Genetic and biochemical approach to early prenatal diagnosis in a family with mut methylmalonic aciduria

Comparing amniotic fluid mass spectrometry assays and amniocyte gene analyses for the prenatal diagnosis of methylmalonic aciduria

Background

Methylmalonic aciduria (MMA), a rare inherited disorder, is the most common organic aciduria in China, and prenatal diagnosis has contributed to its prevention. However, the prenatal diagnosis of MMA using cultured amniocytes or chorionic villi to detect gene mutations is exclusively applicable to families with a definite genetic diagnosis. To evaluate the reliability of mass spectrometry assays for the prenatal diagnosis of MMA, we conducted a retrospective study of our 10 years’ experience.

Materials and methods

This retrospective compare study reviewed the medical records for maternal and fetuses data for 287 mothers with a family history of MMA from June 2010 to December 2020. Methylmalonate and propionylcarnitine in cell-free amniotic fluid were measured using a stable isotope dilution method (GC/MS) and MS/MS-based method (LC/MS/MS). Total homocysteine (tHcy) was measured by fluorescence polarization immunoassay. Depending on the presence of disease-causing gene mutations in probands, gene studies on amniocytes from 222 pregnant women were performed.

Results

For 222 fetuses of the families with definite genetic diagnosis, gene analyses were performed using cultured amniocytes. 52 fetuses were affected by MMA, whereas 170 were “unaffected”. For GC/MS and LC/MS/MS, the specificity was 96.5% and 95.9%, sensitivity was 71.2% and 84.6%, respectively. The positive and negative predictive values were 86.0% and 91.6% and 86.3% and 95.3%, respectively. Propionylcarnitine/butyrylcarnitine ratio showed the highest accuracy and could thus serve as a sensitive indicator to identify those at a risk for MMA. When GC/MS and LC/MS/MS were performed in parallel, the specificity was 92.5% and sensitivity was 95.6%. When evaluating tHcy, the positive and negative predictive values were 95.0% and 96.1%, respectively. In 65 fetuses without family genetic diagnosis, 11 were finally confirmed to have MMA and 54 were “unaffected” by amniotic fluid biochemical assays. The 54 children showed normal urine organic acids and healthy development after birth.

Conclusions

Amniotic fluid biochemical assays using GC/MS and LC/MS/MS in parallel increased the accuracy of prenatal diagnosis of MMA. Propionylcarnitine is a more reliable marker than methylmalonic acid in amniotic fluid. Further, tHcy is recommended for the prenatal diagnosis of combined MMA and homocysteinemia.

Prenatal Diagnosis of Two Common Inborn Errors of Metabolism by Genetic and Mass Spectrometric Analysis of Amniotic Fluid

Methylmalonic acidaemia (MMA) and ornithine transcarbamylase deficiency (OTCD) are both intoxication-type inborn errors of metabolism (IEM). Presently, genetic testing is the primary method for prenatally diagnosing these diseases. However, some reports have demonstrated that mass spectrometry approaches can prenatally diagnose some forms of inborn errors of metabolism using amniotic fluid. Therefore, in this study, genetic and mass spectrometry approaches were used for prenatally diagnosing MMA and OTCD. We collected amniotic fluid samples from 19 foetuses referred, 15 cases were referred for MMA and 4 for OTCD. Of the 15 MMA cases, seven were affected, as determined by genetic testing and the metabolite levels; the characteristic metabolites propionylcarnitine (C3), C3/acetylcarnitine (C2) ratio, methylmalonic acid and methylcitrate levels were significantly higher than the reference range. Eight foetuses were unaffected, and the C3, C3/C2 ratio, methylmalonic acid and methylcitrate levels were within the reference range. The C3, C3/C2, methylmalonic acid, and methylcitrate levels in the amniotic fluid significantly differed between the affected and unaffected foetuses (P = 0.0014, P = 0.0014, P = 0.0003, P = 0.0014, respectively). Moreover, the homocysteine level increased in the amniotic fluid of affected foetuses with MMACHC gene mutations. Of the four OTCD cases, genetic testing confirmed that two foetuses were affected and two were unaffected. However, the characteristic metabolite levels were within the reference range for all foetuses, including citrulline, orotic acid, and uracil. The genetic testing results were confirmed to be correct through the abortion tissue of the foetus and the postnatal follow-up. Our results suggest that mass spectrometry approaches are convenient method for improving the prenatal diagnosis of MMA. The characteristic metabolites C3, C3/C2, methylmalonic acid, and methylcitrate levels in amniotic fluid were reliable biochemical markers for the prenatal diagnosis of MMA.

Prenatal diagnosis of methylmalonic aciduria from amniotic fluid using genetic and biochemical approaches

OBJECTIVES

This study reported the clinical prenatal diagnosis experience of families affected by methylmalonic acidemia (MMA) evaluated at a single prenatal diagnosis center over 8 years, and the reliability of a biochemical approach for prenatal diagnosis was analyzed.

METHODS

Prenatal diagnosis data for 187 MMA families referred to our center from 2009 to 2016 were reviewed retrospectively. The results of the genetic analysis and biochemical approach were compared.

RESULTS

A total of 41 MMA-affected pregnancies (21%) were identified. The biochemical analysis could identify the true status of 99.5% of fetuses. The diagnostic sensitivities of the propionylcarnitine (C3) level, the C3 to acetylcarnitine (C2) ratio (C3/C2), the methylmalonic acid, and methylcitrate levels in the amniotic fluid were 95.1%, 100%, 100%, and 82.9%, respectively, and the specificities were 98.7%, 99.3%, 97.4%, and 96.7%, respectively.

CONCLUSIONS

The biochemical analysis could be optionally used in the prenatal diagnosis of MMA, especially in cases where the genetic results are inconclusive. Among the four tested biochemical markers, C3/C2 appeared to be the most reliable.

Prenatal diagnosis using genetic sequencing and identification of a novel mutation in MMACHC

BACKGROUND

Combined methylmalonic aciduria and homocystinuria, cobalamin(cbl)C deficiency, is a rare disorder of intracellular vitamin B12(cbl) metabolism caused by mutations in the MMACHC gene. Both genetic and biochemical approach have been established to diagnose children and fetuses with cblC deficiency, while in China there is no report of prenatal genetic diagnosis of cblC deficiency. The aim of the present study was to characterize the mutational spectrum of cblC deficiency and investigate the feasibility of genetic-sequencing-based prenatal diagnosis for cblC deficiency.

METHODS

10 pedigrees were recruited in this study with the probands clinically and biochemically confirmed combined methymalonic aciduria and homocystinuria. Peripheral blood samples were collected for MMACHC genetic test from the probands and their parents (4 probands had already dead) and 50 control subjects. The entire coding region and adjacent splice sites of MMACHC were sequenced. After the genotypes of the pedigrees were identified, chorionic villi sampling were performed for 3 high-risk pregnant women for prenatal genetic diagnosis.

RESULTS

A total of 7 mutations were identified: c.217C > T (R73X), c.394C > T (R132X), c.463G > C (G155R), c.609G > A (W203X), c.616C > T (R206W), c.658-660delAAG (220delK), and c.567dupT (I190YfsX13), as well as 2 polymophsims: c.321G > A(V107V), c.-302G > T. And G155R is a novel mutation that haven't been reported in the literatures. All the 6 probands identified with compound heterozygous mutations or homozygous mutations of MMACHC gene, and all the parents of the probands were found to have one MMACHC mutation at a heterozygous level. Prenatal diagnosis of fetuses from 3 families with a child affected cblC deficiency showed that one fetus had the same compound heterozygous mutations as the proband, one did not have MMACHC mutation, and the third fetus had a mutation at a heterozygous level of MMACHC gene. Results from the follow-ups were consistent with the prenatal diagnosis.

CONCLUSION

A novel mutation p.G155R of the MMACHC gene is identified. Genetic diagonsis is an accurate and convenient method for prenatal diagnosis and early intervention of combined methylmalonic aciduria and homocystinuria.

Proposed guidelines for the diagnosis and management of methylmalonic and propionic acidemia

Methylmalonic and propionic acidemia (MMA/PA) are inborn errors of metabolism characterized by accumulation of propionic acid and/or methylmalonic acid due to deficiency of methylmalonyl-CoA mutase (MUT) or propionyl-CoA carboxylase (PCC). MMA has an estimated incidence of ~ 1: 50,000 and PA of ~ 1:100’000 -150,000. Patients present either shortly after birth with acute deterioration, metabolic acidosis and hyperammonemia or later at any age with a more heterogeneous clinical picture, leading to early death or to severe neurological handicap in many survivors. Mental outcome tends to be worse in PA and late complications include chronic kidney disease almost exclusively in MMA and cardiomyopathy mainly in PA. Except for vitamin B₁₂ responsive forms of MMA the outcome remains poor despite the existence of apparently effective therapy with a low protein diet and carnitine. This may be related to under recognition and delayed diagnosis due to nonspecific clinical presentation and insufficient awareness of health care professionals because of disease rarity.

These guidelines aim to provide a trans-European consensus to guide practitioners, set standards of care and to help to raise awareness. To achieve these goals, the guidelines were developed using the SIGN methodology by having professionals on MMA/PA across twelve European countries and the U.S. gather all the existing evidence, score it according to the SIGN evidence level system and make a series of conclusive statements supported by an associated level of evidence. Although the degree of evidence rarely exceeds level C (evidence from non-analytical studies like case reports and series), the guideline should provide a firm and critical basis to guide practice on both acute and chronic presentations, and to address diagnosis, management, monitoring, outcomes, and psychosocial and ethical issues. Furthermore, these guidelines highlight gaps in knowledge that must be filled by future research. We consider that these guidelines will help to harmonize practice, set common standards and spread good practices, with a positive impact on the outcomes of MMA/PA patients.

Prenatal pharmacotherapy for fetal anomalies: a 2011 update

Fetal therapy can be defined as any prenatal treatment administered to the mother with the primary indication to improve perinatal or long-term outcomes for the fetus or newborn. This review provides an update of the pharmacological therapies that are solely directed at the fetus with anomalies and outlines a future transcriptomic approach. Fetal anomalies targeted with prenatal pharmacotherapy are a heterogeneous group of structural, endocrine, and metabolic conditions, including congenital cystic adenomatoid malformation (CCAM), congenital adrenal hyperplasia, congenital heart block, fetal tachyarrhythmias, inborn errors of metabolism, fetal thyroid disorders, and polyhydramnios. To date, the majority of pharmacotherapies for fetal anomalies have been evaluated only in retrospective, uncontrolled studies. The way forward will be with an evidence-based approach to prenatal pharmacological interventions.

Multiple roles of ATP:cob(I)alamin adenosyltransferases in the conversion of B12 to coenzyme B12

Our mechanistic understanding of the conversion of vitamin B(12) into coenzyme B(12) (a.k.a. adenosylcobalamin, AdoCbl) has been substantially advanced in recent years. Insights into the multiple roles played by ATP:cob(I)alamin adenosyltransferase (ACA) enzymes have emerged through the crystallographic, spectroscopic, biochemical, and mutational analyses of wild-type and variant proteins. ACA enzymes circumvent the thermodynamic barrier posed by the very low redox potential associated with the reduction of cob(II)alamin to cob(I)alamin by generating a unique four-coordinate cob(II)alamin intermediate that is readily converted to cob(I)alamin by physiological reductants. ACA enzymes not only synthesize AdoCbl but also they deliver it to the enzymes that use it, and in some cases, enzymes in which its function is needed to maintain the fidelity of the AdoCbl delivery process have been identified. Advances in our understanding of ACA enzyme function have provided valuable insights into the role of specific residues, and into why substitutions of these residues have profound negative effects on human health. From an applied science standpoint, a better understanding of the adenosylation reaction may lead to more efficient ways of synthesizing AdoCbl.