Claudio Rabacchi

Spectrum of mutations and phenotypic expression in patients with autosomal dominant hypercholesterolemia identified in Italy

OBJECTIVE To determine the spectrum of gene mutations and the genotype-phenotype correlations in patients with Autosomal Dominant Hypercholesterolemia (ADH) identified in Italy. METHODS The resequencing of LDLR, PCSK9 genes and a selected region of APOB gene were conducted in 1018 index subjects clinically heterozygous ADH and in 52 patients clinically homozygous ADH. The analysis was also extended to 1008 family members of mutation positive subjects. RESULTS Mutations were detected in 832 individuals: 97.4% with LDLR mutations, 2.2% with APOB mutations and 0.36% with PCSK9 mutations. Among the patients with homozygous ADH, 51 were carriers of LDLR mutations and one was an LDLR/PCSK9 double heterozygote. We identified 237 LDLR mutations (45 not previously reported), 4 APOB and 3 PCSK9 mutations. The phenotypic characterization of 1769 LDLR mutation carriers (ADH-1) revealed that in both sexes independent predictors of the presence of tendon xanthomas were age, the quintiles of LDL cholesterol, the presence of coronary heart disease (CHD) and of receptor negative mutations. Independent predictors of CHD were male gender, age, the presence of arterial hypertension, smoking, tendon xanthomas, the scalar increase of LDL cholesterol and the scalar decrease of HDL cholesterol. We identified 13 LDLR mutation clusters, which allowed us to compare the phenotypic impact of different mutations. The LDL cholesterol raising potential of these mutations was found to vary over a wide range. CONCLUSIONS This study confirms the genetic and allelic heterogeneity of ADH and underscores that the variability in phenotypic expression of ADH-1 is greatly affected by the type of LDLR mutation.

Lysosomal lipase deficiency: Molecular characterization of eleven patients with Wolman or cholesteryl ester storage disease

Wolman Disease (WD) and cholesteryl ester storage disease (CESD) represent two distinct phenotypes of the same recessive disorder caused by the complete or partial deficiency of lysosomal acidic lipase (LAL), respectively. LAL, encoded by the LIPA gene, hydrolyzes cholesteryl esters derived from cell internalization of plasma lipoproteins. WD is a rapidly progressive and lethal disease characterized by intestinal malabsorption, hepatic and adrenal failure. CESD is characterized by hepatic fibrosis, hyperlipidemia and accelerated atherosclerosis. Aim of the study was the identification of LIPA mutations in three WD and eight CESD patients. The WD patients, all deceased before the first year of age, were homozygous for two novel mutations (c.299+1G>A and c.419G>A) or a mutation (c.796G>T) previously reported as compound heterozygosity in a CESD patient. The two mutations (c.419G>A and c.796G>T) resulting in truncated proteins (p.W140* and p.G266*) and the splicing mutation (c.229+1G>A) were associated with undetectable levels of LIPA mRNA in fibroblasts. All eight CESD patients carried the common mutation c.894G>A known to result not only in a major non-functional transcript with the skipping of exon 8 (p.S275_Q298del), but also in a minor normally spliced transcript producing 5-10% residual LAL activity. The c.894G>A mutation was found in homozygosity in four patients and, as compound heterozygosity, in association with a known (p.H295Y and p.G342R) or a novel (p.W140*) mutation in four other CESD patients. Segregation analysis performed in all patients harboring c.895G>A showed its occurrence on the same haplotype suggesting a common founder ancestor. The other WD and CESD mutations were associated with different haplotypes.

Leucine 10 allelic variant in signal peptide of PCSK9 increases the LDL cholesterol-lowering effect of statins in patients with familial hypercholesterolaemia.

BACKGROUND AND AIMS In the normal population, carriers of an additional leucine residue in a stretch of nine leucines in the signal peptide of PCSK9 (L10) have lower total (TC) and low-density lipoprotein cholesterol (LDL-C) than homozygotes for the wild-type allele (L9/L9). A similar effect was detected in familial hypercholesterolaemia (FH) patients with the p.C681X mutation of LDL-receptor (LDLR). We investigated the effect of L10 variant on basal lipid profile and response to statins in molecularly characterised FH patients. METHODS AND RESULTS Plasma lipids were determined in 322 FH patients screened for the L9/L10/L11 polymorphism and in a subgroup of 54 patients carrying the same LDLR mutation (p.Q474HfsX63). Plasma lipids were also determined in 42 FH patients carrying the L10 variant and in a parallel group of 42 FH patients, L9/L9 homozygotes, matched for gender, age, type of LDLR gene mutation, as well as for type, dose and duration of statin treatment. In FH patients, no difference in the basal plasma TC and LDL-C levels was observed between carriers of L10 variant (L9/L10+L10/L10) and L9/L9 homozygotes. The same was true in FH patients carrying the p.Q474HfsX63 LDLR mutation. In the subgroups of statin-treated patients, the reduction of TC and LDL-C was greater in carriers of L10 (-34.0% and -42.5%, respectively) than in L9/L9 homozygotes (-27.5% and -34.3%, respectively) (P<0.001). CONCLUSION The variant L10 of the leucine repeats in PCSK9 signal peptide is to be considered as a factor capable of modulating the lipid-lowering effects of statins in FH.

Novel mutations of ABCA1 transporter in patients with Tangier disease and familial HDL deficiency

The objective of the study was the characterization of ABCA1 gene mutations in 10 patients with extremely low HDL-cholesterol. Five patients (aged 6 months to 76 years) presented with splenomegaly and thrombocytopenia suggesting the diagnosis of Tangier disease (TD). Three of them were homozygous for novel mutations either in intron (c.4465-34A>G) or in exons (c.4376delT and c.5449C>T), predicted to encode truncated proteins. One patient was compound heterozygous for a nucleotide insertion (c.1758_1759insG), resulting in a truncated protein and for a nucleotide substitution c.4799A>G, resulting in a missense mutation (p.H1600R). The last TD patient, found to be heterozygous for a known mutation (p.D1009Y), had a complete defect in ABCA1-mediated cholesterol efflux in fibroblasts, suggesting the presence of a second undetected mutant allele. Among the other patients, four were asymptomatic, but one, with multiple risk factors, had severe peripheral artery disease. Three of these patients were heterozygous for known mutations (p.R130K+p.N1800H, p.R1068C, p.N1800H), while two were carriers of novel mutations (c.1195-27G>A and c.396_397insA), predicted to encode truncated proteins. The pathogenic effect of the two intronic mutations (c. 1195-27G>A and c.4465-34A>G) was demonstrated by the analysis of the transcripts of splicing reporter mutant minigenes expressed in COS-1 cells. Both mutations activated an intronic acceptor splice site which resulted in a partial intron retention in mature mRNA with the production of truncated proteins. This study confirms the allelic heterogeneity of TD and suggests that the diagnosis of TD must be considered in patients with an unexplained splenomegaly, associated with thrombocytopenia and hypocholesterolemia.

Severe Hypertriglyceridemia in a Newborn with Monogenic Lipoprotein Lipase Deficiency: An Unconventional Therapeutic Approach with Exchange Transfusion

Severe hypertriglyceridemia (sHTG) (plasma triglyceride level > 10 mmol/L) due to lipoprotein lipase (LPL) deficiency is a known risk factor for acute pancreatitis. A 23-day-old male with sHTG was admitted to the Neonatal Intensive Care Unit for plasmapheresis being at high risk for acute pancreatitis. Given the potential hazard of an extracorporeal technique in a very young infant, we decided to perform an exchange transfusion (ET), a procedure widely used by neonatologists and less invasive than plasmapheresis. ET led to a dramatic reduction in plasma triglyceride level, from 93.2 to 3.8 mmol/L at the end of the procedure, without adverse events. The subsequent administration of a special formula low in fat and high in medium-chain triglycerides was effective in keeping fasting plasma triglyceride level below 5.6 mmol/L during the first 5 months of life. The sequence of LPL gene revealed that the patient was apparently homozygous for a novel nucleotide deletion (c.840delG) in exon 6 leading to a premature termination codon (p.N281Mfs*23). However, family studies revealed that while the patients mother was heterozygous for this mutation, the father was heterozygous for a novel deletion eliminating the whole LPL gene. The patient therefore turned out to be a compound heterozygous for two LPL gene mutations predicted to abolish LPL activity. This is the first case of sHTG treated with ET in a neonate reported in the literature. ET appears to be a safe procedure, alternative to plasmapheresis, to prevent acute pancreatitis in young infants with sHTG due to LPL deficiency.

An apparent inconsistency in parent to offspring transmission of point mutations of LDLR gene in Familial Hypercholesterolemia

BACKGROUND Familial Hypercholesterolemia (FH), the most common form of autosomal co-dominant hypercholesterolemia, is due to mutations in the LDLR gene, mostly minute or point mutations in the coding sequence. METHODS Analysis of LDLR gene was performed by direct resequencing and multiplex ligation-dependent probe amplification (MLPA). RESULTS LDLR gene resequencing showed that proband I.G., with the clinical diagnosis of homozygous FH, was homozygous for a mutation in exon 12 (c.1775 G>A, G571E) known to be pathogenic, and heterozygous for a mutation in intron 14 (c.2140 +5G>A). Probands daughter with heterozygous FH carried only the intron 14 mutation. To explain this inconsistency we assumed that the proband was a carrier of a gene deletion. MLPA showed that the proband and her daughter were heterozygous for a deletion of exons 11 and 12. This explains the apparent homozygosity of the c.1175 G>A mutation in the proband. Ex 11-12 deletion was linked to the c.2140 +5G>A mutation. Other FH patients, heterozygotes for c.2140 +5G>A, were found to carry the Ex 11-12 deletion found in the proband or other pathogenic mutations. CONCLUSIONS Inconsistencies in the parent to offspring transmission of point mutations in LDLR gene may be due to a large deletion not detected by resequencing.

Effects of miglustat treatment in a patient affected by an atypical form of Tangier disease.

BackgroundTangier disease (TD) is a rare autosomal recessive disorder, resulting from mutations in the ATP binding cassette transporter (ABCA1) gene. The deficiency of ABCA1 protein impairs high density lipoprotein (HDL) synthesis and cholesterol esters trafficking.Case ReportA 58 year-old female, presenting with complex clinical signs (splenomegaly, dysarthria, dysphagia, ataxia, tongue enlargement, prurigo nodularis, legs lymphedema, pancytopenia and bone marrow foam cells), was misdiagnosed as Niemann-Pick C (NPC) and treated with miglustat (300 mg/day), normalizing neurological symptoms and improving skin lesions and legs lymphedema. Subsequently filipin-staining and molecular analysis for NPC genes were negative. Lipid profiling showed severe deficiency of HDL, 2 mg/dl (n.v. 45-65) and apoAI, 5.19 mg/dl (n.v. 110-170), suggesting TD as a probable diagnosis. Molecular analysis of ABCA1 gene showed the presence of a novel homozygous deletion (c.4464-486_4698 + 382 Del). Miglustat treatment was then interrupted with worsening of some neurological signs (memory defects, slowing of thought processes) and skin lesions. Treatment was restarted after 7 months with neurological normalization and improvement of skin involvement.ConclusionsThese results suggest miglustat as a possible therapeutic approach in this untreatable disease. The mechanisms by which miglustat ameliorates at least some clinical manifestations of TD needs to be further investigated.

Functional analysis of two novel splice site mutations of APOB gene in familial hypobetalipoproteinemia.

Familial hypobetalipoproteinemia (FHBL) is a co-dominant disorder characterized by reduced plasma levels of low density lipoprotein cholesterol (LDL-C) and its protein constituent apolipoprotein B (apoB), which may be due to mutations in APOB gene, mostly located in the coding region of this gene. We report two novel APOB gene mutations involving the acceptor splice site of intron 11 (c.1471-1G>A) and of intron 23 (c.3697-1G>C), respectively, which were identified in two patients with heterozygous FHBL associated with severe fatty liver disease. The effects of these mutations on APOB pre-mRNA splicing were assessed in COS-1 cells expressing the mutant APOB minigenes. The c.1471-1G>A APOB minigene generated two abnormal mRNAs. In one mRNA the entire intron 11 was retained; in the other mRNA exon 11 joined to exon 12, in which the first nucleotide was deleted due to the activation of a novel acceptor splice site. The predicted products of these mRNAs are truncated proteins of 546 and 474 amino acids, designated apoB-12.03 and apoB-10.45, respectively. The c.3697-1G>C APOB minigene generated a single abnormal mRNA in which exon 23 joined to exon 25, with the complete skipping of exon 24. This abnormal mRNA is predicted to encode a truncated protein of 1220 amino acids, designated apoB-26.89. These splice site mutations cause the formation of short truncated apoBs, which are not secreted into the plasma as lipoprotein constituents. This secretion defect is the major cause of severe fatty liver observed in carriers of these mutations.

A novel deletion of BRCA1 gene that eliminates the ATG initiation codon without affecting the promoter region

BACKGROUND Point mutations in the highly penetrant cancer susceptibility gene BRCA1 are responsible for the majority of hereditary breast and/or ovarian cancer. We describe a novel large rearrangement of the BRCA1 gene identified in an Italian woman affected by an early onset bilateral breast cancer and a family history of hereditary breast cancer. The proband and her parents were negative for the presence of point mutations in BRCA1 and BRCA2 genes. METHODS Multiplex ligation-dependent probe amplification (MLPA) was used to detect rearrangements in the BRCA1 gene. The breakpoint of the rearrangement identified in the proband was defined by restriction mapping and PCR amplification. BRCA1 mRNA encoded by the mutant allele was isolated from peripheral blood. RESULTS The proband was heterozygous for a 9.1 kb deletion spanning from intron 1 to intron 3 (g.1238_10350del) that eliminates exons 2 and 3 in the mature mRNA. In mutant mRNA exon 1a joins directly to exon 5 with no disruption of the reading frame. CONCLUSIONS This deletion that eliminates the ATG initiation site in exon 2 and the sequence located in exons 2 and 3 encoding part of the RING finger domain of BRCA1 protein, is expected to abolish the function of this protein.

Incidental finding of severe hypertriglyceridemia in children. Role of multiple rare variants in genes affecting plasma triglyceride

BACKGROUND The incidental finding of severe hypertriglyceridemia (HyperTG) in a child may suggest the diagnosis of familial chylomicronemia syndrome (FCS), a recessive disorder of the intravascular hydrolysis of triglyceride (TG)-rich lipoproteins. FCS may be due to pathogenic variants in lipoprotein lipase (LPL), as well as in other proteins, such as apolipoprotein C-II and apolipoprotein A-V (activators of LPL), GPIHBP1 (the molecular platform required for LPL activity on endothelial surface) and LMF1 (a factor required for intracellular formation of active LPL). OBJECTIVE Molecular characterization of 5 subjects in whom HyperTG was an incidental finding during infancy/childhood. METHODS We performed the parallel sequencing of 20 plasma TG-related genes. RESULTS Three children with severe HyperTG were found to be compound heterozygous for rare pathogenic LPL variants (2 nonsense, 3 missense, and 1 splicing variant). Another child was found to be homozygous for a nonsense variant of APOA5, which was also found in homozygous state in his father with longstanding HyperTG. The fifth patient with a less severe HyperTG was found to be heterozygous for a frameshift variant in LIPC resulting in a truncated Hepatic Lipase. In addition, 1 of the patients with LPL deficiency and the patient with APOA-V deficiency were also heterozygous carriers of a pathogenic variant in LIPC and LPL gene, respectively, whereas the patient with LIPC variant was also a carrier of a rare APOB missense variant. CONCLUSIONS Targeted parallel sequencing of TG-related genes is recommended to define the molecular defect in children presenting with an incidental finding of HyperTG.