Mehmet Gündüz

Reversible pulmonary arterial hypertension in cobalamin-dependent cobalamin C disease due to a novel mutation in the MMACHC gene

Methylmalonic aciduria and homocystinuria, cobalamin C (CblC) disease (OMIM 277400), is the most frequent inborn error of vitamin B12 (cobalamin, Cbl) metabolism and is caused by an inability of the cell to convert Cbl to its active forms (MeCbl and AdoCbl). More than 75 mutations have been identified in the MMACHC gene which is responsible for CblC disease. We present a case with CblC disease and pulmonary arterial hypertension (PAH) as the main symptom. The patient improved dramatically with parenteral hydroxocobalamin treatment. Most cases of CblC disease have a multisystemic disease with failure to thrive, developmental delay, hypotonia, visual impairment, and hematologic manifestations. This patient had isolated pulmonary hypertension and hyperhomocysteinemia which is thought to be an important factor in the pathogenesis of PAH. Genetic analysis identified a novel homozygous mutation (c.484G > T; p.Gly162Trp) in the MMACHC gene. Conclusion: CblC disease should be considered in the differential diagnosis of pulmonary hypertension.

Severe renal tubulopathy in a newborn due to BCS1L gene mutation: effects of different treatment modalities on the clinical course.

Very early onset Toni-Debré-Fanconi Syndrome, a disorder of proximal renal tubules of the kidney which results in the increased urinary excretion of glucose, amino acids, uric acid, phosphate and bicarbonate, could be the manifestation of various inborn errors. Defects of oxidative phosphorylation are a heterogeneous group of disorders with various clinical presentations. Recently, patients with early liver failure, renal tubulopathy and encephalopathy due to the mutations in the BCS1L gene coding for a structural protein in mitochondrial complex III have been described. Ten-day-old female newborn was referred to our clinic because of intractable acidosis. Physical examination revealed severe hypotonia, and hepatomegaly. The laboratory examinations revealed lactic acidosis, increased blood alanine, alanine aminotransferase and aspartate aminotransferase levels, generalized aminoaciduria and glucosuria. The tubular reabsorption of phosphate was reduced. Because of multisystem involvement, mitochondrial disease was suspected and the mutational analysis of the BCS1L gene revealed homozygous P99L mutation. As the patient was unresponsive to bicarbonate replacement, oral dichloroacetate and peritoneal dialysis, continuous high dose intravenous sodium bicarbonate therapy with a dose up to 1.25 mEq/kg/h was started. The patient got on well until the age of 9 months when she died of sepsis. It was stressed that high dose intravenous continuous sodium bicarbonate therapy could be an alternative treatment option in patients with severe acidosis and renal tubulopathy resistant to dichloroacetate and peritoneal dialysis. Patients with BCS1L mutations should be considered in the differential diagnosis of severe tubulopathy in the newborn period.

A Novel Abetalipoproteinemia Missense Mutation Highlights the Importance of N-Terminal β-Barrel in Microsomal Triglyceride Transfer Protein Function

Background—The use of microsomal triglyceride transfer protein (MTP) inhibitors is limited to severe hyperlipidemias because of associated hepatosteatosis and gastrointestinal adverse effects. Comprehensive knowledge about the structure–function of MTP might help design new molecules that avoid steatosis. Characterization of mutations in MTP causing abetalipoproteinemia has revealed that the central &agr;-helical and C-terminal &bgr;-sheet domains are important for protein disulfide isomerase binding and lipid transfer activity. Our aim was to identify and characterize mutations in the N-terminal domain to understand its function. Methods and Results—We identified a novel missense mutation (D169V) in a 4-month-old Turkish male child with severe signs of abetalipoproteinemia. To study the effect of this mutation on MTP function, we created mutants via site-directed mutagenesis. Although D169V was expressed in the endoplasmic reticulum and interacted with apolipoprotein B (apoB) 17, it was unable to bind protein disulfide isomerase, transfer lipids, and support apoB secretion. Computational modeling suggested that D169 could form an internal salt bridge with K187 and K189. Mutagenesis of these lysines to leucines abolished protein disulfide isomerase heterodimerization, lipid transfer, and apoB secretion, without affecting apoB17 binding. Furthermore, mutants with preserved charges (D169E, K187R, and K189R) rescued these activities. Conclusions—D169V is detrimental because it disrupts an internal salt bridge leading to loss of protein disulfide isomerase binding and lipid transfer activities; however, it does not affect apoB binding. Thus, the N-terminal domain of MTP is also important for its lipid transfer activity.

3-HMG Coenzyme A Lyase Deficiency: Macrocephaly and Left Ventricular Noncompaction with a Novel Mutation.

Abstract3-hydroxy-3-methylglutaryl-coenzyme A (HMG-CoA) lyase deficiency, an inborn error of ketone body synthesis and leucine degradation, is a rare autosomal recessive disease. There are a few reports demonstrating clinical and neuroradiologic findings of this condition. The authors report case of an 8-mo-old infant with HMG-CoA lyase deficiency, who presented with macrocephaly, left ventricular noncompaction, recurrent pulmonary infections, nonketotic hypoglycemia, seizure and metabolic acidosis. There was no significant difference in brain magnetic resonance imaging after leucine-restricted diet and carnitine therapy and neurologic deterioration was not observed. Left ventricular noncompaction is an interesting finding for HMG-CoA lyase deficiency which has not been reported in the literature. The genetic analysis revealed a novel homozygote deletion in exon 3 and 4 in HMGCL gene. HMG-CoA lyase deficiency should be thought in the patients with hypoketotic hypoglycemia, hyperammonemia, elevated liver function tests, noncompaction left ventricle and characteristic white matter changes and in the differential diagnosis of macrocephaly.

Novel Abetalipoproteinemia Missense Mutation Highlights the Importance of the N-Terminal β-Barrel in Microsomal Triglyceride Transfer Protein FunctionCLINICAL PERSPECTIVE

Background—The use of microsomal triglyceride transfer protein (MTP) inhibitors is limited to severe hyperlipidemias because of associated hepatosteatosis and gastrointestinal adverse effects. Comprehensive knowledge about the structure–function of MTP might help design new molecules that avoid steatosis. Characterization of mutations in MTP causing abetalipoproteinemia has revealed that the central &agr;-helical and C-terminal &bgr;-sheet domains are important for protein disulfide isomerase binding and lipid transfer activity. Our aim was to identify and characterize mutations in the N-terminal domain to understand its function. Methods and Results—We identified a novel missense mutation (D169V) in a 4-month-old Turkish male child with severe signs of abetalipoproteinemia. To study the effect of this mutation on MTP function, we created mutants via site-directed mutagenesis. Although D169V was expressed in the endoplasmic reticulum and interacted with apolipoprotein B (apoB) 17, it was unable to bind protein disulfide isomerase, transfer lipids, and support apoB secretion. Computational modeling suggested that D169 could form an internal salt bridge with K187 and K189. Mutagenesis of these lysines to leucines abolished protein disulfide isomerase heterodimerization, lipid transfer, and apoB secretion, without affecting apoB17 binding. Furthermore, mutants with preserved charges (D169E, K187R, and K189R) rescued these activities. Conclusions—D169V is detrimental because it disrupts an internal salt bridge leading to loss of protein disulfide isomerase binding and lipid transfer activities; however, it does not affect apoB binding. Thus, the N-terminal domain of MTP is also important for its lipid transfer activity.