Isolated Methylmalonic Acidemia

Clinical characteristics.

Isolated methylmalonic acidemia/aciduria, the topic of this GeneReview, is caused by complete or partial deficiency of the enzyme methylmalonyl-CoA mutase (mut⁰ enzymatic subtype or mut^– enzymatic subtype, respectively), a defect in the transport or synthesis of its cofactor, adenosyl-cobalamin (cblA, cblB, or cblD-MMA), or deficiency of the enzyme methylmalonyl-CoA epimerase. Onset of the manifestations of isolated methylmalonic acidemia/aciduria ranges from the neonatal period to adulthood. All phenotypes are characterized by periods of relative health and intermittent metabolic decompensation, usually associated with intercurrent infections and stress.

• In the neonatal period the disease can present with lethargy, vomiting, hypotonia, hypothermia, respiratory distress, severe ketoacidosis, hyperammonemia, neutropenia, and thrombocytopenia and can result in death within the first four weeks of life.
• In the infantile/non-B₁₂-responsive phenotype, infants are normal at birth, but develop lethargy, vomiting, dehydration, failure to thrive, hepatomegaly, hypotonia, and encephalopathy within a few weeks to months of age.
• An intermediate B₁₂-responsive phenotype can occasionally be observed in neonates, but is usually observed in the first months or years of life; affected children exhibit anorexia, failure to thrive, hypotonia, and developmental delay, and sometimes have protein aversion and/or vomiting and lethargy after protein intake.
• Atypical and "benign"/adult methylmalonic acidemia phenotypes are associated with increased, albeit mild, urinary excretion of methylmalonate.

Major secondary complications of methylmalonic acidemia include: intellectual impairment (variable); tubulointerstitial nephritis with progressive renal failure; "metabolic stroke" (acute and chronic basal ganglia injury) causing a disabling movement disorder with choreoathetosis, dystonia, and para/quadriparesis; pancreatitis; growth failure; functional immune impairment; and optic nerve atrophy.

Diagnosis/testing.

Diagnosis of isolated methylmalonic acidemia relies on analysis of organic acids in plasma and/or urine by gas-liquid chromatography and mass spectrometry. Establishing the specific subtype of methylmalonic acidemia requires cellular biochemical studies (including ¹⁴C propionate incorporation and B₁₂ responsiveness, complementation analysis, and cobalamin distribution assays) and molecular genetic testing. The finding of biallelic pathogenic variants in one of the five genes (MMUT, MMAA, MMAB, MCEE, and MMADHC) associated with isolated methylmalonic acidemia – with confirmation of carrier status in the parents – can establish the diagnosis.

Management.

Treatment of manifestations: Critically ill individuals are stabilized by restoring volume status and acid-base balance; reducing or eliminating protein intake; providing increased calories via high glucose-containing fluids and insulin to arrest catabolism; and monitoring serum electrolytes and ammonia, venous or arterial blood gases, and urine output. Management includes a high-calorie diet low in propiogenic amino acid precursors; hydroxocobalamin intramuscular injections; carnitine supplementation; antibiotics such as neomycin or metronidazole to reduce propionate production from gut flora; gastrostomy tube placement as needed; and aggressive treatment of infections. Other therapies used in a limited number of patients include N-carbamylglutamate for the treatment of acute hyperammonemic episodes; liver, kidney, or combined liver and kidney transplantation; and antioxidants for the treatment of optic nerve atrophy.

Prevention of primary manifestations: In some cases, newborn screening allows for presymptomatic detection of affected newborns and early treatment.

Agents/circumstances to avoid: Fasting and increased dietary protein.

Other: Medic Alert^® bracelets and up-to-date, easily accessed, detailed emergency treatment protocols facilitate care.

Genetic counseling.

Isolated methylmalonic acidemia is inherited in an autosomal recessive manner. At conception, each sib of an affected individual has a 25% chance of being affected, a 50% chance of being an asymptomatic carrier, and a 25% chance of being unaffected and not a carrier. Carrier testing for at-risk family members and prenatal testing for pregnancies at increased risk are possible using molecular genetic techniques if the pathogenic variants in the family are known. In some circumstances, prenatal diagnosis for pregnancies at increased risk is possible by enzyme analysis and metabolite measurements on cultured fetal cells (obtained by chorionic villus sampling or amniocentesis).

Suggestive Findings

Because the presenting signs and symptoms of isolated methylmalonic acidemia are nonspecific, suggestive findings can include the following:

• In neonates: lethargy, vomiting, hypotonia, hypothermia, respiratory distress, severe ketoacidosis, hyperammonemia, neutropenia, and thrombocytopenia
  Note: In states with an expanded newborn screening program, isolated methylmalonic acidemia can be diagnosed in well-appearing newborns prior to an episode of acute decompensation.
• In older infants and children: failure to thrive, renal syndromes and hypotonia, intellectual disability or other acute (basal ganglia stroke) and chronic neurologic symptoms

In patients with partial mut enzymatic deficiency, cblA, or cblB, suggestive findings at various ages can include the following:

• An attenuated MMA phenotype [Lerner-Ellis et al 2004, Lerner-Ellis et al 2006, Hörster et al 2007]
• Isolated renal tubular acidosis or chronic renal failure [Dudley et al 1998, Coman et al 2006]
• Metabolic stroke of the basal ganglia [Korf et al 1986, Heidenreich et al 1988]
• Catastrophic/lethal ketoacidosis following an intercurrent illness [Ciani et al 2000]

Establishing the Diagnosis

An overview of the process of intracellular propionate and cobalamin metabolism is depicted in Figure 1. A flowchart for the work up of a person with elevated methylmalonic acid in urine and/or plasma is provided in Figure 2, a modified algorithm that includes the consideration of methylmalonyl-CoA epimerase deficiency, succinyl-CoA ligase deficiency, and other rare defects in the pathway, as well as the use of in vivo vitamin B₁₂ responsiveness in the work up of an individual who is found to have methylmalonic acidemia at any age.

Figure 2.

An algorithm of conditions to be considered in the differential diagnosis of elevated serum or urine methylmalonic acid detected either during the follow up of an increased propionylcarnitine (C3) on newborn screening or a positive urine organic acid (more...)

Step 1. In a proband with suspicious clinical findings and a positive urine organic acid screen for MMA, laboratory testing that can help to establish the diagnosis includes: glucose, electrolytes, ammonia, blood gas, lactate, CBC, and urine ketones, plasma MMA, tHcy, and B₁₂ levels, plasma amino acids, and acylcarnitine profile. Relevant findings:

• High plasma and urine MMA with normal B₁₂, tHcy, and methionine levels
• Elevated propionylcarnitine (C3)
• High anion gap metabolic acidosis in arterial or venous blood gas testing and huge quantities of ketone bodies and lactate in the urine
• Hyperammonemia
• Hyperglycinemia
• Lactic acidosis
• CBC showing neutropenia, thrombocytopenia, anemia

Step 2. In newborns found to have elevation of propionylcarnitine (C3) by expanded newborn screening and in individuals at high genetic risk for the disorder (e.g., sibs of a proband), the first priority is to establish the presence of significantly elevated methylmalonic acid, which is best done by urine organic acid analysis (by GC/MS) and plasma acylcarnitine profile (by TMS). Note: At the same time, obtaining levels of plasma MMA, amino acids, plasma homocysteine, and serum vitamin B₁₂ (in both the newborn and the mother) helps further differentiate the cause of methylmalonic acidemia should that be confirmed (see Step 3).

In addition to elevated methylmalonic acid, the following biochemical findings may also be seen:

• Presence of 3-hydroxypropionate, 2-methylcitrate, and tiglylglycine detected on GC/MS analysis of urine
• Elevated plasma concentration of glycine on plasma amino acid analysis
• Elevated plasma concentration of propionylcarnitine (C3) and variable elevations in C4-dicarboxylic or methylmalonic/succinylcarnitine (C4DC) measured by TMS

Step 3. Once elevation of methylmalonic acidemia and aciduria have been established, a normal plasma homocysteine and vitamin B₁₂ level can help differentiate isolated MMA from other disorders (see Figure 2, left two columns). Note: Although plasma and/or urine methylmalonic acid concentration can be precisely quantitated (Table 1), this is generally not needed immediately for diagnostic purposes.

Step 4. In vivo responsiveness to vitamin B₁₂ should be determined in all affected individuals. No standard regimen has been documented. When stable, affected individuals can be given 1.0 mg of hydroxocobalamin (OH-Cbl) (see Note) intramuscularly or intravenously every day for one to two weeks followed by assessment of production of MMA and related metabolites (3-OH-propionic, 2-methylcitrate) by serial urine organic acid analyses and/or measurement of plasma concentrations of MMA, propionylcarnitine, and homocysteine. A significant (>50%) reduction in metabolite production and plasma concentration(s) is considered to indicate responsiveness [Fowler et al 2008, Kruszka et al 2013]. In vivo response was reported in all individuals with cblA and only rare individuals with cblB [Hörster et al 2007].

Note: Hydroxocobalamin (not cyanocobalamin) is the preferred preparation for treatment of methylmalonic acidemia; thus, if the in vivo response to intramuscular hydroxocobalamin is questionable or borderline, vitamin B₁₂ administration should be continued and a skin biopsy should be obtained to isolate fibroblasts to assess B₁₂ responsiveness by ¹⁴C propionate incorporation in vitro.

Step 5. Molecular genetic testing (Table 2) can be used to establish the diagnosis of isolated MMA by identifying biallelic pathogenic variants in one of the five genes (MMUT, MMAA, MMAB, MCEE, and MMADHC) and confirming carrier status in the parents. In addition, the enzymatic subtype of isolated methylmalonic acidemia is mostly determined by molecular genetic testing due to the limited access to, cost of, and invasive nature of cellular biochemical testing.

Molecular testing approaches can include the following:

• Tiered single-gene testing. Because the phenotype of isolated methylmalonic acidemia can be identical regardless of the mutated gene, molecular genetic testing can be performed in the following order:

  1.

  UT and MMAB in vitamin B₁₂-non-responsive individuals

  2.

  MMAA in vitamin B₁₂-responsive individuals

  3.

  MCEE and MMADHC testing if results of testing of the first three genes (MMUT, MMAB, and MMAA) are unrevealing

  Note: For all genes, sequence analysis is performed first, followed by deletion/duplication analysis if only one pathogenic variant has been detected.
• Use of a multigene panel that includes these five genes and other genes in the metabolic pathway (see Differential Diagnosis). Note: The genes included in the panel and the diagnostic sensitivity of the testing used for each gene vary by laboratory and are likely to change over time.
  For an introduction to multigene panels click here. More detailed information for clinicians ordering genetic tests can be found here.

Step 6. Cellular biochemical testing on skin fibroblasts is the gold standard for determining the MMA subtype and B₁₂ responsiveness in vitro and is useful when the above testing methods fail to provide a firm diagnosis to guide management. For details of biochemical testing, click here (pdf).

Clinical Description

The phenotypes of isolated methylmalonic acidemia described below that are associated with the mut⁰ enzymatic subtype, mut^– enzymatic subtype, cblA, cblB, and cblD-MMA share clinical presentations and a natural history characterized by periods of relative health and intermittent metabolic decompensation, usually associated with intercurrent infections and stress [Zwickler et al 2012]. Each such decompensation can be life-threatening. Of note, the natural history of isolated methylmalonic acidemia requires further study, particularly with respect to medical complications including renal disease, the effect of solid organ transplantation, and molecular pathology.

Infantile/non-B₁₂-responsive phenotype (mut⁰ enzymatic subtype, cblB). The most common phenotype of isolated methylmalonic acidemia presents during infancy. Infants are normal at birth but rapidly develop lethargy, vomiting, and dehydration on initiation of protein-containing feeds. At presentation, they exhibit hepatomegaly, hypotonia, and in many, hyperammonemic encephalopathy. Laboratory findings typically show a severe, high anion-gap metabolic acidosis, ketosis and ketonuria (highly abnormal in neonates and strongly suggestive of an organic aciduria), hyperammonemia, and hyperglycinemia [Matsui et al 1983, Kölker et al 2015a]. Dialysis may be needed especially if hyperammonemia is significant and persistent.

Thrombocytopenia and neutropenia, suggestive of neonatal sepsis, can be seen.

The catastrophic neonatal presentation of isolated methylmalonic acidemia can result in death, despite aggressive intervention. Infants with the B₁₂-responsive mut^– enzymatic subtype or cblA can also present with an acute neonatal crisis.

Partially deficient or B₁₂-responsive phenotypes (mut^– enzymatic subtype, cblA, cblB [rare], cblD-MMA). This intermediate phenotype of isolated methylmalonic acidemia can occur in the first few months or years of life. Affected infants can exhibit feeding problems (typically anorexia and vomiting), failure to thrive, hypotonia, and developmental delay. Some have protein aversion and/or clinical symptoms of vomiting and lethargy after protein intake.

Until the diagnosis is established and treatment initiated these infants are at risk for a catastrophic decompensation (like that in neonates) [Shapira et al 1991, Lerner-Ellis et al 2004, Lerner-Ellis et al 2006, Hörster et al 2007].

During such an episode of metabolic decompensation, the child may die despite intensive intervention if prompt treatment specific for MMA is not instituted and the symptoms are misdiagnosed as, for example, diabetic ketoacidosis [Ciani et al 2000].

Before the onset of newborn screening, infants with the subtypes cblA or mut^– would present with a devastating injury in the basal ganglia (more specifically lacunar infarcts in the globus pallidus) resulting in a debilitating movement disorder [Korf et al 1986, Heidenreich et al 1988].

Patients with partial mut enzymatic deficiency, cblA, or cblB can also present with isolated renal tubular acidosis or chronic renal failure [Dudley et al 1998, Coman et al 2006].

Methylmalonyl-CoA epimerase deficiency. Pathogenic variants in MCEE are a very rare cause of persistent moderate methylmalonic aciduria. Findings in infants/children with mutation of MCEE have ranged from complete absence of symptoms to severe metabolic acidosis with increased MMA and 2-methylcitrate and ketones in the urine at initial presentation [Dobson et al 2006, Gradinger et al 2007]. Symptoms include ataxia, dysarthria, hypotonia, mild spastic paraparesis, and seizures; however, many affected persons were from consanguineous unions — including the first identified individual, who also had a DOPA-responsive dystonia resulting from homozygous pathogenic variants of SPR, the gene encoding sepiapterin reductase [Bikker et al 2006].

Secondary complications. Despite increased knowledge about isolated methylmalonic acidemia and possibly earlier symptomatic diagnosis, isolated methylmalonic acidemia continues to be associated with substantial morbidity and mortality [de Baulny et al 2005, Dionisi-Vici et al 2006, Kölker et al 2015b] that correlates with the underlying defect [Hörster et al 2007]. Individuals with the mut⁰ enzymatic subtype and the cblB subtype have a higher rate of mortality and neurologic complications than those with the mut^– enzymatic subtype and cblA.

The major secondary complications include:

• Intellectual disability. Intellectual disability may or may not be present even in those with severe disease. In a retrospective, survey-based review, about 50% of individuals with the mut⁰ enzymatic subtype and 25% of those with the cblA/cblB enzymatic subtype had an IQ below 80 and significant neurologic impairment [Baumgarter & Viardot 1995].
  In another study about 50% of individuals with mut⁰, 85% with mut^–, 48% with cblA, and 70% with cblB had an IQ above 90 [Hörster et al 2007].
  In a recent natural history study, the mean FSIQ of all individuals with isolated methylmalonic acidemia (n = 37) was 85.0 ± 20.68, which is in the low average range (80 ≤ IQ ≤89). Individuals with cblA (n = 7), cblB (n = 6), and mut diagnosed prenatally or by newborn screening (n = 3) had mean FSIQs in the average range (90 ≤ IQ ≤109). The age of disease onset, the presence of severe hyperammonemia at diagnosis, and a history of seizures were associated with more severe impairments [O'Shea et al 2012].
• Tubulointerstitial nephritis with progressive impairment of renal function. All individuals with isolated methylmalonic acidemia, even those who are mildly affected or who have received a liver allograft [Nyhan et al 2002], are at risk of developing renal insufficiency [Walter et al 1989, Kruszka et al 2013]. End-stage renal disease (ESRD) was common in individuals with the mut⁰ enzymatic subtype (61%) and the cblB (66%) enzymatic subtype, and occurred less frequently in those with the cblA (21%) enzymatic subtype [Hörster et al 2007].
  Secondary mitochondrial dysfunction rather than direct nephrotoxicity of methylmalonic acid is hypothesized. Cell-specific mitochondrial pathology primarily in the proximal tubules, associated with cytochrome c oxidase deficiency and increased markers of oxidative stress in the urine and plasma, have been shown in human and mouse studies [Atkuri et al 2009, Mc Guire et al 2009, Manoli et al 2013, Zsengellér et al 2014].
  An acute renal syndrome, seen in the setting of metabolic decompensation, may also exist [Stokke et al 1967] and requires further clinical delineation. Moreover, renal tubular dysfunction presenting as a decrease in urine concentrating ability and acidification, hyporeninemic hypoaldosteronism, tubular acidosis type 4, and hyperkalemia have been reported in a number of affected individuals, and are supported by murine studies [Walter et al 1989, D'Angio et al 1991, Pela et al 2006, Manoli et al 2013].
• Neurologic findings. Some individuals develop a "metabolic stroke" or infarction of the basal ganglia (characteristically the globus pallidus externa) during acute metabolic decompensation, which can produce an incapacitating movement disorder [Korf et al 1986, Heidenreich et al 1988]. The reported incidence in different cohorts is 17%-30% [Baumgarter & Viardot 1995, Hörster et al 2007]. Distinct segments of the globus pallidus (and sometimes the substrantia nigra in the cerebral peduncles) are affected, suggesting a non-uniform, cell-specific sensitivity to the mechanism of infarct [Baker et al 2015].
  Delayed myelination, incomplete opercularization, subcortical white matter changes, and brain stem and cerebellar changes have been described [Harting et al 2008, Radmanesh et al 2008].
  Of note, individuals who have undergone liver and/or kidney transplantation can develop acute lesions without overt metabolic decompensation, suggesting that the enzyme deficiency in the brain remains unchanged and trapping of toxic metabolites in the CNS compartment can lead to injury despite other systemic benefits of the transplantation [Chakrapani et al 2002, Kaplan et al 2006, Vernon et al 2014].
• Pancreatitis. The incidence of pancreatitis in isolated methylmalonic acidemia is unknown, but it is a well-recognized complication [Kahler et al 1994]. It can occur acutely or chronically. Pancreatitis may be under-recognized because it can manifest nonspecifically with vomiting and abdominal pain.
• Growth failure. Growth failure is frequent and multifactorial. It is the result of severe chronic illness and perhaps relative protein malnutrition that is complicated further by chronic renal failure. Many infants are less than three standard deviations below normal for both length and weight.
  Some children have documented growth hormone (GH) deficiency, but response to GH therapy may vary (see Management).
• Functional immune impairment. This results in an increased susceptibility to severe infections, particularly by fungal and gram-negative organisms [Oberholzer et al 1967, Wong et al 1992].
• Bone marrow failure. During episodes of metabolic decompensation patients can exhibit pancytopenia, with bone marrow hypoplasia and/or dysplasia that most frequently revert to normal with supportive care.
• Optic nerve atrophy. Late-onset optic atrophy associated with acute visual loss, resembling the presentation of the mitochondrial disorder Leber hereditary optic neuropathy (LHON), has been reported in isolated methylmalonic acidemia [Wasserstein et al 1999, Williams et al 2009, Pinar-Sueiro et al 2010, Traber et al 2011], as well as in propionic acidemia [Williams et al 2009, Martinez Alvarez et al 2016].
• Hepatoblastoma. Isolated instances of hepatoblastoma have been reported in the native or donor liver in individuals with mut MMA; however, the overall incidence of cancer in these patients is unknown [Cosson et al 2008, Chan et al 2015]

Survival in isolated methylmalonic acidemia has improved over time [Matsui et al 1983, van der Meer et al 1994, Baumgarter & Viardot 1995, Nicolaides et al 1998, Kölker et al 2015a].

In those with the mut⁰ enzymatic subtype, survival at age one year has improved from 65% in the 1970s to more than 90% in the 1990s; five-year survival has improved from 33% in the 1970s to more than 80% in the 1990s.

In one series, the median age of death of those with the mut⁰ enzymatic subtype was compared over time: 100% died at a median age of 1.6 years in the 1970s, 50% died at a median age of 7.6 years in the 1980s, and 20% died at a median age of 2.2 years in the 1990s. Overall mortality was about 50% for those with the mut⁰ enzymatic subtype (median age of death 2 years) as compared to 50% for the cblB enzymatic subtype (median age of death 2.9 years), 40% for the mut^– enzymatic subtype (median age of death 4.5 years), and about 5% for the cblA enzymatic subtype (1 death at 14 days) [Hörster et al 2007].

The effect of early organ transplantation on overall survival has not been systematically studied.

Effect of newborn screening. The limited number of infants detected by newborn screening (NBS) and the short duration of their follow up do not allow conclusions regarding the effect of NBS on the long-term outcome of methylmalonic acidemia [Leonard et al 2003, Dionisi-Vici et al 2006]. Moreover, it must be emphasized that a significant number of infants with the mut⁰ enzymatic subtype may present clinically before the NBS results become available. Limited observations in sibs with the cblA enzymatic subtype suggest that the IQs of the individuals treated from the newborn period were significantly better than those of their older affected sibs who were diagnosed after the onset of symptoms [Hörster et al 2007].

Of note, before the availability of newborn screening individuals with cblA and some with cblB often manifested in early childhood with encephalopathy and globus pallidus injury, which in theory could have been avoided if they had been detected by NBS and treated before symptoms appeared.

Genotype-Phenotype Correlations

Precise genotype-phenotype correlations are difficult to determine since most affected individuals are compound heterozygotes.

Homozygosity for the p.Asn219Tyr MMUT pathogenic variant is frequently associated with severe mutase deficiency (i.e., the mut⁰ enzymatic subtype) [Acquaviva et al 2001]. p.Arg108Cys, which is also associated with a mut⁰ enzymatic subtype, is more common in individuals of Hispanic descent [Worgan et al 2006].

Homozygosity for the p.Gly717Val MMUT pathogenic variant, which is associated with the mut^– enzymatic subtype [Worgan et al 2006], is more common in individuals of African descent.

The clinical phenotype depends on a number of factors that cannot be accurately predicted by the genotype, including whole-body enzyme activity, in vivo responsiveness to cobalamin, environmental factors, and perhaps the efficiency and activation of alternative propionyl-CoA disposal pathways. It is possible that better understanding of clinical correlations in isolated methylmalonic acidemia could be achieved by estimating the amount of whole-body residual metabolic capacity based on stable isotope studies [Leonard 1997].

Prevalence

Several studies have estimated the birth prevalence of isolated methylmalonic acidemia [Sniderman et al 1999]. Urine screening for isolated methylmalonic acidemia in Quebec identified "symptomatic methylmalonic aciduria" in approximately 1:80,000 newborns screened [Sniderman et al 1999], which approximates the observation of Chace et al [2001] of ten cases of isolated methylmalonic acidemia identified in a sample of 908,543 newborns screened by mass spectrometry in the US.

In Japan, the birth prevalence may be as high as 1:50,000 [Shigematsu et al 2002].

It appears that the prevalence of isolated methylmalonic acidemia may therefore fall between 1:50,000 and 1:100,000; confirmation, however, would require larger studies.