spliced StAR mRNA, as observed on Northern
analysis.
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To determine the details of the abnormally spliced StAR mRNA, we
performed RT-PCR analysis using the RNAs that were used for the
Northern blot analysis as templates. For this analysis, we designed
primers so that we might amplify the region around the junction of
exons 2 and 3. The result was that the StAR mRNA from the patient was
definitely 300 bp longer than that from the normal subject (see Fig. 4
). For further confirmation, the 500- and 800-bp bands
were each retrieved and subjected to sequencing analyses. The results
indicated that the former exactly spliced exons 2 and 3, whereas the
latter could not remove intron 2 (see Fig. 5
). This
unspliced intron 2 resulted in a premature termination codon. In this
analysis, we noticed that not all of the StAR mRNA from the patient was
abnormally spliced; some was spliced correctly. This was verified by
Southern blot analysis (data not shown).
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| Discussion |
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Certain oxygenated derivatives of cholesterol are effective steroidogenic substrates, because the greater solubility of these substrates might permit their transport to the inside of mitochondria, bypassing the process of cholesterol movement to cytochrome P450scc (42, 43, 44, 45, 46). Previous reports indicated that this process, i.e. the traverse of cholesterol from the outer to the inner mitochondrial membranes in response to ACTH stimulation, is dependent on de novo protein synthesis (21, 23, 24, 26, 27, 47), and this protein was termed StAR (26). As the StAR protein has nothing to do with the transport of oxygenated derivatives of cholesterol (32, 33), even with a deficiency of the StAR protein, adrenal and gonadal steroidogeneses seem to occur. On the contrary, if cytochrome P450scc does not function, steroidogenesis in these organs will not be able to occur at all, because even oxygenated derivatives of cholesterol as well as cholesterol must pass through cytochrome P450scc for steroidogenesis. The symptoms of lipoid CAH are considerably diversified; namely, some patients die soon after birth unless steroid replacement therapy is begun immediately, and others, including our patient, thrive for several months or more after birth without this therapy (10, 34). Therefore, we cannot rule out the possibility that mutations in the StAR gene are not the sole cause of lipoid CAH; in other words, mutations in the cytochrome P450scc gene can also account for this disease, although several reports proposed that mutations in the cytochrome P450scc gene were not the cause of lipoid CAH (14, 15, 16, 33). A previous report that a deletion in the cytochrome P450scc gene causes congenital adrenal hyperplasia in the rabbit supports this possibility (48). Previously, we examined patients with the serious lipoid CAH phenotype, who died within a few months of birth. This may be the case for a mutation(s) in the cytochrome P450scc gene. Therefore, we are now investigating cytochrome P450scc genes as well as StAR genes from the parents of those patients. Such studies will facilitate understanding of the cause of lipoid CAH.
| Acknowledgments |
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| Footnotes |
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Received November 25, 1996.
Revised February 4, 1997.
Revised March 13, 1997.
Accepted March 17, 1997.
| References |
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-cholesterol-hydroxylase activity in
adrenal tissue of a patient with lipoid adrenal hyperplasia. Acta
Endocrinol (Copenh). 71:512518.[Medline]
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