Identification of the gene altered in Berardinelli-Seip congenital lipodystrophy on chromosome 11q13.

Congenital generalized lipodystrophy, or Berardinelli-Seip syndrome (BSCL), is a rare autosomal recessive disease characterized by a near-absence of adipose tissue from birth or early infancy and severe insulin resistance. Other clinical and biological features include acanthosis nigricans, hyperandrogenism, muscular hypertrophy, hepatomegaly, altered glucose tolerance or diabetes mellitus, and hypertriglyceridemia. A locus (BSCL1) has been mapped to 9q34 with evidence of heterogeneity. Here, we report a genome screen of nine BSCL families from two geographical clusters (in Lebanon and Norway). We identified a new disease locus, designated BSCL2, within the 2.5-Mb interval flanked by markers D11S4076 and D11S480 on chromosome 11q13. Analysis of 20 additional families of various ethnic origins led to the identification of 11 families in which the disease cosegregates with the 11q13 locus; the remaining families provide confirmation of linkage to 9q34. Sequence analysis of genes located in the 11q13 interval disclosed mutations in a gene homologous to the murine guanine nucleotide-binding protein (G protein), gamma3-linked gene (Gng3lg) in all BSCL2-linked families. BSCL2 is most highly expressed in brain and testis and encodes a protein (which we have called seipin) of unknown function. Most of the variants are null mutations and probably result in a severe disruption of the protein. These findings are of general importance for understanding the molecular mechanisms underlying regulation of body fat distribution and insulin resistance.

Genetic exclusion of 14 candidate genes in lipoatropic diabetes using linkage analysis in 10 consanguineous families.

Lipoatropic diabetes (LD) is a rare recessive autosomal disorder, mainly characterized by lipoatrophy with alterations in lipid metabolism and extreme insulin resistance. To identify molecular defects responsible for this disease, we tested the implication of 14 candidate genes coding for proteins involved either in insulin action, i.e. insulin receptor, insulin receptor substrate 1, insulin-like growth factor I receptor, diabetes-associated ras-like protein (Rad), and glycogen synthase, or in lipid metabolism, i.e. lipoprotein lipase; apolipoproteins CII, AII, and CIII; hepatic lipase; hormone-sensitive lipase; the beta 3-adrenergic receptor; leptin; and fatty acid-binding protein 2. To this end, haplotype and linkage analyses using genotyping with microsatellites in 10 consanguineous families provided us with powerful genetic tools. Our results show that in most families, lod scores at a null recombination fraction were less than -2. Haplotype analysis also argues against the involvement of these genes in LD. This implies that mutations in these genes are unlikely to make a major genetic contribution to LD.

Extremely low values of serum leptin in children with congenital generalized lipoatrophy.

Congenital generalized lipoatrophy (CGL) is a syndrome with multiple clinical manifestations and complete atrophy of adipose tissue. The exact mechanism of this disease remains unknown. One hypothesis presupposes an abnormal development of adipocytes. Leptin, the adipocyte-specific product of the ob gene, acts as a regulatory factor of body weight. In children, as in adults, leptin levels are correlated with body mass index (BMI) and body fat mass. Some authors have demonstrated that adults with congenital or acquired generalized lipoatrophy have decreased leptin concentrations. In order to study serum leptin profile during childhood in this disease, we measured serum leptin concentrations in six children aged 5.5-11 years suffering from CGL, and investigated the relationship between metabolic parameters and the variations in leptin levels. Serum leptin concentrations (1.19+/-0.32 ng/ml (+/- S.D.)) were extremely low compared with those observed in normal children. No significant correlation was found with BMI, which is known to be one of the major determinants of serum leptin. Serum leptin values were significantly correlated with fasting insulin levels (r=0.83, P=0.024). In conclusion, extremely low leptin values measured in children with CGL could be regarded as one among other diagnostic parameters. However, the detectable levels observed in all of these children support the evidence that a small amount of body fat is likely to be present in these patients, despite complete subcutaneous lipoatrophy. Our data suggest that this small amount of adipose tissue could be metabolically active and, at least in part, sensitive to insulin. Further investigations are required to uncover the pathophysiological mechanisms of this syndrome, known to be commonly associated with insulin resistance.

[Early hypomagnesemia, hypercalciuria and nephrocalcinosis: two cases in a family]. / Hypomagnésémie, hypercalciurie et néphrocalcinose de révélation précoce: deux observations familiales.

BACKGROUND: Hypomagnesemia-hypercalciuria and nephrocalcinosis is a rare inherited syndrome which is characterized by persistent hypomagnesemia despite supplementation, hypercalciuria, nephrocalcinosis and progressive renal failure. OBSERVATIONS: Case 1. A girl was referred at the age of 18 months because of polyuria, polydipsia and vitamin-resistant rickets. There was hypomagnesemia, hypercalciuria and mild renal insufficiency; ultrasonography showed nephrocalcinosis. For two years, she received hydrochlorothiazide and the course of the disease was marked by a significant reduction of urine output and hypercalciuria, recurrent urinary tract infections and a progression toward chronic renal failure. Case 2. The brother of this child was investigated at the age of nine months because of polyuria and polydipsia. He also had hypomagnesemia, hypercalciuria and nephrocalcinosis. Renal function was initially normal. After two years on continuous treatment with hydrochlorothiazide, hypercalciuria decreased without deterioration of renal function. No signs of rickets were noted and nephrocalcinosis remained stable. CONCLUSION: To our knowledge, these two patients are the youngest reported in the literature. The long-term deterioration of renal function is hazardous but rickets may be avoided by early administration of hydrochlorothiazide.

[Autoimmunity and insulin-dependent diabetes mellitus. Experimental data and therapeutic prospects]. / Auto-immunité et diabète insulinodépendant. Données expérimentales et perspectives thérapeutiques.

Experimental results and therapeutic strategies. Insulin-dependent diabetes mellitus (IDDM) results from an autoimmune aggression toward beta cells in genetically predisposed individuals. Examination of the frequency of the different antigens coded by the major histocompatibility complex reveals an increased proportion of DR3-DQ2 and DR4-DQ8 haplotypes in IDDM subjects. Sequencing DQ-beta chains in such patients indicates the absence of aspartate in position 57 when compared to control individuals. Islet cell cytoplasmic autoantibodies are early markers of ongoing autoimmunity in addition to insulin autoantibodies before administration of exogenous insulin. Experimental models of autoimmune diabetes like the NOD (NonObese Diabetes) mouse underline the predominant role of T lymphocytes in the constitution of both insulitis and beta cell destruction. In humans, an increased proportion of activated T lymphocytes can be observed but is not specific of the disease. This underlines the need for new cellular markers of the autoimmune process. Transgenic mice allow studies on the consequences of abnormal expression of new molecules on beta cell surface like cytokines or MHC class II molecules which represent a new field of investigation on the pathogenesis of IDDM. Prospective studies in first degree relatives of type I diabetic patients indicate the existence of an asymptomatic phase of beta cell destruction where specific autoimmune markers can be individualized. In some individuals abnormal insulin response to glucose--loss of first phase insulin release during intravenous glucose tolerance test--precedes insulin deficiency. The identification of an autoimmune process leading to beta cell destruction allows new therapeutic approaches with immunointervention at early stages of the disease.

Mutational analysis in Lebanese patients with congenital adrenal hyperplasia due to a deficit in 21-hydroxylase.

Molecular defects in the gene encoding steroid 21-hydroxylase (CYP21) result in impairment of adrenal steroid synthesis in patients affected with autosomal-recessive congenital adrenal hyperplasias (CAH). In this study, we report on the molecular screening of six point mutations, large deletions, gene conversion events and duplications in 25 unrelated Lebanese families affected by CAH due to steroid 21-hydroxylase. The methods used (PCR-digestion and southern blot) allowed the detection of 96% of the disease chromosomes. In classical forms, the most frequent mutation was the splice site mutation in intron 2 accounting for 39% of the disease alleles. Gene conversion events accounted for 14% of the alleles, but no large deletions were found. In nonclassical forms, the V281L mutation in exon 7 represent 86% of the tested alleles. Genotype-phenotype correlations were as expected: Delta 8nt, Q318X and gene conversion correspond to SW forms, whereas the intron 2 splice site mutation may give either SW or SV forms; the V281L mutation was responsible for nonclassical forms. The spectrum of mutations underlines the genetic diversity of the Lebanese population. No correlation could be drawn out between mutations and some specific religious communities, except for the Delta 8nt mutation, which is present only in the Christian Maronite group. Molecular study of the CYP21 gene might constitute a good support for clinicians, especially in consanguineous families, for whom we could provide genetic counselling.