Newborn Screening for Methylmalonic Acidemia in a Chinese Population: Molecular Genetic Confirmation and Genotype Phenotype Correlations

Spectrum analysis of inborn errors of metabolism for expanded newborn screening in Xinjiang, China

To determine the disease spectrum and genetic characteristics of inborn errors of metabolism (IEM) in Xinjiang province in the northwest of China, 41,690 newborn babies were screening by tandem mass spectrometry from November 2018 to December 2021. Of these, 57 newborn babies were referred for genetic analysis by next-generation sequencing, which was validated by Sanger sequencing. A total of 36 newborn babies and one relative were diagnosed with IEM, and the overall positive predictive value was 29.03%. The overall incidence of IEM in Xinjiang was 1:1,158 (36/41,690). The incidence of amino acidemias, organic acidemias, and fatty acid oxidation disorder were 1:1,668 (25/41,690), 1:4,632 (9/41,690), and 1:20,845 (2/41,690), respectively. Phenylketonuria and methylmalonic acidemia were the two most common inborn errors of metabolism (IEM), accounting for 83% (30/36) of all confirmed cases. Some hotspot mutations were observed for several IEMs, including PAH gene c.158G > A (p.Arg53His) and c.688G > A (p.Val230Ile) for hyperphenylalaninemia. Four mutation types of the MMACHC gene (e.g., c.609G > A (p.Trp203Ter), c.567dupT (p.Ile190fs)) and six mutation types of the MMUT gene (e.g., c.729_730insT (p.Asp244fs)) were found for methylmalonic acidemia. We also found 11 mutations in six genes: PCCB, IVD, GCDH, MCCC1, SLC22A5, and ACADS in this region. This study combined tandem mass spectrometry and next-generation sequencing technology for the screening and diagnosis of IEM. The study provides effective clinical guidance, and the data provide a basis for expanding newborn screening, genetic screening, and IEM gene consultation in Xinjiang, China.

Clinical spectrum and genetic variation of six patients with methylmalonic aciduria (MMA); a report from Iran

OBJECTIVE

Methylmalonic acidemia (MMAs) is known as a severe, complex, and lethal disorder of methylmalonate and cobalamin. The patients with MMA may have developmental, neurological, and metabolic disorders such as liver disease. Here, we aim to evaluate 6 Iranian patients suspected to MMA disorder.

STUDY DESIGN

We will provide genetic results, biochemical analysis and treatment for these patients. Liquid chromatography-tandem mass spectrometry (LC-MS/MS) and variant screening in probands by whole exome sequencing (WES) were performed.

RESULTS

A total of six homozygous variants were identified, including five previously identified variants and one novel variant, in the two MMA-causing genes as follows: c.577G > C, c.290 + 69G > T, c.662T > A, c.290 + 69G > T of MMAB, and c.100dupA, c.394 C > T of MMACHC. Sanger sequencing confirmed the identified variants. Additionally, metabolomics data analysis reliably identified elevated C3 and MMA levels, as well as abnormalities in the amino acid profile, indicating the presence of pathogenic variants.

CONCLUSIONS

Our findings expand the global spectrum of genotypes in MMA. While WES, combined with metabolomics and biochemical analysis, offers valuable insights for accurate diagnosis and subtyping of MMA, it is most beneficial in complex cases where clinical findings are unclear.

A case series of Cypriot patients with CblC defect: Clinical, biochemical and molecular characteristics

Methylmalonic aciduria and homocystinuria, CblC type, is an inborn error of intracellular vitamin B12 (cobalamin) metabolism caused, in the majority of cases, by mutations in the MMACHC gene. Five Cypriot patients (four males and one female) were diagnosed with a CblC defect. Age at diagnosis ranged from 10 days to 9 months. We present here the clinical, biochemical and molecular findings of these patients. Our retrospective study indicates that all patients were carriers of the known p.Arg91LysfsTer14 variant in either a homozygous or compound heterozygous state with other known MMACHC pathogenic variants. Out of three patients sharing the same genotype the one diagnosed and initiated treatment in the neonatal period displayed an improved clinical outcome.

Clinical outcomes of patients with mut-type methylmalonic acidemia identified through expanded newborn screening in China

BACKGROUND

Isolated methylmalonic acidemia, an autosomal recessive disorder of propionate metabolism, is usually caused by mutations in the methylmalonyl-CoA mutase gene (mut-type). Because no universal consensus was made on whether mut-type methylmalonic acidemia should be included in newborn screening (NBS), we aimed to compare the outcome of this disorder detected by NBS with that detected clinically and investigate the influence of NBS on the disease course.

DESIGN & METHODS

In this study, 168 patients with mut-type methylmalonic acidemia diagnosed by NBS were compared to 210 patients diagnosed after disease onset while NBS was not performed. Clinical data of these patients from 7 metabolic centers in China were analyzed retrospectively, including initial manifestations, biochemical metabolites, the responsiveness of vitamin B12 therapy, and gene variation, to explore different factors on the long-term outcome.

RESULTS

By comparison of the clinically-diagnosed patients, NBS-detected patients showed younger age at diagnosis, less incidence of disease onset, better responsiveness of vitamin B12, younger age at start of treatment, lower levels of biochemical features before and after treatment, and better long-term prognosis (P < 0.01). Onset of disease, blood C3/C2 ratio and unresponsiveness of vitamin B12 were more positively associated with poor outcomes of patients whether identified by NBS. Moreover, the factors above as well as older age at start of treatment were positively associated with mortality.

CONCLUSIONS

This research highly demonstrated NBS could prevent major disease-related events and allow an earlier treatment initiation. As a key prognostic factor, NBS is beneficial for improving the overall survival of infants with mut-type methylmalonic acidemia.

Using metabolic abnormalities of carriers in the neonatal period to evaluate the pathogenicity of variants of uncertain significance in methylmalonic acidemia

Objective

To accurately verify the pathogenicity of variants of uncertain significance (VUS) in MUT and MMACHC genes through mass spectrometry and silico analysis.

Methods

This multicenter retrospective study included 35 participating units (ClinicalTrials.gov ID: NCT06183138). A total of 3,071 newborns (within 7 days of birth) were sorted into carrying pathogenic/likely pathogenic (P/LP) variants and carrying VUS, non-variant groups. Differences in metabolites among the groups were calculated using statistical analyses. Changes in conservatism, free energy, and interaction force of MMUT and MMACHC variants were analyzed using silico analysis.

Results

The percentage of those carrying VUS cases was 68.15% (659/967). In the MMUT gene variant, we found that C3, C3/C2, and C3/C0 levels in those carrying the P/LP variant group were higher than those in the non-variant group (p < 0.000). The conservative scores of those carrying the P/LP variant group were >7. C3, C3/C0, and C3/C2 values of newborns carrying VUS (c.1159A>C and c.1286A>G) were significantly higher than those of the non-variant group and the remaining VUS newborns (p < 0.005). The conservative scores of c.1159A>C and c.1286A>G calculated by ConSurf analysis were 9 and 7, respectively. Unfortunately, three MMA patients with c.1159A>C died during the neonatal period; their C3, C3/C0, C3/C2, and MMA levels were significantly higher than those of the controls.

Conclusion

Common variants of methylmalonic acidemia in the study population were categorized as VUS. In the neonatal period, the metabolic biomarkers of those carrying the P/LP variant group of the MUT gene were significantly higher than those in the non-variant group. If the metabolic biomarkers of those carrying VUS are also significantly increased, combined with silico analysis the VUS may be elevated to a likely pathogenic variant. The results also suggest that mass spectrometry and silico analysis may be feasible screening methods for verifying the pathogenicity of VUS in other inherited metabolic diseases.

Disease spectrum, prevalence, genetic characteristics of inborn errors of metabolism in 21,840 hospitalized infants in Chongqing, China, 2017-2022

Inborn errors of metabolism (IEMs) are uncommon. Although some studies have explored the distribution and characteristics of IEMs in newborns, the impact of these disorders on hospitalized newborns remains unclear. In this study, we gathered data from 21,840 newborn patients admitted for various medical conditions at the Children's Hospital of Chongqing Medical University from January 2017 and December 2022. Liquid chromatography-tandem mass spectrometry (LC-MS/MS), gas chromatography-mass spectrometry (GC-MS/MS), and genetic analysis were used to elucidate the disease spectrum, incidence rate, and genetic characteristics of IEMs in hospitalized newborns. The results revealed that the incidence of IEMs in hospitalized newborns was 1/377 (58/21,840), with a higher incidence in full-term infants (1/428) than in premature infants (1/3,120). Among the diagnosed genetic metabolic diseases, organic acid metabolism disorders (1/662), amino acid metabolism disorders (1/950), and fatty acid oxidation disorders (1/10,920) were the most prevalent. Methylmalonic acidemia (MMA), especially the isolated form, emerged as the most common IEM, while neonatal intrahepatic cholestasis caused by citrin deficiency (NICCD) and ornithine transcarbamylase deficiency (OTCD) were prevalent in premature infants. Of the 58 confirmed cases of IEMs, 72 variants were identified, of which 31.94% (23/72) had not been reported previously. This study contributes to understanding the incidence and clinical features of IEMs in hospitalized newborns, offering more efficient strategies for screening and diagnosing these disorders.

Clinical and electroencephalogram characteristics of methylmalonic acidemia with MMACHC and MUT gene mutations

OBJECTIVE

This study investigated the clinical, imaging, and electroencephalogram (EEG) characteristics of methylmalonic acidemia (MMA) with nervous system damage as the primary manifestation.

METHODS

From January 2017 to November 2022, patients with nervous system injury as the main clinical manifestation, diagnosed with methylmalonic acidemia by metabolic and genetic testing, were enrolled and analyzed. Their clinical, imaging, and electroencephalogram data were analyzed.

RESULTS

A total of 18 patients were enrolled, including 15 males and 3 females. The clinical symptoms were convulsions, poor feeding, growth retardation, disorder of consciousness, developmental delay, hypotonia, and blood system changes. There were 6 cases (33%) of hydrocephalus, 9 (50%) of extracerebral space widened, 5 (27%) of corpus callosum thinning, 3 (17%) of ventricular dilation, 3 (17%) of abnormal signals in the brain parenchyma (frontal lobe, basal ganglia region, and brain stem), and 3 (17%) of abnormal signals in the lateral paraventricular. In addition, there were 3 cases (17%) of cerebral white matter atrophy and 1 (5%) of cytotoxic edema in the basal ganglia and cerebral peduncle. EEG data displayed 2 cases (11%) of hypsarrhythmia, 3 (17%) of voltage reduction, 12(67%) of abnormal discharge, 13 (72%) of abnormal sleep physiological waves or abnormal sleep structure, 1 (5%) of immature (delayed) EEG development, and 8 (44%) of slow background. There were 2 cases (11%) of spasms, 1 (5%) of atonic seizures, and 1 (5%) of myoclonic seizures. There were 16 patients (89%) with hyperhomocysteinemia. During follow-up, 1 patient was lost to follow-up, and 1 died. In total, 87.5% (14/16) of the children had varying developmental delays. EEG was re-examined in 11 cases, of which 8 were normal, and 3 were abnormal. Treatments included intramuscular injections of vitamin B12, L-carnitine, betaine, folic acid, and oral antiepileptic therapy. Acute treatment included anti-infective, blood transfusion, fluid replacement, and correcting acidosis. The other treatments included low-protein diets and special formula milk powder.

CONCLUSION

Methylmalonic acidemia can affect the central nervous system, leading to structural changes or abnormal signals on brain MRI. Metabolic screening and genetic testing help clarify the diagnosis. EEG can reflect changes in brain waves during the acute phase.

Genetic analysis of isolated methylmalonic acidemia in Henan, China: c.1663G>A variant of MMUT prevalent in the Henan population

BACKGROUND

Methylmalonic acidemia (MMA) is the most common organic acidemia in China, and isolated MMA accounts for approximately 30 % of all types of MMA. Common variants of the MMUT gene vary greatly around the world. The present study aims to determine the high-frequency and novel genetic variants of the MMUT gene in the Henan population of China and evaluate the prognosis of patients carrying the c.1663G>A (p.Ala555Thr) variant.

METHODS

We performed next-generation sequencing for 41 patients with isolated MMA screened by tandem mass spectrometry (MS/MS) and analysed the genetic results. We also evaluated the prognosis of patients with the c.1663G>A variant. We used Jalview software for multispecies sequence alignment and Missense3D and DynaMut to predict the protein function of the detected novel variants.

RESULTS

A total of 43 variants from 41 patients with isolated MMA were detected, of which c.1663G>A (14.63 %), c.729_730insTT (10.98 %), and c.1106G>A (8.53 %) are high-frequency variants of the MMUT gene in the Henan population. The patients carrying the c.1663G>A variant tended to be responsive to vitamin B12, have a low mortality rate. We also identified 5 novel variants (c.479C>T, c.811G>C, c.965T>A, c.1142G>A and c.1667C>T).

CONCLUSION

The rare variant c.1663G>A is prevalent in the Henan population, and infants with this variant tend to have good prognosis. Our findings, especially novel variants, will help broaden the spectrum of genetic variants and facilitate clinical diagnosis and genetic counselling for affected families.

Evaluation of the clinical, biochemical, genotype and prognosis of mut-type methylmalonic acidemia in 365 Chinese cases

BACKGROUND

Methylmalonic acidemia (MMA), which results from defects in methylmalonyl-CoA mutase (mut type) or its cofactor, is the most common inherited organic acid metabolic disease in China. This study aimed to investigate the phenotype and genotype of mut-type MMA in Chinese patients.

METHODS

We recruited 365 patients with mut-type MMA; investigated their disease onset, newborn screening (NBS) status, biochemical metabolite levels, gene variations and prognosis; and explored the relationship between phenotype and genotype.

RESULTS

There were 152 patients diagnosed by tandem mass spectrometry (MS/MS) expanded NBS, 209 patients diagnosed because of disease onset without NBS and 4 cases diagnosed because of sibling diagnosis. The median age of onset was 15 days old, with a variety of symptoms without specificity. Urinary levels of methylmalonic acid and methylcitric acid (MCA) decreased after treatment. Regarding the prognosis, among the 152 patients with NBS, 50.6% were healthy, 30.3% had neurocognitive impairment and/or movement disorders and 13.8% died. Among the 209 patients without NBS, 15.3% were healthy, 45.9% had neurocognitive impairment and/or movement disorders and 33.0% died. In total, 179 variants were detected in the MMUT gene, including 52 novel variations. c.729_730insTT, c.1106G>A, c.323G>A, c.914T>C and c.1663G>A were the five most frequent variations. The c.1663G>A variation led to a milder phenotype and better prognosis.

CONCLUSION

There is a wide spectrum of variations in the MMUT gene with several common variations. Although the overall prognosis of mut-type MMA was poor, participation in MS/MS expanded NBS, vitamin B12 responsive and late onset are favourable factors for the prognosis.

Late-onset cblC defect: clinical, biochemical and molecular analysis

BACKGROUND

cblC defect is the most common type of methylmalonic acidemia in China. Patients with late-onset form (>1 year) are often misdiagnosed due to heterogeneous symptoms. This study aimed to describe clinical characteristics and evaluate long-term outcomes of Chinese patients with late-onset cblC defect.

METHODS

A total of 85 patients with late-onset cblC defect were enrolled. Clinical data, including manifestations, metabolites, molecular diagnosis, treatment and outcome, were summarized and analyzed.

RESULTS

The age of onset ranged from 2 to 32.8 years old (median age 8.6 years, mean age 9.4 years). The time between first symptoms and diagnosis ranged from a few days to 20 years (median time 2 months, mean time 20.7 months). Neuropsychiatric symptoms were presented as first symptoms in 68.2% of cases, which were observed frequently in schoolchildren or adolescents. Renal involvement and cardiovascular disease were observed in 20% and 8.2% of cases, respectively, which occurred with the highest prevalence in preschool children. Besides the initial symptoms, the disease progressed in most patients and cognitive decline became the most frequent symptom overall. The levels of propionylcarnitine, propionylcarnitine / acetylcarnitine ratio, methylmalonic acid, methylcitric acid and homocysteine, were decreased remarkably after treatment (P<0.001). Twenty-four different mutations of MMACHC were identified in 78 patients, two of which were novel. The c.482G>A variant was the most frequent mutated allele in this cohort (25%). Except for 16 patients who recovered completely, the remaining patients were still left with varying degrees of sequelae in a long-term follow-up. The available data from 76 cases were analyzed by univariate analysis and multivariate logistic regression analysis, and the results showed that the time from onset to diagnosis (OR = 1.025, P = 0. 024) was independent risk factors for poor outcomes.

CONCLUSIONS

The diagnosis of late-onset cblC defect is often delayed due to poor awareness of its various and nonspecific symptoms, thus having an adverse effect on the prognosis. It should be considered in patients with unexplained neuropsychiatric and other conditions such as renal involvement, cardiovascular diseases or even multiple organ damage. The c.482G>A variant shows the highest frequency in these patients. Prompt treatment appears to be beneficial.

Newborn screening for inborn errors of metabolism in a northern Chinese population

OBJECTIVES

Newborn screening (NBS) for inborn errors of metabolism (IEMs) has been successfully implemented in China. However, the data on the IEM profiles in many regions are lacking. This study aimed to report the incidence, disease spectrum, and genetic profile of IEMs in northern China.

METHODS

A total of 36,590 newborns were screened using tandem mass spectrometry between January 2016 and April 2022. Newborns with positive results were referred for confirmatory testing.

RESULTS

Ten patients were confirmed to have IEMs, with an overall incidence of 1:3,539 in the Rizhao region. Five types of IEMs were detected, including four patients with propionic acidemia (PA), three patients with methylmalonic acidemia (MMA), one of each with citrin deficiency, primary carnitine deficiency, and isobutyryl-CoA dehydrogenase deficiency. PA was the most common IEM, with an unexpectedly high incidence of 1:8,848, followed by MMA, with an incidence rate of 1:11,797. All patients had abnormal screening markers and harbored biallelic variants in their respective causative genes. Two novel PCCB variants (c.505G>A and c.1123_1124insG) were identified in patients with PA. In silico analyses predicted that these two variants were potentially pathogenic.

CONCLUSIONS

This study preliminarily clarified the incidence, disease spectrum, and genetic profile of IEMs in the Rizhao region. PA is the most common IEM and MMA is the second most common in our region. The two novel identified PCCB variants further expand the variant spectrum of PA. More attention should be paid to NBS, early diagnosis, and management of PA and MA.

Methylmalonic acid levels in serum, exosomes, and urine and its association with cblC type methylmalonic acidemia-induced cognitive impairment

Background

The cblC type methylmalonic acidemia is the most common methylmalonic acidemia (MMA) in China. The biochemical characteristics of this disease include elevated methylmalonic acid and homocysteine (HCY), increased propionylcarnitine (C3), decreased free carnitine (C0). In this study, we aimed to clarify the roles of these biomarkers in cblC-MMA induced cognitive impairment and evaluate the capacity of methylmalonic acid in different fluids or exosomes to distinguish cblC-MMA induced cognitive impairment.

Methods

15 non-inherited hyperhomocysteinemia (HHcy) patients, 42 cblC-MMA patients and 57 age- and sex-matched healthy children were recruited in this study. The levels of HCY were detected by an automatic immune analyzer. The levels of acylcarnitines and methylmalonic acid were detected by tandem mass spectrometer.

Results

The main findings were all biomarkers as HCY, acylcarnitines and methylmalonic acid had capacities for distinguishing patients with cblC-MMA induced cognitive impairment from healthy children. The methylmalonic acid in different fluids or exosomes had good performances for distinguishing patients with cblC-MMA induced cognitive impairment from HHcy patients. The methylmalonic acid in serum exosomes and neuronal-derived exosomes were able to distinguishing cblC-MMA patients with cognitive impairment from patients without cognitive impairment. The methylmalonic acid in neuronal-derived exosomes might be helpful to evaluate the severity of cblC-MMA induced cognitive impairment.

Discussion

Methylmalonic acid levels in serum exosomes, especially in serum neuronal-derived exosomes, serve as potential biomarkers for distinguishing cblC-MMA induced cognitive impairment.

Versatile Enzymology and Heterogeneous Phenotypes in Cobalamin Complementation Type C Disease

Nutritional deficiency and genetic errors that impair the transport, absorption, and utilization of vitamin B₁₂ (B₁₂) lead to hematological and neurological manifestations. The cblC disease (cobalamin complementation type C) is an autosomal recessive disorder caused by mutations and epi-mutations in the MMACHC gene and the most common inborn error of B₁₂ metabolism. Pathogenic mutations in MMACHC disrupt enzymatic processing of B₁₂, an indispensable step before micronutrient utilization by the two B₁₂-dependent enzymes methionine synthase (MS) and methylmalonyl-CoA mutase (MUT). As a result, patients with cblC disease exhibit plasma elevation of homocysteine (Hcy, substrate of MS) and methylmalonic acid (MMA, degradation product of methylmalonyl-CoA, substrate of MUT). The cblC disorder manifests early in childhood or in late adulthood with heterogeneous multi-organ involvement. This review covers current knowledge on the cblC disease, structure–function relationships of the MMACHC protein, the genotypic and phenotypic spectra in humans, experimental disease models, and promising therapies.

Clinical, phenotypic and genetic landscape of case reports with genetically proven inherited disorders of vitamin B12 metabolism: A meta-analysis

Inherited disorders of B₁₂ metabolism produce a broad spectrum of manifestations, with limited knowledge of the influence of age and the function of related genes. We report a meta-analysis on 824 patients with a genetically proven diagnosis of an inherited disorder of vitamin B₁₂ metabolism. Gene clusters and age categories are associated with patients' manifestations. The "cytoplasmic transport" cluster is associated with neurological and ophthalmological manifestations, the "mitochondrion" cluster with hypotonia, acute metabolic decompensation, and death, and the "B₁₂ availability" and "remethylation" clusters with anemia and cytopenia. Hypotonia, EEG abnormalities, nystagmus, and strabismus are predominant in the younger patients, while neurological manifestations, such as walking difficulties, peripheral neuropathy, pyramidal syndrome, cerebral atrophy, psychiatric disorders, and thromboembolic manifestations, are predominant in the older patients. These results should prompt systematic checking of markers of vitamin B₁₂ status, including homocysteine and methylmalonic acid, when usual causes of these manifestations are discarded in adult patients.

Identification of MMACHC and ZEB2 mutations causing coexistent cobalamin C disease and Mowat-Wilson syndrome in a 2-year-old girl

Cobalamin C (cblC) disease and Mowat-Wilson syndrome (MWS) are rare hereditary diseases. To date, there have been no reports of people suffering from these two genetic diseases, or whether there is any correlation between the two diseases. We reported a 2-year-old girl with both cblC disease and MWS. The patient initially manifested as slow weight gain, hypotonia, broad nasal bridge, high forehead, high palate arch, ear crease, patent ductus arteriosus, atrial and ventricular septal defect and bilateral mild ventriculomegaly in the neonatal period. However, as the baby grew older, the typical facial features became more prominent, and overall developmental delays were noted at the subsequent follow-up, with the motor and cognitive development significantly lagging behind that of other children of the same age. At 26 days old, laboratory tests revealed remarkably elevated levels of serum homocysteine, C3/C2 and urine organic acid. Whole-exome sequencing detected compound heterozygous variants in MMACHC, including one previously reported mutation [c.609G > A (p.W203X) and a novel missense mutation[ c.643 T > C (p.Y215H)]. The computer simulations of the protein structure analysis of the novel missense mutation showed the variant p.Y215H replaced a neutral amino acid with a strongly basic lysine, which broken the local structure by changing the carbon chain skeleton and decreasing the interaction with adjacent amino acid. This is expected to damage the utilization of vitamin B12 and influence the synthesis of AdoCbl and MeCbl, contributing to its pathogenicity. Thus, clinical and genetic examinations confirmed the cblC disease. Another heterozygous variant in ZEB2 [NM_014795; loss1(exon:2-10)(all); 127901 bp] was detected by whole-exome sequencing. The heterozygous 3.04 Mb deletion in EB2 [GRCH37]del(2)(q22.2q22.3) (chr2:142237964-145274917) was also confirmed by genome-wide copy number variations (CNVs) scan, which was pathogenic and led to the diagnosis of Mowat-Wilson syndrome. The biochemical indicators associated with cblC disease in the patient were well controlled after treatment with vitamin B12 and betaine. Here, a patient with coexisting cblC disease and MWS caused by different pathogenic genes was reported, which enriched the clinical research on these two rare genetic diseases.

Inherited defects of cobalamin metabolism

Cobalamin (vitamin B₁₂) is required for activity of the enzymes methylmalonyl-CoA mutase and methionine synthase in human cells. Inborn errors affecting cobalamin uptake or metabolism are characterized by accumulation of the substrates for these enzymes, methylmalonic acid and homocysteine, in blood and urine. Inborn errors affecting synthesis of the adenosylcobalamin coenzyme required by methylmalonyl-CoA mutase (cblA and cblB) result in isolated methylmalonic aciduria; inborn errors affecting synthesis of the methylcobalamin coenzyme required by methionine synthase (cblE and cblG) result in isolated homocystinuria. Combined methylmalonic aciduria and homocystinuria is seen in patients with impaired intestinal cobalamin absorption (intrinsic factor deficiency, Imerslund-Gräsbeck syndrome) and with defects affecting synthesis of both cobalamin coenzymes (cblC, cblD, cblF and cblJ). A series of disorders caused by pathogenic variant mutations affecting gene regulators (transcription factors) of the MMACHC gene have recently been described (HCFC1 [cblX disorder] and deficiencies of THAP11, and ZNF143 [the cblK disorder]).

Recent research on inherited metabolic diseases in children

With the improvement in the research level and the diagnosis and treatment technology of inherited metabolic diseases (IMD), the research on pediatric IMD in China has made great progress, but there is still some distance from the international level. Due to the vast territory of China and the uneven distribution of medical resources, the regional characteristics of IMD remain unclear in China, and there are many problems and difficulties in early diagnosis and treatment. Therefore, it is necessary to improve the understanding of pediatric IMD among pediatricians, so as to improve the diagnosis and treatment level, achieve an early identification, diagnosis, and treatment of pediatric IMD, and effectively reduce the fatality and disability rates of children with IMD. This article reviews the research progress of IMD in children in China, and analyzes the features of representative IMDs. Citation:Chinese Journal of Contemporary Pediatrics, 2022, 24(3): 326-331.

The Follow-Up of Chinese Patients in cblC Type Methylmalonic Acidemia Identified Through Expanded Newborn Screening

Objective: The cblC type of combined methylmalonic acidemia and homocystinuria, an inherited disorder with variable phenotypes, is included in newborn screening (NBS) programs at multiple newborn screening centers in China. The present study aimed to investigate the long-term clinical benefits of screening individual.

Methods: A national, retrospective multi-center study of infants with confirmed cblC defect identified by NBS between 2004 and 2020 was conducted. We collected a large cohort of 538 patients and investigated their clinical data in detail, including disease onset, biochemical metabolites, and gene variation, and explored different factors on the prognosis.

Results: The long-term outcomes of all patients were evaluated, representing 44.6% for poor outcomes. In our comparison of patients with already occurring clinical signs before treatment to asymptomatic ones, the incidence of intellectual impairment, movement disorders, ocular complications, hydrocephalus, and death were significantly different (p < 0.01). The presence of disease onset [Odd ratio (OR) 12.39, 95% CI 5.15–29.81; p = 0.000], variants of c.609G>A (OR 2.55, 95% CI 1.49–4.35; p = 0.001), and c.567dupT (OR 2.28, 95% CI 1.03–5.05; p = 0.042) were independently associated with poor outcomes, especially for neurodevelopmental deterioration.

Conclusion: NBS, avoiding major disease-related events and allowing an earlier treatment initiation, appeared to have protective effects on the prognosis of infants with cblC defect.

Noninvasive Prenatal Testing of Methylmalonic Acidemia cblC Type Using the cSMART Assay for MMACHC Gene Mutations

Noninvasive prenatal testing (NIPT) for monogenic disorders has been developed in recent years; however, there are still significant technical and analytical challenges for clinical use. The clinical feasibility of NIPT for methylmalonic acidemia cblC type (cblC type MMA) was investigated using our circulating single-molecule amplification and re-sequencing technology (cSMART). Trios molecular diagnosis was performed in 29 cblC type MMA-affected children and their parents by traditional Sanger sequencing. In the second pregnancy, invasive prenatal diagnosis (IPD) of the pathogenic MMACHC gene was used to determine fetal genotypes, and NIPT was performed using a novel MMACHC gene–specific cSMART assay. Maternal–fetal genotypes were deduced based on the mutation ratio in maternal plasma DNA. Concordance of fetal genotypes between IPD and NIPT, and the sensitivity and specificity of NIPT were determined. After removing two cases with a low P value or reads, the concordance ratio for NIPT and IPD was 100.00% (27/27), and the sensitivity and specificity were 100.00% (54.07–100.00%) and 100.00% (83.89–100.00%), respectively. This study demonstrates that NIPT using the cSMART assay for cblC type MMA was accurate in detecting fetal genotypes. cSMART has a potential clinical application as a prenatal diagnosis and screening tool for carrier and low-risk genotypes of cblC type MMA and other monogenic diseases.

Case Report: A Case of Late-Onset Combined Methylmalonic Acidemia and Hyperhomocysteinemia Induced by a Vegetarian Diet

Methylmalonic acidemia is a rare autosomal recessive metabolic disease. However, because of the atypical clinical symptoms, the type of late-onset methylmalonic academia is often misdiagnosed. Especially when the blood vitamin B12 and folic acid levels are normal, it is not easy to think of this disease. Herein we report a 9-year-old girl who developed normally on a relatively balanced diet before 7 years of age. However, she presented with fatigue and attention deficit when she followed a vegetarian diet. Laboratory examination showed moderate macrocytic anemia, high levels of homocysteine, high level of propionylcarnitine/acetylcarnitine, urinary methylmalonic acid and methyl citrate. Gene mutation analysis showed c.609G > A and c.80A > G compound heterozygous mutations in the MMACHC gene, supported late-onset combined methylmalonic academia with homocysteinemia. Then treatment performed with add meat to the diet, vitamin B12, folic acid betaine and L-carnitine supplement. One week later, the child's clinical symptoms and the laboratory examinations were significantly improved.

Application of the Artificial Intelligence Algorithm Model for Screening of Inborn Errors of Metabolism

Inborn errors of metabolism (IEMs) are strongly related to abnormal growth and development in newborns and can even result in death. In total, 94,648 newborns were enrolled for expanded newborn screening using tandem mass spectrometry (MS/MS) from 2016 to 2020 at the Neonatal Disease Screening Center of the Maternal and Child Health Hospital in Shaoyang City, China. A total of 23 confirmed cases were detected in our study with an incidence rate of 1:4,115. A total of 10 types of IEM were identified, and the most common IEMs were phenylalanine hydroxylase deficiency (PAHD; 1:15,775) and primary carnitine deficiency (PCD; 1:18,930). Mutations in phenylalanine hydroxylase (PAH) and SLC22A5 were the leading causes of IEMs. To evaluate the application effect of artificial intelligence (AI) in newborn screening, we used AI to retrospectively analyze the screening results and found that the false-positive rate could be decreased by more than 24.9% after using AI. Meanwhile, a missed case with neonatal intrahepatic cholestasis citrin deficiency (NICCD) was found, the infant had a normal citrulline level (31 μmol/L; cutoff value of 6-32 μmol/L), indicating that citrulline may not be the best biomarker of intrahepatic cholestasis citrin deficiency. Our results indicated that the use of AI in newborn screening could improve efficiency significantly. Hence, we propose a novel strategy that combines expanded neonatal IEM screening with AI to reduce the occurrence of false positives and false negatives.

Genetic testing is necessary for correct diagnosis and treatment in patients with isolated methylmalonic aciduria: a case report

Background

Isolated methylmalonic aciduria can be caused by pathogenic mutations in the gene for methylmalonyl-CoA mutase or in the genes encoding enzymes involved in the intracellular metabolism of cobalamin. Some of these mutations may be cobalamin responsive. The type of methylmalonic aciduria cannot always be assumed from clinical manifestation and the responsiveness to cobalamin has to be assessed for appropriate cobalamin administration, or to avoid unnecessary treatment. The cases presented herein highlight the importance of genetic testing in methylmalonic aciduria cases and the need for standardisation of the in vivo cobalamin-responsiveness assessment.

Case presentation

We describe two patients who presented in the first week of life with rapid neurological deterioration caused by metabolic acidosis with severe hyperammonaemia requiring extracorporeal elimination in addition to protein restriction, energy support, carnitine, and vitamin B12 treatment. The severity of the clinical symptoms and high methylmalonic acid concentrations in the urine (>30,000 μmol/mmol of creatinine) without hyperhomocysteinaemia in both of our patients suggested isolated methylmalonic aciduria. Based on the neonatal manifestation and the high methylmalonic acid urine levels, we assumed the cobalamin non-responsive form. The in vivo test of responsiveness to cobalamin was performed in both patients. Patient 1 was evaluated as non-responsive; thus, intensive treatment with vitamin B12 was not used. Patient 2 was responsive to cobalamin, but the dose was decreased to 1 mg i.m. every two weeks with daily oral treatment due to non-compliance. Genetic tests revealed bi-allelic mutations in the genes MMAB and MMAA in Patient 1 and 2, respectively. Based on these results, we were able to start intensive treatment with hydroxocobalamin in both patients. After the treatment intensification, there was no acute crisis requiring hospitalisation in Patient 1, and the urine methylmalonic acid levels further decreased in Patient 2.

Conclusions

Despite carrying out the in vivo test of responsiveness to cobalamin in both patients, only the results of molecular genetic tests led us to the correct diagnosis and enabled intensive treatment with hydroxocobalamin. The combination of the standardized in vivo test of cobalamin responsiveness and genetic testing is needed for accurate diagnosis and appropriate treatment of isolated methylmalonic aciduria.

Preimplantation Genetic Testing for Rare Inherited Disease of MMA-CblC: an Unaffected Live Birth

Methylmalonic acidemia combined with homocysteinemia and cobalamin C type (MMA-CblC, MIM # 277400) is a rare inherited disease with cobalamin metabolic disorder, which are caused by deficiency in the MMACHC gene. A couple with a proband child carried with compound heterozygous mutations of MMACHC (c.609G>A and c.567 dup T, NM_015506) sought for assisted reproductive technology to avoid the transmission of pathogenic genetic variants and unnecessary induction of labor. Thus, in vitro fertilization (IVF), preimplantation genetic testing (PGT), and prenatal genetic diagnosis were applied to fulfill this clinical demand. In this study, seven embryos were biopsied and carried out whole-genome amplification using multiple annealing and looping-based amplification cycle (MALBAC) method. Sanger sequencing together with copy number variation (CNV) analysis and single-nucleotide polymorphism (SNP) haplotyping was conducted to detect the mutated alleles and chromosomal abnormalities simultaneously. Three embryos (E07, E06, and E02) were confirmed without CNVs and inherited mutations at MMACHC gene. Embryo E07 with the best embryo ranking of 5BB was selected preferentially to transfer which led to a successful pregnancy and an unaffected live birth. Prenatal genetic diagnosing with amniotic fluid cells, Sanger sequencing with cord blood cells, and neonate MMA screening further verified our successful application of PGT in preventing mutated allele transmission for this rare inherited disease.

Value of amniotic fluid homocysteine assay in prenatal diagnosis of combined methylmalonic acidemia and homocystinuria, cobalamin C type

BACKGROUND

Combined methylmalonic acidemia and homocystinuria, cobalamin C type (cblC defect) is the most common inborn error of cobalamin metabolism, and different approaches have been applied to its prenatal diagnosis. To evaluate the reliability of biochemical method for the prenatal diagnosis of cblC defect, we conducted a retrospective study of our 10-year experience at a single center.

METHODS

248 pregnancies whose probands were diagnosed as cblC defect were referred to our center for prenatal diagnosis from January 2010 to December 2019. Prenatal data of Hcy levels determined by enzymatic cycling assay, acylcarnitine analysis using liquid chromatography tandem mass spectrometry, organic acid analysis using gas chromatography mass spectrometry, and genetic analysis by direct sequencing of 248 at-risk fetuses were retrospectively reviewed.

RESULTS

For 2.0 and 16.0 μmol/L levels of Hcy AF samples, the relative errors were - 2.5% and 2.8%, respectively. The respective measurement uncertainties were 13.07% and 14.20%. For the 248 at-risk fetuses, 63 fetuses were affected and 185 fetuses were unaffected. Hcy level of 13.20 (6.62-43.30) μmol/L in 63 affected fetuses was significantly higher than that in 185 unaffected fetuses of 2.70 (0.00-5.80) μmol/L, and there was no overlap between the affected and unaffected groups. The diagnostic sensitivity and specificity of Hcy were 100% and 92.05%, respectively. The positive and negative predictive values of the combination of Hcy, propionylcarnitine (C3), ratio of C3 to acetylcarnitine (C2; C3/C2), methylmalonic acid (MMA), and methylcitric acid (MCA) were both 100%. Sixteen fetuses displayed inconclusive genetic results of MMACHC variants, in which seven fetuses were determined to be affected with elevated levels of Hcy, C3, C3/C2 and MMA, and their levels were 18.50 (6.70-43.30) μmol/L, 8.53(5.02-11.91) μmol/L, 0.77 (0.52-0.97), 8.96 (6.55-40.32) mmol/mol Cr, respectively. The remaining nine fetuses were considered unaffected based on a normal amniotic fluid metabolite profile.

CONCLUSIONS

Hcy appears to be another characteristic biomarker for the prenatal diagnosis of cblC defect. The combination of Hcy assay with acylcarnitine and organic acid analysis is a fast, sensitive, and reliable prenatal diagnostic biochemical approach. This approach could overcome the challenge of the lack of genetic analysis for families with at-risk cblC defect fetuses.

A high frequency and geographical distribution of MMACHC R132* mutation in children with cobalamin C defect

Cobalamin C defect is caused by pathogenic variants in the MMACHC gene leading to impaired conversion of dietary vitamin B₁₂ into methylcobalamin and adenosylcobalamin. Variants in the MMACHC gene cause accumulation of methylmalonic acid and homocysteine along with decreased methionine synthesis. The spectrum of MMACHC gene variants differs in various populations. A total of 19 North Indian children (age 0-18 years) with elevated methylmalonic acid and homocysteine were included in the study, and their DNA samples were subjected to Sanger sequencing of coding exons with flanking intronic regions of MMACHC gene. The genetic analysis resulted in the identification of a common pathogenic nonsense mutation, c.394C > T (R132*) in 85.7% of the unrelated cases with suspected cobalamin C defect. Two other known mutations c.347T > C (7%) and c.316G > A were also detected. Plasma homocysteine was significantly elevated (> 100 µmol/L) in 75% of the cases and methionine was decreased in 81% of the cases. Propionyl (C3)-carnitine, the primary marker for cobalamin C defect, was found to be elevated in only 43.75% of cases. However, the secondary markers such as C3/C2 and C3/C16 ratios were elevated in 87.5% and 100% of the cases, respectively. Neurological manifestations were the most common in our cohort. Our findings of the high frequency of a single MMACHC R132* mutation in cases with combined homocystinuria and methylmalonic aciduria may be proven helpful in designing a cost-effective and time-saving diagnostic strategy for resource-constraint settings. Since the R132* mutation is located near the last exon-exon junction, this is a potential target for the read-through therapeutics.

Spectrum analysis of inborn errors of metabolism for expanded newborn screening in a northwestern Chinese population

Expanded newborn screening facilitates early identification and intervention of patients with inborn errors of metabolism (IEMs), There is a lack of disease spectrum data for many areas in China. To determine the disease spectrum and genetic characteristics of IEMs in Xi'an city of Shaanxi province in northwest China, 146152 newborns were screening by MSMS from January 2014 to December 2019 and 61 patients were referred to genetic analysis by next generation sequencing (NGS) and validated by Sanger sequencing. Seventy-five newborns and two mothers were diagnosed with IEMs, with an overall incidence of 1:1898 (1:1949 without mothers). There were 35 newborns with amino acidemias (45.45%, 1:4176), 28 newborns with organic acidurias (36.36%, 1:5220), and 12 newborns and two mothers with FAO disorders (18.18%; 1:10439 or 1:12179 without mothers). Phenylketonuria and methylmalonic acidemia were the two most common disorders, accounting for 65.33% (49/75) of all confirmed newborn. Some hotspot mutations were observed for several IEMs, including PAH gene c.728G>A for phenylketonuria; MMACHC gene c.609G>A and c.567dupT, MMUT gene c.323G>A for methylmalonic acidemia and SLC25A13 gene c.852_855del for citrin deficiency. Our study provides effective clinical guidance for the popularization and application of expanded newborn screening, genetic screening, and genetic counseling of IEMs in this region.

A rare mutation c.1663G > A (p.A555T) in the MMUT gene associated with mild clinical and biochemical phenotypes of methylmalonic acidemia in 30 Chinese patients

Background

Methylmalonic acidemia is an inherited organic acid metabolic disease. It involves multiple physiological systems and has variable manifestations. The primary causative gene MMUT carries wide range of mutations, and one of them, c.1663G > A (p.A555T), is considered to be a rare type, which is seen more frequently in Asian than other populations. So far, little is known about the clinical features of patients carrying this mutation. In the present study, we aimed to define the clinical and biochemical features of the patients with this genotype.

Methods

Among 328 mut type methylmalonic acidemia patients from multiple hospitals in China, we collected 30 compound heterozygous patients sharing the mutation c.1663G > A (p.A555T) in the MMUT gene. Their clinical characteristics and biochemical index were described in detail and compared with methylmalonic acidemia patients without this variant.

Results

Most of these patients were diagnosed via newborn screening (26/30), treated in a timely manner, and kept healthy (24/30). Disease onset occurred in 7 patients. Developmental delay or intellectual impairment occurred in 4 patients. 100% of these patients (29/29) were responsive to Vitamin B12 administration. The blood propionylcarnitine, blood propionylcarnitine/acetylcarnitine ratio, urinary methylmalonic acid, urinary methylcitric acid before and after treatment in c.1663G > A (p.A555T) carrying patients were much lower than those in non-c.1663G > A (p.A555T) carrying patients.

Conclusion

Compared to patients with other mutations in the MMUT gene, patients with the c.1663G > A (p.A555T) mutation showed later onset, milder clinical phenotype, lighter biochemical abnormalities, better vitamin B12 responsiveness, lower morbidity, easier metabolic control, and thereby better prognosis. Newborn screening project plays an important role in early diagnosis, treatment, and prognosis of these patients.

Analysis of 70 patients with hydrocephalus due to cobalamin C deficiency

OBJECTIVE

To analyze the clinical characteristics of patients with hydrocephalus secondary to cobalamin C (cblC) deficiency and to discuss the optimal strategies for assessing and treating such patients by performing clinical and laboratory studies in 70 patients.

METHODS

A total of 1,211 patients were clinically diagnosed with methylmalonic acidemia (MMA) from 1998 to 2019. Among them, cblC deficiency was confirmed in 70 patients with hydrocephalus by brain imaging and biochemical and genetic analysis.

RESULTS

Of the 70 patients, 67 (95.7%) had early-onset MMA and homocystinuria. The patients typically had high blood propionylcarnitine and total homocysteine, low methionine, and methylmalonic aciduria. Signs of intracranial hypertension were relatively rare. We measured ventricular dilatation early in the disease by cranial ultrasound and MRI and/or CT. Eighteen different MMACHC mutations, including 4 novel mutations (c.427C>T, c.568insT, c.599G>A, and c.615C>A), were identified biallelically in all 70 patients. c.609G>A was the most frequent mutation, followed by c.658_660del, c.217C>T, and c.567dupT. Three cases were diagnosed by postmortem study. Metabolic therapy, including cobalamin injections supplemented with oral l-carnitine and betaine, was administered in the remaining 67 cases. A ventriculoperitoneal shunt was performed in 36 cases. During the follow-up, psychomotor development, nystagmus, impaired vision, and sunset eyes improved gradually.

CONCLUSION

Hydrocephalus is a severe condition with several different causes. In this study, ventriculomegaly was found in 70 patients with cblC deficiency. Early diagnosis, etiologic treatment, and prompt surgical intervention are crucial to improve the prognosis of patients.

Mutation analysis, treatment and prenatal diagnosis of Chinese cases of methylmalonic acidemia

Methylmalonic acidemia (MMA)-affected patients may have developmental, hematological, neurological, metabolic, ophthalmological, and dermatological clinically abnormal findings. This study aimed to identify mutations in 13 Chinese MMA cases. We provided genetic counseling, treatment, and prenatal diagnosis for the families with MMA. Liquid chromatography-tandem mass spectrometry (LC–MS/MS) was performed and the results were confirmed by gas chromatography and mass spectrometry (GC/MS). Variant screening in probands was performed by targeted next-generation sequencing. Identified variants were confirmed by Sanger sequencing. Of these 13 MMA cases, seven were isolated MMA, and among them, six were caused by variants in MMUT and one was caused by a variant in MCEE. The other six cases were MMA with homocystinuria, which was caused by variants in MMACHC. We found six novel variants in three MMA-causing genes as follows: c.2008G>A, c.301_302insTA, c.984delC, and c.319A>T of MMUT; c.445T>C of MMACHC; and c.296T>C of MCEE. We provided prenatal diagnosis for two families with MMA at their next pregnancy, and one family had a healthy newborn. In conclusion, our findings expand the spectrum of genotypes in MMA. Effective genetic counseling is required to allow awareness of the patients’ families that MMA disease is treatable and a good prognosis can be obtained.

A study on a cohort of 301 Chinese patients with isolated methylmalonic acidemia

Methylmalonic acidemia (MMA) is the most common organic acidemia in China. This study aimed to characterise the genotypic and phenotypic variabilities, and the molecular epidemiology of Chinese patients with isolated MMA. Patients (n = 301) with isolated MMA were diagnosed by clinical examination, biochemical assays, and genetic analysis. Fifty-eight patients (19.3%) were detected by newborn screening and 243 patients (80.7%) were clinically diagnosed after onset. Clinical onset ranged from the age of 3 days to 23 years (mean age = 1.01 ± 0.15 years). Among 234 MMA patients whose detailed clinical data were available, 170 (72.6%) had early onset disease (before the age of 1 year), and 64 (27.4%) had late-onset disease. The 234 MMA patients manifested with neuropsychiatric impairment (65.4%), haematological abnormality (31.6%), renal damage (8.5%), and metabolic crises (67.1%). Haematological abnormality was significantly more common in early-onset patients than that in late-onset patients. The incidence of metabolic crises was significantly high (P < 0.001) in patients with mut type than those with other types of isolated MMA. Variations (n = 122) were identified in MMUT, MMAA, MMAB, MMADHC, SUCLG1, and SUCLA2, of which 45 were novel. c.729_730insTT was the most frequent MMUT mutation, with a significantly higher frequency in our patients than that in 151 reported European patients. The frequency of c.914T>C in MMUT in our cohort was also higher than that in 151 European patients. MMUT mutations c.729_730insTT and c.914T>C are specific for the Chinese population. Our study expanded the spectrum of phenotypes and genotypes in isolated MMA.

Rapid screening of MMACHC gene mutations by high-resolution melting curve analysis

Background

Cobalamin (cbl) C is a treatable rare hereditary disorder of cbl metabolism with autosomal recessive inheritance. It is the most common organic acidemia, manifested as methylmalonic academia combined with homocysteinemia. Early screening and diagnosis are important. The mutation spectrum of the MMACHC gene causing cblC varies among populations. The mutation spectrum in Chinese population is notably different from that in other populations.

Methods

A PCR followed by high‐resolution melting curve analysis (PCR‐HRM) method covering all coding exons of MMACHC gene was designed to verify 14 pathogenic MMACHC gene variants found in patients with cblC, including all common mutations in Chinese patients with cblC.

Result

By PCR‐HRM analysis, 14 pathogenic variants of MMACHC showed distinctly different melting curves, which were consistent with Sanger sequencing. The homozygous type of the most common mutation c.609G > A (p.Trp203Ter) can also be analyzed by specially designed PCR‐HRM.

Conclusion

The established PCR‐HRM method for screening common pathogenic MMACHC variants in Chinese patients with cblC has the advantages of high accuracy, high throughput, low cost, and high speed. It is suitable for the large‐sample screening of suspected children with methylmalonic acidemia and carriers in population.

Targeted SLC19A3 gene sequencing of 3000 Saudi newborn: a pilot study toward newborn screening

Background

Biotin–thiamine‐responsive basal ganglia disease (BTBGD) is an autosomal recessive neurometabolic disorder mostly presented in children. The disorder is described as having subacute encephalopathy with confusion, dystonia, and dysarthria triggered by febrile illness that leads to neuroregression and death if untreated. Using biotin and thiamine at an early stage of the disease can lead to significant improvement.

Methods

BTBGD is a treatable disease if diagnosed at an early age and has been frequently reported in Saudi population. Keeping this in mind, the current study screened 3000 Saudi newborns for the SLC19A3 gene mutations using target sequencing, aiming to determine the carrier frequency in Saudi Population and whether BTBGD is a good candidate to be included in the newborn‐screened disorders.

Results

Using targeted gene sequencing, DNA from 3000 newborns Saudi was screened for the SLC19A3 gene mutations using standard methods. Screening of the SLC19A3 gene revealed a previously reported heterozygous missense mutation (c.1264A>G (p.Thr422Ala) in six unrelated newborns. No probands having homozygous pathogenic mutations were found in the studied cohort. The variant has been frequently reported previously in homozygous state in Saudi population, making it a hot spot mutation. The current study showed that the carrier frequency of SLC19A3 gene mutation is 1 of 500 in Saudi newborns.

Conclusion

For the first time in the literature, we determined the carrier frequency of SLC19A3 gene mutation in Saudi population. The estimated prevalence is too rare in Saudi population (at least one in million); therefore, the data are not in favor of including such very rare disorders in newborn screening program at population level. However, a larger cohort is needed for a more accurate estimate.