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INSERM U-402, Faculté de Médecine Saint-Antoine (C.V., C.B., J.C., J.M.); Service dEndocrinologie Pédiatrique, Hôpital Necker-Enfants Malades (J.-J.R.); and Service dEndocrinologie et de Diabétologie, Hôpital Robert Debré (N.T.-R.), Paris, France; Service de Pédiatrie, Hôpital Hôtel-Dieu de France (E.K.), Beirut, Lebanon; Service dEndocrinologie Pédiatrique, Hôpital Sud (M.d.K.), Rennes, France; and CNRS URA 1922, Généthon (S.F., J.W.), Evry, France
Address all correspondence and requests for reprints to: Dr. J. Magré, INSERM U-402, Faculté de Médecine Saint-Antoine, 27 rue Chaligny, 75571 Paris Cedex 12, France. E-mail: magre{at}st-antoine.inserm.fr
| Abstract |
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| Introduction |
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Although the pathophysiology of LD is not clearly understood at the moment, insulin resistance and lipoatrophy appear to be the main features. Therefore, to identify the gene(s) responsible for LD, we performed a linkage study in affected families using a functional approach. We studied candidate genes involved in insulin action and lipid metabolism, most of which have been shown to be related to insulin resistance in noninsulin-dependent diabetes mellitus (NIDDM) or obese patients.
Defective insulin action may be due to alterations in any protein involved in signal transduction between the receptor and the final insulin-regulated proteins. We previously excluded the implication of the insulin receptor gene (INSR) in two families with LD (6), in accordance with other reports (7, 8, 9), and we now extend this study to eight more families. We also examined the insulin-like growth factor I receptor (IGFIR), which shares with the insulin receptor several steps of intracellular signaling (10). We previously described a reduction in IGF-I-stimulated receptor autophosphorylation in cultured fibroblasts from LD patients (11). We studied insulin receptor substrate 1 (IRS1), an important step in the insulin and IGF-I signaling pathways (12). Diabetes-associated ras-like protein (rad) is overexpressed in muscle of NIDDM patients and could be implicated in insulin action (13). Muscle glycogen synthase (GSY1), the rate-limiting enzyme in the glycogen synthesis pathway, has been shown not to be activated by insulin in a LD patient (14).
Genes affecting lipid metabolism are also candidate genes for contributing to the development of LD. Lipoprotein lipase (LPL), its cofactor apolipoprotein CII (APOC2), and hepatic lipase (LIPC) catalyze the hydrolysis of triglycerides at the endothelial surfaces of extrahepatic and hepatic tissue, respectively, thus providing free fatty acids for adipose, muscular, and hepatic cells. Hormone-sensitive lipase (LIPE) plays a prominent role in the mobilization of free fatty acids from adipose tissue by controlling the rate of lipolysis of the stored triglycerides. Increased activation of this enzyme could produce triglyceride depletion of adipocytes. We also studied apolipoproteins AII (APOA2) and CIII (APOC3), which play an important role in the regulation of plasma lipids. It has been suggested that the APOA2 locus is linked to a gene controlling free fatty acid levels (15). A gene linked to the fatty acid-binding protein 2 locus (FABP2) on chromosome 4q may contribute to insulin resistance in Pima Indians (16). The ß3-adrenergic receptor (ADRB3), located mainly in adipose tissue, is involved in the regulation of lipolysis and thermogenesis. Finally, as leptin is secreted by adipose tissue and is involved in regulation of body fat, it is of interest to examine its gene, ob, for linkage in LD families.
According to the homozygosity mapping theory (17), consanguineous families provide a powerful tool to test the role of a candidate gene in a rare autosomal recessive disorder. Linkage analysis was, therefore, carried out in 10 inbred families using simple tandem repeat DNA polymorphisms at these 14 candidate genes loci.
| Subjects and Methods |
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We studied 10 consanguineous families with 13 affected
children, originating from Senegal, Tunisia, Algeria, Portugal, and
Lebanon. Pedigrees of these families are shown in Fig. 1
. Patients were born of first cousin
unions in 6 families (He, Jr, Ma, Ou, Sa, and Ta), second cousin
marriages in 2 families (Di and Fa), and a more complex consanguinity
is in families Ha and Wa. These latter families, Ha and Wa, cluster in
a very small geographic area of Northern Lebanon. The main clinical and
biological features of each patient are shown in Table 1
. All 15 patients exhibited typical
features of LD; in addition to lipoatrophy, most of them presented with
acanthosis nigricans, hepatomegaly, and muscular hypertrophy. Other
clinical characteristics were observed in about half of the patients:
acromegaloid dysmorphy (7 of 15), external genitalia enlargement (8 of
15), and mild mental retardation (8 of 15). Polycystic ovary disease
was diagnosed in all postpubertal females (Di-1, Ta-1, and Ou-1) with
marked hirsutism. Insulin resistance was always present, most
frequently worsening with aging; hyperinsulinemia existed in fasting
and/or in postabsorptive states, leading to glucose intolerance or
diabetes. Di-3, who was 9 months old at the time of the examination,
had only mild postabsorptive hyperinsulinemia and mild fasting
hypertriglyceridemia. Diagnosis was based essentially on the
lipoatropic aspect and the familial antecedents. It is noteworthy that
her brother, Di-2, from the age of 7 yr progressively developed frank
hyperinsulinemia and clinical features of LD in addition to neonatal
lipoatrophy. When insulin therapy was used, very high doses were
necessary to lower plasma glucose; patients Di-1, Ou-1, and Jr-1
received 5 U/kg or more daily insulin, without sufficient glycemic
control. Two patients exhibited the delayed onset form of the
lipoatrophy (Ou-1 and Ta-1), although all other patients were
lipoatropic at birth or within the very first months of life.
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The patients and their relatives gave their informed consent to participate in the study. This work was approved by an institutional review board.
Genetic studies
DNA was prepared from peripheral white blood cells using
standard procedures (21). Subjects were genotyped at the candidate
genes loci, using simple tandem repeat DNA polymorphisms. Published
intragenic polymorphisms were used to test INSR, IRS1, IGFIR,
FABP2, GSY1, LPL, APOC2, LIPC, APOA2, APOC3, and
LIPE. ADRB3, rad, and ob
were analyzed using anonymous microsatellites (22) located at,
respectively, 02.3 centimorgans (cM) (23), 02.6 cM (22, 24), and 0
cM (25) from the corresponding gene. Markers were amplified by PCR,
using the primer sequences and conditions shown in Table 2
, following a multiplexing procedure
(26).
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When two microsatellites were tested to study a single locus (studies of ADRB3, ob, LIPE, andrad), haplotype analyses were interpreted using the Lander and Botstein theory (17): in rare recessive genetic diseases, affected children from consanguineous families are expected to be homozygous for the defective allele inherited from a common ancestor.
| Results |
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) of 0.00
and, when the microsatellites were not intragenic, also at
corresponding to the maximal possible distance from the genes. Maximal
lod scores at
= 0.00 have been calculated by simulating perfect
segregation between a rare allele of a fully informative putative
marker and the disease.
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ADRB3 and rad loci were studied using two
microsatellites located, respectively, 02.3 and 02.6 cM from the
gene. Lod scores calculated at
corresponding to the maximal genetic
distance were, as expected, less significant than those at
= 0.00.
However, in addition to lod score calculations, haplotype analysis was
performed in each family. With regard to the Lander and Botstein theory
(17), affected subjects born from consanguineous unions are expected to
be homozygous for the morbid allele inherited from each of their
parents. Therefore, assuming a complete penetrance of the disease in
the context of full informativity of the markers, we could exclude
linkage at a locus because 1) the two parents do not share a common
allele; 2) patients are heterozygous; 3) an affected child has the same
haplotype as a normal sibling; or 4) affected siblings have different
haplotypes. Haplotype analysis formally excluded the involvement of
ADRB3 and rad loci in most families. Similar
results were obtained for LIPE and ob.
These two analyses of lod scores and haplotype transmission allowed us to exclude most of the loci in each family. Definite exclusion could not be affirmed in only four cases: for the ADRB3 locus in families Ha and He, for the LIPE locus in family He, and for the rad locus in family Sa. In these cases, sufficient informativity of markers was not achieved, as one of the parents or both were homozygous for one or several microsatellite at these loci. Furthermore, as the ADRB3 locus was excluded in family Wa, it is very unlikely that it is involved in family Ha.
| Discussion |
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Several hypotheses have been proposed to explain both insulin resistance and failure of fat storage in adipocytes. On the one hand, a defect in the insulin transduction pathway could lead to severe insulin resistance with increased lipolysis (28). On the other hand, a primary abnormality in adipose tissue metabolism could explain lipoatrophy and the presence of high levels of nonesterified fatty acids that would trigger secondary insulin resistance. As the primary defect is unknown, genes involved in the mechanism of action of insulin or in lipid metabolism could be considered candidate genes for this disease.
Linkage analysis in inbred families provides a powerful strategy to investigate the roles of these genes in rare recessive disorders. Indeed, in these diseases, affected subjects are expected to inherit 2 copies of the morbid allele transmitted from a common ancestor. The affected individuals are, therefore, said to be homozygous by descent (rather than by state) for the disease allele (17). The 10 consanguineous families described here provided us with substantial family resource to study the implication of candidate genes in LD. Many lod scores were infinitely negative, allowing definite exclusion of corresponding loci in a large proportion of families. In addition, when two microsatellites were typed at a locus, haplotype analysis could be used in a way complementary to lod score calculations to study allele transmission. These 2 approaches led us to exclude the involvement, in most families, of 14 candidate genes coding for proteins involved either in insulin action, i.e. INSR, IRS1, IGFIR, rad, and GSY1, or in lipid metabolism, i.e. LPL, APOCII, APOAII, APOCIII, LIPC, LIPE, ADRB3, leptin, and FABP2. Although a few genes could not be ruled out in a minority of families, these nonexcluded genes differed among those families. This implies that mutations in these genes are unlikely to make a major genetic contribution to LD regardless of whether the form of the disease is congenital or retarded.
Other candidate genes should be studied, such as genes that influence insulin receptor tyrosine kinase activity (i.e. tyrosine phosphatases) or the insulin antilipolytic effect, or those involved in fatty acid transport and adipocyte differentiation. Henceforth, this functional approach will be combined with whole genome scanning, presently performed, in which anonymous markers are tested for homozygosity in these patients. Only candidate genes mapped in a genetic interval where most individuals are homozygous will be retained for further analyses.
The identification of the genetic abnormality in LD will help to understand the pathophysiology of this disease. It will also provide new data concerning insulin resistance and lipid alterations that are common features in more frequent diseases, such as NIDDM.
| Acknowledgments |
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| Footnotes |
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Received March 7, 1997.
Revised May 23, 1997.
Accepted June 18, 1997.
| References |
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