Postprandial lipid absorption in seven heterozygous carriers of deleterious variants of MTTP in two abetalipoproteinemic families.

BACKGROUND: Abetalipoproteinemia, a recessive disease resulting from deleterious variants in MTTP (microsomal triglyceride transfer protein), is characterized by undetectable concentrations of apolipoprotein B, extremely low levels of low-density lipoprotein cholesterol in the plasma, and a total inability to export apolipoprotein B-containing lipoproteins from both the intestine and the liver. OBJECTIVE: To study lipid absorption after a fat load and liver function in 7 heterozygous relatives from 2 abetalipoproteinemic families, 1 previously unreported. RESULTS: Both patients are compound heterozygotes for p.(Arg540His) and either c.708_709del p.(His236Glnfs*11) or c.1344+3_1344+6del on the MTTP gene. The previously undescribed patient has been followed for 22 years with ultrastructure analyses of both the intestine and the liver. In these 2 families, 5 relatives were heterozygous for p.(Arg540His), 1 for p.(His236Glnfs*11) and 1 for c.1344+3_1344+6del. In 4 heterozygous relatives, the lipid absorption was normal independent of the MTTP variant. In contrast, in 3 of them, the increase in triglyceride levels after fat load was abnormal. These subjects were additionally heterozygous carriers of Asp2213 APOB in-frame deletion, near the cytidine mRNA editing site, which is essential for intestinal apoB48 production. Liver function appeared to be normal in all the heterozygotes except for one who exhibited liver steatosis for unexplained reasons. CONCLUSION: Our study suggests that a single copy of the MTTP gene may be sufficient for human normal lipid absorption, except when associated with an additional APOB gene alteration. The hepatic steatosis reported in 1 patient emphasizes the need for liver function tests in all heterozygotes until the level of risk is established.

Microvillous atrophy: atypical presentations.

OBJECTIVES: Microvillous inclusion disease (MVID) is a cause of intractable diarrhea in infancy. In its classic form, the disease is characterized by a severe persistent watery diarrhea starting within the first days of life. Parenteral nutrition and small bowel transplantation are the only known treatments for the affected children. Histologically, periodic acid-Schiff (PAS) staining shows accumulation of periodic acid-Schiff-positive staining material along the apical pole of enterocytes, whereas transmission electron microscopy exhibits microvillus inclusion bodies within the cytoplasm of enterocytes with rarefied and shortened microvilli and secretory granules. The objective of this work was to explore clinical, morphological, and genetic findings in cases of MVID with unusual presentations. METHODS: Clinical, histological, and genetic findings are reported for 8 cases of MVID with atypical presentation. RESULTS: The diarrhea started after several months in 3 cases. It was usually less abundant and 3 patients were weaned off parenteral nutrition. None required intestinal transplantation. Three patients experienced malformations, dysmorphy, sensory disabilities, and severe mental retardation. One had a hydrocephaly. Three patients had a cholestasis with low γ-glutamyl transferase levels. Light microscopy showed histological abnormalities consistent with MVID in all of the cases, but the lesions were sometimes focal or delayed. Transmission electron microscopy retrieved some criteria of MVID in 4 patients. Finally, 6 patients were homozygotes or compound heterozygotes for MYO5B mutations. CONCLUSIONS: This study extends the spectrum of MVID to less severe clinical presentations.

Homozygous MTTP and APOB mutations may lead to hepatic steatosis and fibrosis despite metabolic differences in congenital hypocholesterolemia.

BACKGROUND & AIMS: Non-alcoholic steatohepatitis leading to fibrosis occurs in patients with abetalipoproteinemia (ABL) and homozygous or compound heterozygous familial hypobetalipoproteinemia (Ho-FHBL). We wanted to establish if liver alterations were more frequent in one of both diseases and were influenced by comorbidities. METHODS: We report genetic, clinical, histological and biological characteristics of new cases of ABL (n =7) and Ho-FHBL (n = 7), and compare them with all published ABL (51) and Ho-FHBL (22) probands. RESULTS: ABL patients, diagnosed during infancy, presented mainly with diarrhea, neurological and ophthalmological impairments and remained lean, whereas Ho-FHBL were diagnosed later, with milder symptoms often becoming overweight in adulthood. Despite subtle differences in lipid phenotype, liver steatosis was observed in both groups with a high prevalence of severe fibrosis (5/27 for Ho-FHBL vs. 4/58 for ABL (n.s.)). Serum triglycerides concentration was higher in Ho-FHBL whereas total and HDL-cholesterol were similar in both groups. In Ho-FHBL liver alterations were found to be independent from the apoB truncation size and apoB concentrations. CONCLUSIONS: Our findings provide evidence for major liver abnormalities in both diseases. While ABL and Ho-FHBL patients have subtle differences in lipid phenotype, carriers of APOB mutations are more frequently obese. These results raise the question of a complex causal link between apoB metabolism and obesity. They suggest that the genetic defect in VLDL assembly is critical for the occurrence of liver steatosis leading to fibrosis and shows that obesity and insulin resistance might contribute by increasing lipogenesis.

Low rate of production of apolipoproteins B100 and AI in 2 patients with Anderson disease (chylomicron retention disease).

OBJECTIVE: Anderson disease is a rare inherited lipid malabsorption syndrome associated with hypocholesterolemia and linked to SAR1B mutations. The aim of this article was to analyze the mechanisms responsible for the low plasma apolipoprotein Apo-B100 and Apo-AI in 2 patients with Anderson disease. METHODS AND RESULTS: A primed constant infusion of (13)C-leucine was administered for 14 hours to determine the kinetics of lipoproteins. In the 2 patients, total cholesterol (77 and 85 mg/dL versus 155±32 mg/dL), triglycerides (36 and 59 versus 82±24 mg/dL), Apo-B100 (48 and 43 versus 71±5 mg/dL), and Apo-AI (47 and 62 versus 130±7 mg/dL) were lower compared with 6 healthy individuals. Very-low-density lipoprotein-B100 production rate of the patients was lower (4.08 and 5.52 mg/kg/day versus 12.96±2.88 mg/kg/day) as was the fractional catabolic rate (5.04 and 4.32 day(-1) versus 12.24±3.84 day(-1)). No difference was observed in intermediate-density lipoprotein-B100 and LDL-B100 kinetic data. The production rate of high-density lipoprotein Apo-AI was lower in the patients (7.92 and 8.64 versus 11.96±1.92 mg/kg/day) and the fractional catabolic rate was higher (0.38 and 0.29 versus 0.22±0.01 day(-1)). CONCLUSIONS: The low plasma Apo-B100 and Apo-AI concentrations in the patients with Anderson disease were mainly related to low rates of production.

Molecular and functional analysis of two new MTTP gene mutations in an atypical case of abetalipoproteinemia.

Abetalipoproteinemia (ABL) is an inherited disease characterized by the defective assembly and secretion of apolipoprotein B-containing lipoproteins caused by mutations in the microsomal triglyceride transfer protein large subunit (MTP) gene (MTTP). We report here a female patient with an unusual clinical and biochemical ABL phenotype. She presented with severe liver injury, low levels of LDL-cholesterol, and subnormal levels of vitamin E, but only mild fat malabsorption and no retinitis pigmentosa or acanthocytosis. Our objective was to search for MTTP mutations and to determine the relationship between the genotype and this particular phenotype. The subject exhibited compound heterozygosity for two novel MTTP mutations: one missense mutation (p.Leu435His) and an intronic deletion (c.619-5_619-2del). COS-1 cells expressing the missense mutant protein exhibited negligible levels of MTP activity. In contrast, the minigene splicing reporter assay showed an incomplete splicing defect of the intronic deletion, with 26% of the normal splicing being maintained in the transfected HeLa cells. The small amount of MTP activity resulting from the residual normal splicing in the patient explains the atypical phenotype observed. Our investigation provides an example of a functional analysis of unclassified variations, which is an absolute necessity for the molecular diagnosis of atypical ABL cases.

Anderson's disease/chylomicron retention disease in a Japanese patient with uniparental disomy 7 and a normal SAR1B gene protein coding sequence.

BACKGROUND: Anderson's Disease (AD)/Chylomicron Retention Disease (CMRD) is a rare hereditary hypocholesterolemic disorder characterized by a malabsorption syndrome with steatorrhea, failure to thrive and the absence of chylomicrons and apolipoprotein B48 post-prandially. All patients studied to date exhibit a mutation in the SAR1B gene, which codes for an essential component of the vesicular coat protein complex II (COPII) necessary for endoplasmic reticulum to Golgi transport. We describe here a patient with AD/CMRD, a normal SAR1B gene protein coding sequence and maternal uniparental disomy of chromosome 7 (matUPD7). METHODS AND RESULTS: The patient, one of two siblings of a Japanese family, had diarrhea and steatorrhea beginning at five months of age. There was a white duodenal mucosa upon endoscopy. Light and electron microscopy showed that the intestinal villi were normal but that they had lipid laden enterocytes containing accumulations of lipid droplets in the cytoplasm and lipoprotein-size particles in membrane bound structures. Although there were decreased amounts in plasma of total- and low-density lipoprotein cholesterol, apolipoproteins AI and B and vitamin E levels, the triglycerides were normal, typical of AD/CMRD. The presence of low density lipoproteins and apolipoprotein B in the plasma, although in decreased amounts, ruled out abetalipoproteinemia. The parents were asymptomatic with normal plasma cholesterol levels suggesting a recessive disorder and ruling out familial hypobetalipoproteinemia. Sequencing of genomic DNA showed that the 8 exons of the SAR1B gene were normal. Whole genome SNP analysis and karyotyping revealed matUPD7 with a normal karyotype. In contrast to other cases of AD/CMRD which have shown catch-up growth following vitamin supplementation and a fat restricted diet, our patient exhibits continued growth delay and other aspects of the matUPD7 and Silver-Russell Syndrome phenotypes. CONCLUSIONS: This patient with AD/CMRD has a normal SAR1B gene protein coding sequence which suggests that factors other than the SAR1B protein may be crucial for chylomicron secretion. Further, this patient exhibits matUPD7 with regions of homozygosity which might be useful for elucidating the molecular basis of the defect(s) in this individual. The results provide novel insights into the relation between phenotype and genotype in these diseases and for the mechanisms of secretion in the intestine.

Strategies for proprotein convertase subtilisin kexin 9 modulation: a perspective on recent patents.

IMPORTANCE OF THE FIELD: Proprotein convertase subtilisin kexin 9 (PCSK9) is a new actor discovered in 2003 that is implicated in autosomal dominant hypercholesterolemia, cholesterol homeostasis and coronary heart disease. It has been shown to degrade the low-density lipoprotein (LDL) receptor independently of its catalytic activity. Several pharmacological strategies to reduce PCSK9 are being thoroughly investigated. AREAS COVERED IN THIS REVIEW: This article reviews all different strategies that are presently pursued to modulate the functional activity of PCSK9 which is a prime target for controlling LDL-cholesterol. It also provides a briefing of all the patents up to July 2010 from various organizations including pharmaceutical companies and academic institutions that have been submitted and/or approved. WHAT THE READER WILL GAIN: This review is addressed to researchers from academia and pharmaceutical companies who are engaged in PCSK9 research/cholesterol regulation and in the development of cholesterol lowering drugs. Readers will gain an up-to-date overview of the different strategies that have been investigated to reduce PCSK9 including antisense technology and specific antibodies. TAKE HOME MESSAGE: Clinical trials have been launched using RNA interference approaches to reduce PCSK9 expression or specific antibodies targeting and inhibiting PCSK9 interaction with the LDL receptor. They constitute very promising approaches to reducing cholesterol levels and coronary heart disease.