Liver transplantation is not curative for methylmalonic acidopathy caused by methylmalonyl-CoA mutase deficiency

Abstract

Methylmalonic acidopathy resulting from severe methylmalonyl-CoA mutase deficiency causes acute, potentially lethal ketoacidotic episodes, renal failure, and acute and chronic neurologic disease. As dietary and alkali therapy is suboptimal, liver transplantation during infancy has been touted as a potential cure. However, reports in liver transplant recipients about new onset neurologic disease, in the absence of ketoacidosis, and progressive renal insufficiency have cast doubt about its effectiveness. We report the long-term (9 years) outcome for the first patient with severe methylmalonic acidopathy transplanted in the USA and provide new biochemical data that indicate why transplanted patients are still susceptible to “metabolic strokes”. In our 10-year-old male patient, there is clear evidence that the de novo synthesis of propionyl-CoA within the CNS leads to brain methylmalonate (MMA) accumulation that is largely unaffected by transplantation. Liver replacement is not a cure for methylmalonic acidopathy.

Introduction

Traditional treatment for one of the most common organic acidopathies, methylmalonic acidopathy, results in variable but suboptimal outcomes [1], [2], [3], [4], [5], [6], [7], [8]. An ideal therapy would be enzyme replacement, achieved by intravenous infusion, or organ and cell transplantation, or gene therapy. Liver transplantation was performed in a 19-month-old boy with “classical” methylmalonic acidopathy caused by severe methylmalonyl-CoA mutase deficiency [1]. He underwent transplantation during infancy at his parents’ request, but only after protracted, intensive counseling about the uncertain outcome for resolution of the metabolic disease. We report the long-term outcome showing that liver transplantation has not cured the disease, and demonstrate, for the first time, that transplantation does not eliminate the de novo production of propionyl-CoA, and therefore methylmalonate (MMA), within the central nervous system and, thus, the potential for new onset neurologic damage.

A 10-year-old boy, with consanguineous Turkish parents, was diagnosed with methylmalonic acidopathy at 2 weeks of age. He became symptomatic (poor feeding, vomiting and lethargy) on the 2nd day of life and was hypothermic and dehydrated by the 4th day. There was severe metabolic acidosis, ketosis and mild hyperammonemia. The concentration of MMA in urine was 28,261 μmol/mmol creatinine (normal <10 μmol/mmol creatinine) and in plasma 1666 μmol/L (normal 0.07–0.27 μmol/L). The total 2-methylcitrate concentration in plasma was 24.3 μmol/L (normal 0.06–0.23 μmol/L). Renal function was normal. Magnetic resonance imaging (MRI) of brain, renal ultrasonography and echocardiography was normal. The fibroblasts showed no response to hydroxocobalamin (OHCbl) in the in vitro propionate incorporation assay. Complementation analysis indicated that the mut group was consistent with no functional methylmalonyl-CoA mutase activity [1], [2]. Genotyping showed that the patient is homozygous for the R369H mutase gene mutation [9].

After the metabolic acidosis had been corrected, a protein-restricted diet (1.7 g/kg/day) was started using proprietary formulas (one with complete protein and another that was low in isoleucine, valine, methionine and threonine) and supplemented with protein-free formula. A trial of hydroxycobalmin did not lower the methylmalonate levels. Daily oral bicarbonate was given to maintain plasma total CO₂ above 19 mmol/L. After several months, a gastrostomy tube was inserted because of anorexia and difficulties maintaining metabolic balance. Continuous overnight feeds and daytime bolus feeds were given. The weight, length and head circumference parameters were between the 25th and 50th percentiles.

During the first year, the patient had nine periods of hospitalization for a total of 17 weeks. With the exception of one hospitalization for gastrostomy placement, these occurred because infections caused significant metabolic decompensation despite frequent metabolic monitoring. At 7 months, he developed Candida albicans infection in urine and blood. At 9 months, osteomyelitis developed in the left femur, at a site not used previously for intravenous catheters. Candida tropicalis was cultured in blood and bone from the infected area.

After treatment was started, the average MMA concentration in cerebrospinal fluid (CSF) was 1103 μmol/L (normal 0.17–0.59 μmol/L). From the neonatal period to 19 months of age, the mean ± SD MMA concentration in plasma was 574 ± 431 μmol/L (n = 9) and in urine was 9307 ± 4923 μmol/mmol creatinine (n = 37) (Fig. 1 and Table 1). The levels were higher during periods of infection. The CSF/plasma MMA ratio was 4.5 at 9 months of age. The total 2-methylcitrate concentration in CSF, on one occasion, was 18.2 μmol/L (normal 0.32–1.07 μmol/L) and mean plasma concentration was 9.6 μmol/L.

Liver transplantation was requested by the parents because of concerns about continuing infections and metabolic decompensation, with potentially irreversible neurologic sequelae [10], [11]. Pre-transplant evaluation included: (1) brain MRI, which showed an increased subarachnoid space consistent with a minor degree of volume loss. The basal ganglia appeared normal. (2) Hepatic function—normal alanine-(ALT) and aspartate-(AST) aminotransferases, γ-glutamyltranspeptidase (GGT) and coagulation studies. (3) Simultaneous MMA quantitation in blood, urine and CSF.

At 19 months, the patient underwent orthotopic liver transplantation. On the day of surgery, the plasma MMA concentration was 665 μmol/L. There was no metabolic decompensation during and after the surgery. Immunosuppression with tacrolimus (FK506), 0.3 mg/kg/day, and steroids was started post-operatively. The blood levels of tacrolimus were maintained in the therapeutic range (5–20 ng/mL).

For 1 month after the transplant, there was no evidence of metabolic, hepatic or neurologic dysfunction. Intravenous hyperalimentation, containing 2 g complete amino acids/kg/day, 2–3 g/intralipid/kg/day and dextrose, started 1 day after transplantation, was well tolerated. One day after transplantation, the plasma MMA level was 248 μmol/L and the serum total CO₂ was 22 mmol/L. Twenty-two days post-transplantation, no basal ganglia lesions were seen with MRI. Thirty days post-transplantation, the patient developed severe, coarse, generalized tremors, which were less marked at rest and absent while asleep. The CSF and plasma MMA levels were 1321 and 307 μmol/L, respectively. Blood levels of tacrolimus were still within the therapeutic range. Tacrolimus was discontinued and cyclosporine started but there was no clear improvement of the tremors. Subsequent reinstitution of tacrolimus and discontinuation of cyclosporine produced no immediate or apparent worsening of the movements. The tremors disappeared when the patient was sedated with midazolam, a benzodiazepine drug; therefore, he was treated with oral clonazepam therapy, started at a low dose and slowly increased to 0.9 mg three times daily, with gradual improvement. The tremors resolved after ∼1 year. Eleven days after the tremors started, the patient manifested herpes simplex encephalitis but, with prompt institution of standard treatment of methylmalonic acidopathy for a period of stress, including a high caloric diet with no protein, he became acidotic only once. This might have been secondary to dehydration and electrolyte imbalance.

Seventy-two days post-transplantation, the patient developed fever, gastroenteritis, moderate dehydration, hyperchloremic metabolic acidosis (serum chloride 115 mmol/L and total CO₂ 12 mmol/L) and moderate transaminasemia. MRI with diffusion-weighted imaging of brain demonstrated an acute lesion in the right globus pallidus, consistent with ischemic and/or edematous changes, in the location often affected in non-transplanted patients with MMA [10], [11]. He has never manifested clinical signs of extrapyramidal tract disease. Subsequent MRI 18 months later showed resolution of the basal ganglion lesion. One year after transplantation, he developed varicella, without complications.

Renal size and texture (assessed by ultrasonography at 1 week, 6 and 13 months of age) were normal; at 6 years and 9 years 6 months, there was mildly increased echogenicity. Renal function remained normal until 9 years when the plasma creatinine concentration began to rise; at 10 years, creatinine was 0.8 mg/dL (normal ⩽ 0.7 mg/dL), equivalent to an estimated glomerular filtration rate of 77 mL/min/1.73 m². Cardiac function, assessed with electrocardiograms and echocardiography at 1 week and 6 months after the transplant, has been normal.

At 4 years of age, profound sensorineural hearing loss in the right ear was diagnosed. Tests for the cause of the hearing loss included normal temporal bone CT scan and connexin 26 gene studies. Repeat hearing evaluations have shown no progression of the hearing loss.

Linear growth continued along the 25th percentile until 3 years and then decelerated, reaching the 5th percentile. Bone age determinations, at chronologic ages of 5 years 10 months and 8 years 1 month, were 2 SD below the mean. His father has disproportionate short stature (height is <5th percentile) but the patient is proportionate and does not manifest skeletal abnormalities. He was evaluated by an endocrinologist and the short stature was attributed to his chronic illness and possible constitutional growth delay.

A developmental assessment was performed at 9 years of age. The Wide Range Achievement Test III was administered. He had a raw reading score of 30, giving him a standard score of 92 and second grade equivalent; arithmetic raw score 23, standard score 80, second grade equivalent. On the Elementary WORD test-revised, a test of expressive and semantic language for children aged 7–11, he had a raw score of 11 for associations, giving him a standard score of 86. In semantic absurdities, he had a raw score of 5, giving him a standard score of 63. On the Beery Test of Visual/Motor Integration (VMI), he had a raw score of 14, age equivalent of 6 years 2 months, giving him a standard score of 77. In the past year, there has been a modest improvement in the organization of figures, but his standard score has dropped. His gross and fine motor skills are age-appropriate. He requires help in school for reading and mathematics and receives speech, language and occupational therapy.