Grant A. Mitchell’s research while affiliated with CHU Sainte-Justine and other places

A 2.5-month-old girl admitted for failure to thrive and severe pancytopenia was diagnosed with methylmalonic acidemia (MMA) secondary to transcobalamin II deficiency, an inborn error of vitamin B12 metabolism. Opportunistic Cytomegalovirus and Pneumocystis jirovecii pneumonia led to severe acute respiratory distress syndrome (ARDS) and immune reconstitution inflammatory syndrome (IRIS) after treatment initiation with vitamin B12 supplementation. In children with interstitial pneumonia-related ARDS, normal lymphocyte count should not delay invasive procedures required to document opportunistic infections. MMA can be associated with underlying lymphocyte dysfunction and vitamin B12 supplementation can fully reverse the associated immunodeficiency. IRIS may appear in highly treatment-responsive forms of pancytopenia in children and prompt treatment of dysregulated inflammation with high-dose corticosteroids should be initiated.

The leucine catabolism pathway intermediate, trans-3-methylglutaconyl (3MGC) CoA, is considered to be the precursor of 3MGC acid, a urinary organic acid associated with specific inborn errors of metabolism (IEM). trans-3MGC CoA is an unstable molecule that can undergo a sequence of non-enzymatic chemical reactions that lead to either 3MGC acid or protein 3MGCylation. Herein, the susceptibility of trans-3MGC CoA to protein 3MGCylation was investigated. trans-3MGC CoA was generated through the activity of recombinant 3-methylcrotonyl CoA carboxylase (3MCCCase). Following enzyme incubations, reaction mixtures were spin-filtered to remove 3MCCCase. The recovered filtrates, containing trans-3MGC CoA, were then incubated in the presence of bovine serum albumin (BSA). Following this, sample aliquots were subjected to α-3MGC IgG immunoblot analysis to probe for 3MGCylated BSA. Experiments revealed a positive correlation between trans-3MGC CoA incubation temperature and 3MGCylated BSA immunoblot signal intensity. A similar correlation was observed between incubation time and 3MGCylated BSA immunoblot signal intensity. When trans-3MGC CoA hydratase (AUH) was included in incubations containing trans-3MGC CoA and BSA, 3MGCylated BSA immunoblot signal intensity decreased. Evidence that protein 3MGCylation occurs in vivo was obtained in studies with liver-specific 3-hydroxy-3-methylglutaryl (HMG) CoA lyase knockout mice. Therefore, trans-3MGC CoA is a reactive, potentially toxic metabolite, and under normal physiological conditions, lowering trans-3MGC CoA levels via AUH-mediated hydration to HMG CoA protects against aberrant non-enzymatic chemical reactions that lead to protein 3MGCylation and 3MGC acid production.

An 11-month-old girl with severe acidosis, lethargy and vomiting, was diagnosed with holocarboxylase synthetase deficiency. She received biotin and was stable until age 8 years when vomiting, severe acidosis, hypoglycemia, and hyperammonemia developed. Management with intravenous glucose aiming to stimulate anabolism led to hyperglycemic ketoacidosis. Insulin therapy rapidly corrected biochemical parameters, and clinical status improved. We propose that secondary Krebs cycle disturbances affecting pancreatic beta cells impaired glucose-stimulated insulin secretion, resulting in insulinopenia.

Mutations in MMACHC cause cobalamin C disease (cblC, OMIM 277400), the commonest inborn error of vitamin B12 metabolism. In cblC, deficient activation of cobalamin results in methylcobalamin and adenosylcobalamin deficiency, elevating methylmalonic acid (MMA) and total plasma homocysteine (tHcy). We retrospectively reviewed the medical files of seven cblC patients: three compound heterozygotes for the MMACHC (NM_015506.3) missense variant c.158 T > C p.(Leu53Pro) in trans with the common pathogenic mutation c.271dupA (p.(Arg91Lysfs*14), “compounds”), and four c.271dupA homozygotes (“homozygotes”). Compounds receiving hydroxocobalamin intramuscular injection monotherapy had age-appropriate psychomotor performance and normal ophthalmological examinations. In contrast, c.271dupA homozygotes showed marked psychomotor retardation, retinopathy and feeding problems despite penta-therapy (hydroxocobalamin, betaine, folinic acid, l-carnitine and acetylsalicylic acid). Pretreatment levels of plasma and urine MMA and tHcy were higher in c.271dupA homozygotes than in compounds. Under treatment, levels of the compounds approached or entered the reference range but not those of c.271dupA homozygotes (tHcy: compounds 9.8–32.9 μM, homozygotes 41.6–106.8 (normal (N) < 14); plasma MMA: compounds 0.14–0.81 μM, homozygotes, 10.4–61 (N < 0.4); urine MMA: compounds 1.75–48 mmol/mol creatinine, homozygotes 143–493 (N < 10)). Patient skin fibroblasts all had low cobalamin uptake, but this was milder in compound cells. Also, the distribution pattern of cobalamin species was qualitatively different between cells from compounds and from homozygotes. Compared to the classic cblC phenotype presented by c.271dupA homozygous patients, c.[158 T > C];[271dupA] compounds had mild clinical and biochemical phenotypes and responded strikingly to hydroxocobalamin monotherapy.

Background: Defects in the mitochondrial respiratory chain (MRC) can lead to combined MRC dysfunctions (COXPDs) with heterogenous genotypes and clinical features. We report a patient carrying heterozygous variants in the TUFM gene who presented with clinical features compatible with COXPD4 and radiological findings mimicking multiple sclerosis (MS). Methods: A 37-year-old French Canadian woman was investigated for recent onset of gait and balance problems. Her previous medical history included recurrent episodes of hyperventilation associated with lactic acidosis during infections, asymptomatic Wolff-Parkinson-White syndrome, and non-progressive sensorineural deafness. Results: Neurological examinations revealed fine bilateral nystagmus, facial weakness, hypertonia, hyperreflexia, dysdiadochokinesia and dysmetria, and ataxic gait. Brain MRI showed multifocal white matter abnormalities in cerebral white matter as well as cerebellar hemispheres, brainstem, middle cerebellar peduncles, some of which mimicking MS. Analysis of native-state OXPHOS showed a combined decrease in CI/CII, CIV/CII, and CVI/CII. Exome sequencing detected two heterozygous TUFM gene variants. Little clinical progression was noted over a 5-year follow-up. Brain MRI remained unchanged. Conclusions: Our report broadens the phenotypic and radiological spectrum of TUFM-related disorders by adding milder, later-onset forms to the previously known early-onset, severe presentations. The presence of multifocal white matter abnormalities can be misinterpreted as due to acquired demyelinating diseases and thus, TUFM-related disorders should be added to the list of mitochondrial MS mimickers.

A breastfed 3-mo girl referred for high methylmalonate (MMA) on urine screening (>500 mmol/mol creatinine; N, 0-25) had failure to thrive, thrombopenia 8x10^9/L (normal (N), 140-440), anemia (hemoglobin 46 g/L, N, 95-135), MCV 86.9 fL (N, 77-115), reticulocytes 5.3x10^9/L (N, 18-158) and leucocytes 3.98x10^9/L (N, 6-17.5). Pregnancy, delivery and maternal diet were unremarkable. Other pretreatment blood tests: vitamin B12, 657 pM (N, 191-1163); folate, N; ammonia, N; acylcarnitines (C3, 2.9 microM (N, 0.14-0.79); C4DC, N); total homocysteine (tHcy), 23.7 microM (N, 2.9-10); methionine, 25 microM (N 14-38); glycine, N; citrulline, 2 microM (N, 14-32). Urine organic acids (mmol/mol creatinine): MMA 1129 (N, 0-10); methylcitrate 15.7 (N, 1-5); 3-OH-propionate, N. Bone marrow showed large immature myeloid cells with absent megakaryocytes, initially suggesting malignancy. Hydroxocobalamin (1 mg/day im) and folate (1 mg/day po), without protein restriction (treatment day 0, D0), were followed by normalization of tHcy, <1 mM (D6), citrulline, 14 microM (D9), urine MMA, 24 (D12) but high blood elements (10^9/L): platelets (>450 (D11-17), maximum 909 (D16)); leukocytes (>30 (D4-20), maximum 53 (D17)) and reticulocytes (maximum 199 (D10)). Pulmonary cytomegalovirus and Pneumocystis jirovecii infections (D5) received cotrimoxazole, ganciclovir, prednisolone and 4-day intratracheal ventilation. Rapid genome sequencing revealed a homozygous splicing variant in TCN2, encoding transcobalamin 2. Hyper-MMA, hyper-tHcy plus normal B12 suggest B12 activation or TCN2 deficiencies. Hydroxocobalamin coincided with rapid metabolite improvement but hematopoietic overcorrection, thrombocytosis and high levels of immature leukocytes, perhaps causing cytokine imbalance, increased severity of pulmonary inflammation and IRIS.

An 8-year-old girl with holocarboxylase synthetase-deficiency (HLCSD), vomiting and stupor. She presented at age 11m with severe lactic and ketoacidoses (pH 6.95), typical HLCSD metabolite pattern and biallelic pathogenic HLCS variants, and has developed normally with biotin 60 mg/day and L-carnitine. For 12h before admission, she vomited all medications, having abdominal pain, headache, tachycardia and tachypnea but was normotensive, afebrile, with no diarrhea. Testing revealed: capillary pH 6.90 (normal (N) 7.34-7.44), pCO2 21.3 mmHg (N 37-47), bicarbonate 4 mM (N 21-27), anion gap 42.8 mEq/L (N 12-20), lactate 17 mM (N 0.5-2.2), glucose 2.22 mM (N 4.1-5.9), 3-hydroxybutyrate 2.4 mM (N 0-0.6), ammonia 119 microM (N 5-55), leucocytes 29.4x10^9/L (N 4.5-13.5), neutrophils 15.6x10^9/L (N 1.5-7.3), CRP 6.1 mg/L (N 0-1); lipase N; ALT N; abdominal ultrasound N. Nasopharyngeal non-SARS-coronavirus-2 was detected. Initial treatment: fluid replacement, D10%-NS (5.9 mg glucose/kg/min), oral biotin (70 mg), L-carnitine 100 mg/kg/d iv q3h and ondansetron 0.1 mg/kg q8h. Progressive hyperglycemia (peak 21.8 mM after 5 hours of D10%) required insulin treatment for 16h (maximum, 0.08 units/kg/h, after 3h). Metabolites improved after insulin, with normalization of anion gap (4h post-insulin), lactate (10h), 3-hydroxybutyrate (10h), ammonemia (13h), acidosis (28h) and bicarbonate (28h). Ketoacidosis in pyruvate carboxylase (PC) deficiency responds to suppression of PC-mediated gluconeogenesis by glucose. Brief glucose intolerance inhibited cell entry/use of glucose despite hyperglycemia, prolonging demand for PC-mediated gluconeogenesis and its accompanying ketoacidosis, but resolved rapidly with exogenous insulin.

Patient: Male, 10-month-old Final Diagnosis: Tyrosinemia type 2 Symptoms: Decreased appetite • epiphora • irritability • photophobia Medication: — Clinical Procedure: — Specialty: Pediatrics and Neonatology Objective Challenging differential diagnosis Background Tyrosinemia Type II (TYRII) is a rare autosomal recessive inborn error of metabolism caused by deficiency of tyrosine aminotransferase (TAT), leading to hypertyrosinemia. TYRII patients often present in the first year of life with ocular and cutaneous findings, including corneal ulcers, pseudodendritic keratitis, and palmoplantar hyperkeratosis. The corneal involvement is often mistaken for herpes simplex virus (HSV) keratitis, which is a much commoner condition. Case Report A previously healthy 10-month-old male infant was referred to Ophthalmology for acute onset photophobia. Bilateral dendritiform corneal lesions raised the suspicion for herpetic keratitis. Additionally, a papular, crusted lesion was found on his thumb after a few days of hospitalization, also raising concerns about HSV. The patient’s clinical condition seemed to improve under intravenous acyclovir and supportive treatment. A conjunctival swab and crusted lesion on the thumb were tested for HSV using a polymerase chain reaction (PCR) technique, and both were negative. Nevertheless, given the clinical presentation and the favorable course of signs and symptoms, hospital discharge was planned with oral acyclovir. It was halted by an alternative diagnosis of autosomal recessive inborn error of metabolism, tyrosinemia type II, confirmed by elevated plasma tyrosine level and later by molecular analysis requested as a confirmatory investigation by the genetics medical team. Conclusions The corneal involvement in TYRII is often mistaken for HSV keratitis, and clinical course alone should not halt further investigations to rule out TYRII. Clinicians should suspect TYRII clinically when its characteristic ocular dendritiform lesions are present, namely in infancy or early childhood, and even in the absence of its typical cutaneous palmoplantar hyperkeratosis plaques.