Abetalipoproteinemia in Israel: evidence for a founder mutation in the Ashkenazi Jewish population and a contiguous gene deletion in an Arab patient

High carrier frequency for abetalipoproteinemia and evidence of a founder variant in a French-Canadian population

Abetalipoproteinemia (ABL) is a rare recessive genetic disease caused by bi-allelic pathogenic variants in the microsomal triglyceride transfer protein (MTTP) gene. This disease is characterized by a deficiency in the secretion of apolipoprotein B-containing lipoproteins. Patients with ABL present with neurological, hematological, and gastrointestinal symptoms due to fat malabsorption and a deficiency in liposoluble vitamins. In this report, we present a total of four ABL cases, including three new cases, all originating from the same French-Canadian founder population in Saguenay-Lac-Saint-Jean, Québec, Canada. These individuals are homozygous for the same pathogenic variant in the MTTP gene (c.419dup, p.Asn140Lysfs*2). We found that this variant is more common than anticipated in this population, with an estimated carrier frequency of 1:203. Early diagnosis is essential to initiate treatment known to prevent complications associated with ABL. Population carrier screening or newborn screening for ABL should be considered in this French-Canadian founder population.

A novel p.Gly417Valfs*12 mutation in the MTTP gene causing abetalipoproteinemia: Presentation of the first patient in Mexico and analysis of the previously reported cases

BACKGROUND

Our aims were to describe the first Mexican patient with abetalipoproteinemia and to perform a comparative analysis of biochemical, clinical, and genetic characteristics of 100 cases reported in the literature.

METHODS

We performed biochemical and molecular screenings in a Mexican girl with extremely low lipid levels and in her family. Further, we integrated and evaluated the characteristics of the cases with abetalipoproteinemia described in the literature.

RESULTS

Our patient is a six-year-old girl who presented vomiting, chronic diarrhea, failure to thrive, malabsorption, acanthocytosis, anemia, transaminases elevation, and extremely low lipid levels. MTTP gene sequencing revealed homozygosity for a novel mutation p.Gly417Valfs*12 (G deletion c.1250). With the analysis of the reported cases, 60 clinical features (14 classical and 46 non-classical) were observed, being the most common acanthocytosis (57.5%), malabsorption (43.7%), and diarrhea (42.5%); 48.8% of the patients presented only classic clinical features, while the remaining 51.2% developed secondary effects due to a fat-soluble vitamin deficiency. An odds ratio analysis disclosed that patients diagnosed after 10 years of age have an increased risk for presenting clinical complications (OR = 18.0; 95% CI 6.0-54.1, p < 0.0001). A great diversity of mutations in MTTP has been observed (n = 76, being the most common p.G865X and p.N139_E140) and some of them with possible residual activity.

CONCLUSION

The first Mexican patient with abetalipoproteinemia presents a novel MTTP mutation p.Gly417Valfs*12. Three factors that could modulate the phenotype in abetalipoproteinemia were identified: age at diagnosis, treatment, and the causal mutation.

Normal serum ApoB48 and red cells vitamin E concentrations after supplementation in a novel compound heterozygous case of abetalipoproteinemia

BACKGROUND AND AIMS

Abetalipoproteinemia (ABL) is a rare recessive monogenic disease due to MTTP (microsomal triglyceride transfer protein) mutations leading to the absence of plasma apoB-containing lipoproteins. Here we characterize a new ABL case with usual clinical phenotype, hypocholesterolemia, hypotriglyceridemia but normal serum apolipoprotein B48 (apoB48) and red blood cell vitamin E concentrations.

METHODS

Histology and MTP activity measurements were performed on intestinal biopsies. Mutations in MTTP were identified by Sanger sequencing, quantitative digital droplet and long-range PCR. Functional consequences of the variants were studied in vitro using a minigene splicing assay, measurement of MTP activity and apoB48 secretion.

RESULTS

Intestinal steatosis and the absence of measurable lipid transfer activity in intestinal protein extract supported the diagnosis of ABL. A novel MTTP c.1868G>T variant inherited from the patient's father was identified. This variant gives rise to three mRNA transcripts: one normally spliced, found at a low frequency in intestinal biopsy, carrying the p.(Arg623Leu) missense variant, producing in vitro 65% of normal MTP activity and apoB48 secretion, and two abnormally spliced transcripts resulting in a non-functional MTP protein. Digital droplet PCR and long-range sequencing revealed a previously described c.1067+1217_1141del allele inherited from the mother, removing exon 10. Thus, the patient is compound heterozygous for two dysfunctional MTTP alleles. The p.(Arg623Leu) variant may maintain residual secretion of apoB48.

CONCLUSIONS

Complex cases of primary dyslipidemia require the use of a cascade of different methodologies to establish the diagnosis in patients with non-classical biological phenotypes and provide better knowledge on the regulation of lipid metabolism.

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.

Homozygous deletion of TRMT10A as part of a contiguous gene deletion in a syndrome of failure to thrive, delayed puberty, intellectual disability and diabetes mellitus

Two recent reports describe a new syndrome of intellectual disability, short stature, microcephaly, and young onset diabetes or disturbed glucose metabolism in association with inactivating mutations in the TRMT10A gene. We investigated the clinical spectrum presented by a 17-year-old female with a homozygous contiguous gene deletion involving the TRMT10A gene. From infancy, she presented with failure to thrive and microcephaly. Puberty was characterized by a slow and an inconsistent course of progression. Concomitantly, gonadotropin levels fluctuated between low and high levels which were compatible with gonadal failure. Unlike the previous reports, the patient had ketoacidosis at onset of diabetes and islet cell autoantibodies. Nevertheless, glycemic control was excellent (HbA1C 5.0%-6.2%). RT-PCR and Western blot analysis demonstrated a complete abolishment of TRMT10A mRNA and its translated protein. In order to elucidate the nature of diabetes in this patient, endogenous insulin secretion and glycemic control were evaluated by a glucagon stimulation test and continuous glucose monitoring both during insulin treatment and off therapy. Endogenous insulin secretion still persisted 22 months after onset of diabetes and relatively normal glucose levels were kept over 3 days without insulin treatment. The fluctuating course of puberty and diabetes may reflect intermittent apoptotic damages due to sensitization of the relevant cells to various stress agents in the absence of functional TRMT10A.

Insights from human congenital disorders of intestinal lipid metabolism

The intestine must challenge the profuse daily flux of dietary fat that serves as a vital source of energy and as an essential component of cell membranes. The fat absorption process takes place in a series of orderly and interrelated steps, including the uptake and translocation of lipolytic products from the brush border membrane to the endoplasmic reticulum, lipid esterification, Apo synthesis, and ultimately the packaging of lipid and Apo components into chylomicrons (CMs). Deciphering inherited disorders of intracellular CM elaboration afforded new insight into the key functions of crucial intracellular proteins, such as Apo B, microsomal TG transfer protein, and Sar1b GTPase, the defects of which lead to hypobetalipoproteinemia, abetalipoproteinemia, and CM retention disease, respectively. These "experiments of nature" are characterized by fat malabsorption, steatorrhea, failure to thrive, low plasma levels of TGs and cholesterol, and deficiency of liposoluble vitamins and essential FAs. After summarizing and discussing the functions and regulation of these proteins for reader's comprehension, the current review focuses on their specific roles in malabsorptions and dyslipidemia-related intestinal fat hyperabsorption while dissecting the spectrum of clinical manifestations and managements. The influence of newly discovered proteins (proprotein convertase subtilisin/kexin type 9 and angiopoietin-like 3 protein) on fat absorption has also been provided. Finally, it is stressed how the overexpression or polymorphism status of the critical intracellular proteins promotes dyslipidemia and cardiometabolic disorders.

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.

Clinical features and molecular genetics of two Tunisian families with abetalipoproteinemia

Abetalipoproteinemia (ABL) is a rare monogenic disease characterized by very low plasma levels of cholesterol and triglyceride and almost complete absence of apolipoprotein B (apoB)-containing lipoproteins. Typically, patients present with failure to thrive, acanthocytosis, pigmented retinopathy and neurological features. It has been shown that ABL results from mutations in the gene encoding the microsomal triglyceride transfer protein (MTTP). Sanger sequencing of MTTP was performed for two unrelated consanguineous Tunisian families with two affected individuals each, presenting a more severe ABL phenotype than previously reported in the literature. The patients were found to be homozygous for two novel mutations. In the first family, a nonsense mutation, c.2313T>A, leading to a truncated protein (p.Y771X) was identified. In the second family, a splice mutation, IVS 9+2T>G, was found. These mutations are believed to abolish the assembly and secretion of apoB-containing lipoproteins.

Molecular characterization of Tunisian families with abetalipoproteinemia and identification of a novel mutation in MTTP gene

BACKGROUND

Abetalipoproteinemia (ABL; OMIM 200100) is a rare monogenic disorder of lipid metabolism characterized by reduced plasma levels of total cholesterol (TC), low density lipoprotein-cholesterol (LDL-C) and almost complete absence of apolipoprotein B (apoB). ABL results from genetic deficiency in microsomal triglyceride transfer protein (MTP; OMIM 157147). In the present study we investigated two unrelated Tunisian patients, born from consanguineous marriages, with severe deficiency of plasma low-density lipoprotein (LDL) and apo B.

METHODS

Intestinal biopsies were performed and The MTTP gene was amplified by Polymerase chain reaction then directly sequenced in patients presenting chronic diarrhea and retarded growth.

RESULTS

First proband was homozygous for a novel nucleotide deletion (c. 2611delC) involving the exon 18 of MTTP gene predicted to cause a non functional protein of 898 amino acids (p.H871I fsX29). Second proband was homozygous for a nonsense mutation in exon 8 (c.923 G > A) predicted to cause a truncated protein of 307 amino acids (p.W308X), previously reported in ABL patients.

CONCLUSIONS

We discovered a novel mutation in MTTP gene and we confirmed the diagnosis of abetalipoproteinemia in new Tunisian families.

VIRTUAL SLIDES

The virtual slide(s) for this article can be found here: http://www.diagnosticpathology.diagnomx.eu/vs/8134027928652779.

Congenital diarrheal disorders: an updated diagnostic approach

Congenital diarrheal disorders (CDDs) are a group of inherited enteropathies with a typical onset early in the life. Infants with these disorders have frequently chronic diarrhea of sufficient severity to require parenteral nutrition. For most CDDs the disease-gene is known and molecular analysis may contribute to an unequivocal diagnosis. We review CDDs on the basis of the genetic defect, focusing on the significant contribution of molecular analysis in the complex, multistep diagnostic work-up.

Abetalipoproteinemia: two case reports and literature review

Abetalipoproteinemia (ABL, OMIM 200100) is a rare, autosomal recessive disorder, characterized by fat malabsorption, acanthocytosis and hypocholesterolemia in infancy. Later in life, deficiency of fat-soluble vitamins is associated with development of atypical retinitis pigmentosa, coagulopathy, posterior column neuropathy and myopathy. ABL results from mutations in the gene encoding the large subunit of microsomal triglyceride transfer protein (MTP; OMIM 157147). To date at least 33 MTP mutations have been identified in 43 ABL patients. We describe the clinical progress of two patients, both currently in the fifth decade of life, who were diagnosed with ABL as children and were treated with high oral doses of fat soluble vitamins, including vitamin E over the last three decades. Treatment appears to have been associated with arrest of the neuropathy and other complications in both patients. Because pharmacologic inhibition of MTP is being developed as a novel approach to reduce plasma cholesterol for prevention of cardiovascular disease, defining the long-term clinical features of patients with a natural deficiency in MTP might provide some insight into the possible effects of such treatments. We review the range of clinical, biochemical and molecular perturbations in ABL.