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From Department of Medicine (A.F.V, K.K.), University of Helsinki, FIN-00290 Helsinki, Finland and Central Hospital of North Karelia (H.T.), FIN-80210 Joensuu, Finland.
Correspondence to Professor Kimmo Kontula, MD, Department of Medicine, University of Helsinki, FIN-00290 Helsinki, Finland.
| Abstract |
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Key Words: receptors, low-density lipoprotein mutation polymerase chain reaction cord blood North Karelia
| Introduction |
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In principle, the molecular nature of the underlying defect in FH varies from family to family, but in specific populations, including the French Canadians, South African Afrikaners, Christian Lebanese, Ashkenazi Jews as well as Finns, one to four founder mutations seem to account for the majority of mutant LDL receptor genes,1 rendering population-genetic studies feasible. Either of the two deletions of the LDL receptor gene, FH-Helsinki [FH-HKI] or FH-North Karelia [FH-NK], is present in two-thirds of Finnish patients with heterozygous FH, and the FH-NK mutation, deleting seven nucleotides from exon 6 and representing a class 1 (null allele) phenotype of FH, accounts for 90% of FH cases in the Finnish North Karelia.4 This constellation provides extraordinary possibilities for population genetic studies on FH in the province of North Karelia, which has a population base of 180 000 inhabitants and a higher than average prevalence of coronary artery disease in Finland.
The atherosclerotic process appears to seed its origins already in childhood.5 A number of studies have indicated that preventive measures started at a young age may modify risk factors of atherosclerosis in a favorable way. In children with FH, lowering of atherogenic lipoprotein components can be achieved by dietary measures,6,7 supplementation of diet with plant sterols8 or by use of cholesterol-lowering medication.7 Collectively, all these data underscore the importance of an early unequivocal diagnosis in the pediatric management of FH. A number of studies carried out in 1970s and 1980s suggest that FH screening based on lipid measurements in umbilical cord samples is well suited for studies in affected families but may be unreliable for FH screening in general population. The interpretation of these data are, however, hampered by the fact that classification into FH and non-FH categories was based solely on lipid measurements and pedigree information, rendering both wrong negative and wrong positive diagnoses possible. In order to provide insight into the discriminative power of serum lipid levels in early diagnosis of FH and to obtain information on changes of serum lipid levels in molecularly defined FH during the first year of life, we conducted the present prospective study in offspring of documented carriers of a mutant LDL receptor gene.
| Materials and Methods |
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This study was carried out at the North Karelia Central Hospital, Joensuu, Finland, from January 1992 to August 1996. All the parents gave their informed consent to the present study.
Laboratory Methods
DNA was prepared from frozen EDTA-anticoagulated cord or venous
blood samples. DNA analysis was carried out using the duplex
PCR-test described previously, with minor
modifications.10 In short, 5
oligonucleotide primers (P1-P5) were used in a PCR
amplification reaction containing 150 ng of template DNA and the
various ingredients listed in the original
paper.10 After 30 PCR cycles for 1 minute at
95°C, 1 minute at 55°C and 1 minute at 72°C, the amplified DNA
fragments were analyzed by electrophoresis (at 250 V for 5
hours) on nondenaturating 12% polyacrylamide gels using a
buffer containing 0.09 mol/L boric acid, 0.002 mol/L
EDTA, pH 7.5. After electrophoresis, the DNA fragments were visualized
by ethidium bromide staining. Under these conditions, the primer pair
P1-P2 results in the formation of a specific 93-bp band, whenever the
FH-NK mutation is present in the sample, and the primer pair P3-P5
generates an amplified fragment of 159-bp in size, whenever the FH-HKI
gene is present.10
Cord blood was collected in plain and EDTA-anticoagulated test tubes from the placental end of the cord. Because prolonged clamping of the cord is known to increase the possibility of maternal contribution to the cord blood,11 great care was followed to take the samples immediately after cord clamping. Venous blood samples at the age of about 1 year were collected after a 12-hour fast. Serum was separated by low-speed centrifugation of blood. All serum and EDTA-anticoagulated whole-blood samples were stored at -20°C until analysis.
Serum total cholesterol [TC] and triglyceride [TG] concentrations were determined by enzymatic methods using commercial kits obtained from Boehringer-Mannheim. The concentration of serum HDL cholesterol [HDL-C] was measured enzymatically after precipitation of LDL and VLDL fractions with dextran sulfate and MgCl2, and serum LDL cholesterol [LDL-C] level was calculated using the formula of Friedewald et al.12
Statistical Analysis
Data are presented mean±SE. The mean serum lipid levels
in different groups of children were compared using the Mann-Whitney's
nonparametric test. For comparison of intra-individual
lipid values at birth and one year of age, Wilcoxon
matched-pairs signed rank test was used. Before statistical comparison,
serum TG levels were transformed logarithmically.
| Results |
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TC and LDL cholesterol levels in cord serum from newborns
(n=11) with the FH-NK mutation (2.62±0.73 and 1.78±0.57
mmol/L, respectively) and those (n=3) with the FH-HKI mutation
(2.53±0.74 and 1.75±0.64 mmol/L, respectively) were not
significantly different from each other. This data, as well as our
earlier experience showing virtually identical serum TC and LDL-C
levels in adult heterozygous FH-NK and FH-HKI
patients,13 permitted us to pool lipid data in
these two mutation categories (see below). Serum TC, LDL-C, HDL-C, or
TG levels were not significantly different in cord sera of affected
boys and girls (data not shown). Moreover, cord lipid levels in the
affected newborns were independent on whether it was the father or the
mother who was carrier of the mutant gene; for example, the LDL-C
concentrations in cord serum in the former (n=3) and latter (n=11)
group were 1.74±0.68 and 1.78±0.56 mmol/L, respectively
(P=.64). Serum TC and LDL-C levels were distinctly high in 1
nonaffected newborn from a family with FH (Fig 1
). The pregnancy in this case was
complicated by preeclampsia and delivery was induced at the week 34 of
pregnancy; therefore, this case was excluded from analysis of
lipid data at birth but not from comparison of lipid levels at the age
of 1 year. All the other pregnancies ended with a full term
delivery.
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Mean TC and LDL-C levels in cord serum were significantly elevated in
the affected newborns compared to those of nonaffected offspring of
heterozygous FH subjects or those of 30 randomly collected control
pregnancies (Fig 1
, Table 2
). There was,
however, a considerable overlap between the ranges of individual lipid
levels in these three groups. Cholesterol levels tended to
be slightly higher in cord samples from randomly collected controls
than those derived from pregnancies with a nonaffected child to a
parent with heterozygous FH (Fig 1
, Table 2
). The concentrations of
HDL-C and TG in cord serum were not significantly different in affected
and nonaffected newborns of couples with one affected parent (Fig 1
, Table 2
).
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The mean (±SD) serum TC and LDL-C concentrations in the combined two
groups of nonaffected newborns shown in Fig 1
were 1.77±0.43 and
0.97±0.29 mmol/L, respectively, yielding the 95th
percentile values of 2.60 and 1.44 mmol/L, respectively,
for these two variables. If these levels were used as
diagnostic criteria as the upper normal limits, then 6 or
5, respectively, out of the 14 molecularly defined newborn FH subjects
would have been diagnosed as non-FH babies.
Follow-up serum samples for lipid assays became available from 7
nonaffected and 9 nonaffected children at the age of 1 year. There was
a steep, 3- to 4-fold increase of serum total and LDL
cholesterol levels in both groups resulting in two totally
nonoverlapping distributions at 1 year of age (Fig 2
). Both serum HDL-C and TG
concentrations increased significantly (P<.01) from birth
to the age of 1 in both nonaffected and affected infants, but in
absolute terms these increases were less substantial than that seen in
LDL-C levels (Fig 2
). Our previous survey on 400 heterozygous FH
patients in North Karelia (Vuorio et al, unpublished data) had
identified 17 children born to families with FH who were in the same
age class (age between 1 to 2 years). Inspection of their individual TC
and LDL-C levels showed that these concentrations similarly
distinguished gene carriers and noncarriers (Fig 2
). At the age of 1 to
2 years, the mean serum HDL-C concentration of the FH children (n=18)
was significantly lower than that of the non-FH children (n=16) at this
age (Table 2
).
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There was one affected pair of twins carrying the FH-NK deletion in the study cohort. In these girls serum and LDL-C levels were strikingly similar, both at birth and the age of 1 year.
| Discussion |
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While some earlier studies indicated cord blood cholesterol determinations to show promise in early screening for inherited hypercholesterolemia,20,21 other studies have questioned this, with the possible exception of their use in newborn children of affected parents.2224 Some alternative approaches, instead of assaying cholesterol levels only, have been suggested to be useful, such as measurement of LDL-C,25 LDL+VLDL cholesterol,26,27 ß-lipoprotein,28 apolipoprotein B,2931 or apolipoprotein A-I to B ratio32 in cord serum. In none of these earlier studies has FH been diagnosed by molecular genetic techniques.
We took advantage of the special circumstances in the Finnish North
Karelia permitting analysis of cosegregation of
hypercholesterolemia with a defective gene in
affected families using a prospective design. Our data show that while
mean serum TC and LDL-C levels in molecularly defined affected and
nonaffected infants differ significantly from each other both at birth
and age of 1 year, TC and LDL-C levels in individual subjects of the
two groups are markedly overlapping in cord serum but nonoverlapping at
the later date (Fig 1
, 2
). Thus, use of arbitrary cut-off values of
6 mmol/L and 5 mmol/L for serum total and LDL-C
concentrations, respectively, would have differentiated our molecularly
defined FH heterozygotes (n=18) and nonaffected children (n=16) from
each other at the age of 1 year in this relatively small group of
infants (Fig 2
). It should be emphasized that our data were obtained in
a study population that obeyed strict measures to control dietary fat,
and the difference in serum cholesterol levels between
1-year-old affected and nonaffected subjects in a random population
with no previously established diagnosis of parental FH may therefore
not be equally striking.
Use of LDL-C measurements in cord serum, as suggested by Kwiterowich et
al,25 appeared to perform somewhat better in
comparison to total cholesterol measurements to
discriminate FH and non-FH newborns from each other, yet providing
unsatisfactory diagnostic accuracy (Fig 1
). Earlier studies
have indicated that estimation of LDL-C levels in human cord serum
derived by calculation from TC, HDL-C, and TG values using the
Friedewald formula12 correspond well to those
obtained by
ultracentrifugation.33
Previous studies have indicated that a number of maternofetal factors
such as gestational age, prematurity, respiratory distress, maternal
diabetes, and preeclampsia may affect serum lipid levels at
birth.24,34,35 Our material included only one
complicated pregnancy, and lipid data of the respective newborn were
excluded from our data set. We did not see any correlation between
gestational age and cord serum LDL-C levels in our study group, and
lipid levels in cord sera after vaginal deliveries and cesarean
sections were similar. None of the mothers used any type of
lipid-lowering drugs during pregnancy. All mothers of the FH families
had received a similar-type of dietary advisement, regardless of
whether it was the mother or father who was affected and whether the
newborn proved subsequently to have FH or not. This may be the reason
why serum total and LDL-C levels in cord blood samples from pregnancies
with either of the parents affected with FH tended to lower slightly in
comparison to those in cord samples derived from a randomly selected
population material (Fig 1
).
Studies on longitudinal changes in serum lipid levels after birth have shown a rapid increase of serum cholesterol and apolipoprotein B levels during the first week of life, a less pronounced increment during the ensuing month, and relatively stable levels from the age of 6 months to 2 years.3639 Nutritional factors such as fatty acid composition of the diet may modify serum lipid levels or lipoprotein composition during the early weeks of human life37,39 and during later phases of the first year of life.40 We are not aware of similar week-to-week follow-up studies in newborn children with heterozygous FH, but reports on a few individual cases suggest that this increment of serum cholesterol takes place as quickly as in non-FH subjects.22,24
The finding that serum HDL-C levels in affected children at the age of
1 year were 18% lower than those in nonaffected ones (Table 2
) is in
accordance with previous studies in children of age classes from 1 to
19 years.6,19,41 It was of interest, however,
that in our study this phenotypic difference was not yet manifest at
birth (Fig 1
, Table 2
). Our data suggest that the difference in serum
HDL-C levels in affected and nonaffected children is a secondary
phenomenon, and appear when the human fetus switches from making use of
carbohydrates as its principal caloric source42
to utilize fats as an important energy source after birth.
In conclusion, our study relying on the use of molecular methods to distinguish between carriers and noncarriers of a mutant LDL receptor gene indicate that, although serum LDL-C levels in affected children are significantly elevated from birth on, assessment of diagnosis of heterozygous FH using lipid determinations in cord blood is unreliable; a better discrimination is obtained when lipid determinations are carried out at the age of 1 year. In addition, our data suggest that children with FH are born with serum HDL-C levels similar to those in other newborns and that the subsequent rise in HDL-C levels during the first year of life is slightly less pronounced in children with FH than in children without it.
| Acknowledgments |
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Received February 11, 1997; accepted April 10, 1997.
| References |
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