Key Clinical Message
Congenital Methemoglobinemia is a rare neurologic condition which can mimic other diseases such as epilepsy syndromes and leukodystrophies. The responsible gene, CYB5R3, is not typically included on commonly order neurologic and epilepsy panels. We recommend that laboratories include this gene on these tests which often precede larger‐scale genetic studies.
Keywords: Cyanosis, CYB5R3, developmental delay, leukodystrophy, methemoglobin reductase, methemoglobinemia type II, microcephaly
Introduction
Congenital methemoglobinemia (RCM) (MIM #250800) is a rare, autosomal recessive condition associated with alterations in the CYB5R3 gene, located on chromosome 22q13 and containing nine exons and eight introns 3. Mutations in the CYB5R3 gene ultimately lead to decreased or nearly absent activity of the NADH‐cytochrome b5 reductase enzyme. Hemoglobin M disease and cytochrome b5 reductase deficiency are alternate names for inherited methemoglobinemia type II, each reflecting decreased or lack of the cytochrome b5 reductase enzyme and the associated clinical symptoms.
Determining the severity of the enzyme deficiency is one of the primary diagnostic indicators for RCM 21. NADH‐cytochrome b5 reductase is present in an erythrocytic form as well as a membrane‐bound form on mitochondrial and endoplasmic reticulum membranes and is responsible for methemoglobin reduction 13, 15. There are two main forms of RCM (Types I and II) that can develop based on the type of mutation in the CYB5R3 gene and the resulting protein change. Type I RCM has been found to be associated with enzyme deficiency localized only to the erythrocytes, while Type II RCM involves enzyme deficiency in all tissues 6. The localization of the NADH‐cytochrome b5 reductase enzyme deficiency to red blood cells in Type I RCM leads to a less severe form of RCM characterized by cyanosis without neurologic impairment. This form can also be treated more easily than Type II RCM 1. Leroux et al. 15 demonstrated that cytochrome b5 reductase was deficient in cells from a variety of tissues, including red blood cells, leukocytes, muscle, liver, and fibroblasts in a patient with methemoglobinemia and mental retardation. Essentially, this study demonstrated the first correlation between patients with methemoglobinemia associated with neurologic impairment and an enzyme deficiency in all cell types. It was proposed that the deficiency of cytochrome b5 reductase in all cell types leads to abnormalities with fatty acid metabolism, which then leads to neurologic impairments in these patients 15.
We report here a male proband who initially presented to genetics at 6 months of age with concerns of a movement disorder. Subsequent visits to genetics revealed acquired microcephaly, developmental delay, oral aversion, seizures, dystonia, and white matter changes on brain imaging. After an extensive workup, including exome sequencing, his diagnosis remained unknown. After an acute illness at around the age of 3 years for nonhypoxic cyanosis, the concern for methemoglobinemia was raised. Reanalysis of exome sequencing and further testing revealed a previously reported pathogenic nonsense variant 1 and a partial intronic deletion (in trans). To our knowledge, this report introduces the first genetically confirmed case of RCM type II in the United States. In addition, this case introduces another novel deletion in the CYB5R3 gene that has not yet been reported to cause either type of RCM. We report the proband's diagnostic odyssey in order to elucidate the genetic and clinical features of RCM type II in order to assist clinicians with similar patient presentations. A brief review of common epilepsy and neurologic panels do not appear to have CYB5R3 on these tests. We would recommend that this gene be included in some of these frequently ordered panels as they are often a prerequisite to more expensive and broad tests such as exome sequencing.
Clinical Report
The proband (Fig. 1) was initially referred to medical genetics at 6 months of age due to a possible movement disorder. He was the product of a full‐term gestation born to his 30‐year‐old G3P1‐2 mother after a relatively uncomplicated pregnancy. At birth, the proband weighed 3.4 kg (75th centile), with a length of 50 cm (75th centile), and had a head circumference of 36.5 cm (>90th centile). Although the patient was discharged at 2 days of age, he was readmitted at 8 days of life due to feeding difficulty and frequent vomiting. At 2 months of age, the feeding difficulties continued and were accompanied by poor weight gain, lack of a social smile, and hypotonia. He was admitted to the hospital again at the age of 3 months because of frequent vomiting and failure to thrive. At this point, his head circumference was below the third percentile. Nasogastric feeds were initiated. At 5 months, a brain MRI was obtained and showed prominent extra‐axial fluid spaces and supratentorial sulci, enlarged lateral and third ventricles, and decreased white matter volume. A gastrostomy tube was placed at 6 months and was eventually converted to a gastrojejunostomy tube at 7 months. In addition, the proband had staring spells, hyperkinetic movements, spasticity of the lower extremities, and dystonic movements. Torticollis was also noted at this time. By 10 months, the proband was completely averse to any oral stimulation, including sucking on a pacifier.
Figure 1.
The proband at ages 1 month (A), 13 months (B), and 2 years (C). Note progressive microcephaly.
At 12 months, the proband was diagnosed with dysmotility and visceral hyperalgesia. A repeat brain MRI performed at 13 months noted white matter atrophy and further volume loss. At 17 months of age, he was diagnosed with infantile spasm‐type seizures based on abnormal EEG findings, which included disorganization, discontinuity, electrodecrement, and multifocal and generalized spikes and spike‐wave discharges. He was diagnosed with Lennox–Gastaut Syndrome due to the frequency of spastic movements. His seizures were controlled with a ketogenic diet which was introduced at age 28 months.
When he was seen by medical genetics at 3 years and 5 months, he was having four to ten seizures a day that lasted 5–10 min each. In addition, he was not meeting any developmental milestones other than following faces and turning to sounds. He was able to roll and sit by 12 months, but he lost these abilities, along with head control, when the seizures began at 17 months.
Shortly prior to that evaluation in clinic, at 34 months, the proband was hospitalized due to a 2‐week period of nonhypoxic cyanosis. At one point, during a blood draw, the intensivist caring for the proband noted that his blood appeared “chocolate‐colored.” This raised concern for possible methemoglobinemia. Methemoglobin reductase levels were ordered and found to be deficient (<2.6 U/g, normal 6.6–13.3 U/g).
At 3 years and 5 months, the proband was seen at the National Institutes of Health Clinical Center. A brain MRI (Fig. 2) performed at the hospital where he was seen previously was reviewed and showed progressive degeneration of the brain, with both cerebral and cerebellar atrophy, in addition to ventriculomegaly and enlargement of the subarachnoid spaces. Additionally, white matter hypomyelination was present. Developmentally, he was felt to function at the level of a 6 months old. Range of motion was full, but limb strength and tone were described as fluctuating in the upper limbs, trunk, and lower limbs. There was decreased muscle mass, appendicular hypertonia, and axial hypotonia. The proband was also noted to have uncontrolled and rather spastic movements, which were described as myoclonic jerks. In addition to these muscular abnormalities, the proband also had an anterior rib prominence on the left side, dextroscoliotic curvature of the midthoracic region, and levoscoliotic curvature of the upper lumbar region. Ophthalmologic examinations revealed delayed visual maturation, cortical visual impairment, and hyperopic astigmatism. He was reported to have normal hearing, but reduced middle ear system mobility bilaterally.
Figure 2.
Axial (top) and sagittal (bottom) MRI views of the proband's brain performed at 5 months of age (A, B), 13 months of age (C, D), and 19 months of age (E, F) demonstrating diffuse, progressive cerebral white matter volume loss.
Up until this point, extensive genetic testing had been completed and included whole‐exome sequencing, an epilepsy panel, metabolic studies, and mitochondrial genome panels. The epilepsy and mitochondrial panels returned negative. Whole‐exome sequencing was performed at 14 months of age prior to the episode of cyanosis and was initially nondiagnostic.
The proband's clinical diagnosis including cyanosis, “chocolate‐colored” blood, and neurologic symptoms, along with negative genetic testing results led physicians to perform follow‐up targeted analysis of the CYB5R3 gene at age 34 months. These results ultimately were felt to be consistent with the diagnosis of RCM Type II.
Genomic DNA was extracted from whole blood from the proband, mother, and father. Exome sequencing (ES) was performed on exon targets isolated by capture using the Agilent SureSelect Human All Exon V4 (50 Mb) kit (Agilent Technologies, Santa Clara, CA). The sequencing methodology and variant interpretation protocol have been previously described 27. Data were later reanalyzed after being initiated as part of routine clinical care.
The exome sequencing initially performed at 14 months identified three maternally inherited heterozygous variants in SUMF1, BRAT1, and SUCLG2. Exome sequencing at 30 months uncovered a paternally inherited KCNA1 variant. Reanalysis of exome sequencing data after the episode of nonhypoxic cyanosis revealed a previously reported nonsense variant in the CYB5R3 gene (p.R160X). A follow‐up deletion/duplication assay revealed a heterozygous partial deletion of intron 1 of the CYB5R3 gene. This deletion is based on the NM−_000398.6 transcript. The deletion includes noncoding exons in the following transcripts: NM_001129819, NM_007326, and NM_001171661. This deletion also includes coding exon 1 in the NM_001171660 transcript. Samples from the mother and father were analyzed for this deletion, and the mother was found to be heterozygous for this partial deletion of intron 1 in the CYB5R3 gene. The father was negative for the partial deletion. To our knowledge, this deletion has not been reported previously as a cause for either form of RCM.
Discussion
We report here a proband with microcephaly, seizures, dystonia, and white matter changes who ultimately was diagnosed with RCM II after a long diagnostic workup and a relatively incidental finding of the classic “chocolate” appearance to his blood after an episode of nonhypoxic cyanosis. To our knowledge, this is the only published case of RCM Type II from the United States, as well as the first report of this particular combination of a mutation and partial deletion in the CYB5R3 gene leading to disease. The child's phenotype of microcephaly, seizures, spastic movements, and developmental delay is consistent with many of the neurologic impairments that are characteristic of RCM type II. His early presentation of feeding difficulty, with subsequent development axial hypotonia and appendicular hypertonia, is also consistent with other cases of this condition. Brain MRIs showed progression of white matter volume loss, continued CSF space enlargement, and cerebral and cerebellar atrophy. He was able to achieve some developmental milestones, but eventually lost these abilities after his seizures began. The proband was also diagnosed with infantile spasm‐type seizures and eventually began to have cyanotic episodes. Based on his clinical features and “chocolate‐colored” blood, he was finally diagnosed with RCM type II and underwent genetic analysis of the CYB5R3 gene which confirmed his diagnosis. The treatment for acute methemoglobinemia is with methylene blue, which our proband received after the initial diagnosis 4. Treatment for the remainder of the neurologic abnormalities seen in Type II is predominately supportive in nature.
The differential diagnosis of neurologic regression in children can be broad, but may include leukodystrophies (X‐linked adrenoleukodystrophy, metachromatic leukodystrophy), lysosomal or other storage disorders (Krabbe, neuronal ceroid lipofuscinosis, and mucopolysaccharidosis), and a multitude of other syndromes associated which may be associated with microcephaly, seizures, and abnormal movements. Although his relatively nonspecific symptoms made the ultimate diagnosis difficult, a brief review of commonly ordered neurologic panels does not appear to have the CYBR53 gene included. We would recommend that this gene be incorporated into some of these panels to help reduce the almost inevitably long diagnostic process.
The variants that were identified in the proband's initial exome sequencing analysis were not felt to be contributory to the proband's phenotype. The three variants inherited from his healthy, unaffected mother were all autosomal recessive and heterozygous in inheritance. The p.G222R SUMF1 variant has not yet been reported as a disease‐causing variant. However, pathogenic variants in SUMF1 have been reported to cause multiple sulfatase deficiency, which is an autosomal recessive condition that involves many of the clinical features seen in the proband. These features include developmental delay, neurologic abnormalities, ocular abnormalities, and seizures 5. While this was considered in the differential, extensive metabolic studies carried out when the proband was seen initially, including acylcarnitine profiles, plasma amino acids, urine organic acids, ammonia levels, lysosomal enzyme studies, and urine oligosaccharides and glycosaminoglycans all returned normal.
The p.L321R BRAT1 variant has also not been reported as a disease‐causing variant. Pathogenic variants in the BRAT1 gene have been reported to be associated with neonatal rigidity, seizures, microcephaly, and lack of developmental progress 23. Finally, the maternally inherited heterozygous p.N374S variant in the SUCLG2 has not been reported as a disease‐causing variant. Pathogenic variants in the SUCLG1 and SUCLG2 have been reported to cause infantile lactic acidosis along with other mitochondrial abnormalities 24. However, the proband never had any episodes of lactic acidosis or elevated lactate, and due to the zygosity and mode of inheritance of these variants, we do not believe that any of these variants contributed to the phenotype of the proband. Finally, the heterozygous p.Q20H variant in the KCNA1 gene, inherited from his healthy father, has also not been reported as a disease‐causing mutation. Mutations in the KCNA1 gene have been found to be associated with episodic ataxia 2. It was inherited in an autosomal dominant manner and therefore is not thought to be associated with the proband's phenotype.
Many of the phenotypic features of the proband and the previously reported patient with the p.R160X pathogenic variant are similar. These patients both have different mutations on their second allele; however, they have many of the same phenotypic features. These include developmental delay and intellectual disability, short stature, failure to thrive, cyanosis, use of a feeding tube, hypertonia, spastic movements, scoliosis, and seizures. Both of these patients also had similar brain abnormalities including microcephaly, cerebellar/cerebral atrophy, and delayed myelination. Although these patients have many common clinical features, it seems that the proband that we report has a more severe phenotype than the patient reported by Aalfs et al. in 2000. We propose that the novel deletion in combination with the p.R160X pathogenic variant in our proband may have contributed to this increased severity compared to the 1 patient, who had two missense mutations. Deletions, splice site, and terminations appear to create the most severe methemoglobinemia phenotypes leading to type II rather than type I because of the significantly reduced methemoglobin reductase activity.
We have compiled the 34 published cases of RCM type II confirmed by genetic analysis (Table 1) and analyzed the clinical features of each case according to mutation type (Table 2). Many patients with RCM type II and their respective mutations have been reported in the literature. Some of the variants that have been discovered in RCM type II patients cause the severe type II form of the disease if homozygous in the patient, but cause type I RCM if the patient is heterozygous for the variant 9. The most recent reviews of pathogenic variants in the CYB5R3 gene leading to RCM Types I and II were published in 2008. Since that time, three novel missense mutations and an additional four novel deletions leading to clinical features of RCM type II have been published (Table 1). The cases that have been published since the 2008 reviews of RCM type II all report clinical features that are consistent with our proband and the previously reported cases of RCM type II. These features include developmental delay, cyanosis, feeding difficulty, use of a feeding tube, failure to thrive, spastic limb movements, hypotonia, microcephaly, and changes in the brain including cerebellar and/or cerebral atrophy and delayed myelination. However, there does not seem to be a clear correlation between the type/severity of mutation and the severity of the phenotypic features that are manifested (Table 2).
Table 1.
Mutations leading to RCM type II
| Mutation type | Codon change | Amino acid change | Exon/Intron | Publication |
|---|---|---|---|---|
| Missense/Nonsense | c.129C > A | Tyr42Term | Exon 2 | 17 |
| Missense/Nonsense | Unknown | Lys111Met | Exon 4 | 6 |
| Missense/Nonsense | c.287C > A | Pro95His | Exon 4 | 17 |
| Missense/Nonsense | c.247C > T | Arg83Ter | Exon 4 | 10 |
| Missense/Nonsense | c.229C > T | Gln77Ter | Exon 4 | 1 |
| Missense/Nonsense | Unknown | Gly76Ser | Boundary of Exon 4 and Exon 5 | 20 in a type I/6 |
| Missense/Nonsense | Unknown | Leu131Pro | Exon 5 | 6 |
| Missense/Nonsense | c. 382T > C | Ser127Pro | Exon 5 | 12 |
| Missense/Nonsense | c. 379A > G | Met127Val | Exon 5 | 13 |
| Missense/Nonsense | c.478C > T | Arg160Ter | Exon 6 | 1 |
| Missense/Nonsense | c.608G > A | Cys203Tyr | Exon 7 | 16–first reported in Wang 1999 in RCM Type I |
| Missense/Nonsense | c.610T > C | Cys204Arg | Exon 7 | 30 |
| Missense/Nonsense | c.705G > A | Trp235Term | Exon 8 | 16 |
| Missense/Nonsense | A > G (position unknown) | Arg240Gly | Exon 8 | 29 |
| Missense/Nonsense | c.655C > T | Arg219Ter | Exon 8 | 30 |
| Missense/Nonsense | c.708G > A | Trp236Ter | Exon 8 | 31, 32 |
| Missense/Nonsense | c.757G > A | Val253Met | Exon 9 | 13 |
| Missense/Nonsense | c.173C > G | Arg58Pro | Unknown | 22 |
| Splice site | IVS 2 ds + 1 G > A | Unknown | Intron 2 | 6 |
| Splice site | IVS 4 as ‐2 A > G | Loss of exon5 | Intron 4 | 13 |
| Splice site | IVS 5 ds + 2 T > C | Mis‐splicing of exon 5 | Intron 5 | 33 |
| Splice Site | IVS 5 ds + 8 G > C | Loss of exon 5 | Intron 5 | 30; also reported in 8 |
| Splice site | IVS 5 as ‐2 A > C | Loss of exon 6 | Intron 5 | 19 |
| Splice site | IVS 5 ‐2 A > C | Loss of exon 6 | Intron 5 | 18 |
| Splice site | IVS 8 as ‐1 G > T | Mis‐splicing of exon 9 | Intron 8 | 26 |
| Deletion | c.215delG | Gly72AlafsTer | Exon 3 | 11 |
| Deletion | Unknown | Lys173‐Ser174‐Val175del | Exon 6 | 31, 32 MGM |
| Deletion | c.815_817del | Met272del | Exon 9 | 30 |
| Deletion | c.895_897del | Phe298del | Exon 9 | 25, 28 |
| Deletion | c.562_564del | Leu188del | Unknown | 22 |
| Small insertion/Deletion | c. 882_884delinsAA | Thr295fsTer | Exon 9 | 14 |
| Gross deletion | c.22 1320_633 + 1224del | Unknown | Exons 2–7 | 9 |
Table 2.
Clinical features in 35 genetically confirmed cases of RCM type II
| Complications | Heterozygous | Homozygous | |||||||
|---|---|---|---|---|---|---|---|---|---|
| Proband (termination–deletion) | Missense–Missense | Missense–deletion, termination, or splice site | Termination–Termination | Missense | Termination | Deletion | Splice site | Total | |
| General | |||||||||
| Complicated pregnancy | 100% (1/1) | 0% (0/2) | 42.86% (3/7) | 0% (0/1) | 0% (0/7) | 33.33% (1/3) | 0% (0/5) | 11.11% (1/9) | 17.14% (6/35) |
| DD/ID | 100% (1/1) | 50% (1/2) | 42.86% (3/7) | 100% (1/1) | 71.43% (5/7) | 100% (3/3) | 80% (4/5) | 7/9 | 71.43% (25/35) |
| FTT | 100% (1/1) | 0% (0/2) | 0% (0/7) | 100% (1/1) | 28.57% (2/7) | 0% (0/3) | 20% (1/5) | 22.22% (2/9) | 20% (7/35) |
| Short stature | 100% (1/1) | 0% (0/2) | 14.29% (1/7) | 100% (1/1) | 14.29% (1/7) | 0% (0/3) | 20% (1/5) | 0% (0/9) | 14.29% (5/35) |
| Cyanosis | 100% (1/1) | 100% (2/2) | 100% (7/7) | 100% (1/1) | 57.14% (4/7) | 0% (0/3) | 100% (5/5) | 55.55% (5/9) | 71.43% (25/35) |
| Gastrointestinal | |||||||||
| Feeding difficulty | 100% (1/1) | 100% (2/2) | 14.29% (1/7) | 100% (1/1) | 0% (0/7) | 0% (0/3) | 20% (1/5) | 33.33% (3/9) | 22.86% (8/35) |
| Feeding tube | 100% (1/1) | 50% (1/2) | 0% (0/7) | 0% (0/1) | 0% (0/7) | 0% (0/3) | 0% (0/5) | 0% (0/9) | 8.57% (3/35) |
| GI dysmotility | 100% (1/1) | 0% (0/2) | 0% (0/7) | 0% (0/1) | 0% (0/7) | 0% (0/3) | 0% (0/5) | 0% (0/9) | 2.86% (1/35) |
| Musculoskeletal | |||||||||
| Hypotonia | 100% (1/1) | 50% (1/2) | 0% (0/7) | 0% (0/1) | 14.29% (1/7) | 0% (0/3) | 60% (3/5) | 44.44% (4/9) | 28.57% (10/35) |
| Hypertonia | 100% (1/1) | 50% (1/2) | 14.29% (1/7) | 100% (1/1) | 0% (0/7) | 33.33% (1/3) | 40% (2/5) | 11.11% (1/9) | 22.86% (8/35) |
| Scoliosis | 100% (1/1) | 50% (1/2) | 0% (0/7) | 100% (1/1) | 0% (0/7) | 0% (0/3) | 20% (1/5) | 0% (0/9) | 11.43% (4/35) |
| Torticollis | 100% (1/1) | 0% (0/2) | 0% (0/7) | 0% (0/1) | 0% (0/7) | 0% (0/3) | 20% (1/5) | 0% (0/9) | 5.71% (2/35) |
| Decreased muscle mass | 100% (1/1) | 0% (0/2) | 0% (0/7) | 0% (0/1) | 0% (0/7) | 0% (0/3) | 20% (1/5) | 0% (0/9) | 5.71% (2/35) |
| Jerking movements | 100% (1/1) | 50% (1/2) | 57.14% (4/7) | 100% (1/1) | 42.86% (3/7) | 66.66% (2/3) | 60% (3/5) | 77.77% (7/9) | 62.86% (22/35) |
| Quadriparesis/tetraparesis | 0% (0/2) | 42.86% (3/7) | 100% (1/1) | 14.29% (1/7) | 0% (0/3) | 20% (1/5) | 11.11% (1/9) | 20% (7/35) | |
| Ear, Nose, and Throat | |||||||||
| Reduced middle ear mobility | 100% (1/1) | 0% (0/2) | 0% (0/7) | 0% (0/1) | 0% (0/7) | 0% (0/3) | 0% (0/5) | 0% (0/9) | 2.86% (1/35) |
| Neurologic | |||||||||
| Seizures | 100% (1/1) | 0% (0/2) | 0% (0/7) | 100% (1/1) | 14.29% (1/7) | 0% (0/3) | 0% (0/5) | 11.11% (1/9) | 11.43% (4/35) |
| Staring spells | 100% (1/1) | 0% (0/2) | 0% (0/7) | 0% (0/1) | 0% (0/7) | 0% (0/3) | 0% (0/5) | 0% (0/9) | 2.86% (1/35) |
| Microcephaly | 100% (1/1) | 100% (2/2) | 57.14% (4/7) | 100% (1/1) | 57.14% (4/7) | 66.66% (2/3) | 60% (3/5) | 88.88% (8/9) | 71.43% (25/35) |
| Cerebellar/cerebral atrophy | 100% (1/1) | 0% (0/2) | 0% (0/7) | 100% (1/1) | 42.86% (3/7) | 33.33% (1/3) | 0% (0/5) | 22.2% (2/9) | 22.86% (8/35) |
| Enlarged ventricle | 100% (1/1) | 0% (0/2) | 0% (0/7) | 0% (0/1) | 0% (0/7) | 0% (0/3) | 0% (0/5) | 0% (0/9) | 2.86% (1/35) |
| White matter loss | 100% (1/1) | 0% (0/2) | 0% (0/7) | 0% (0/1) | 0% (0/7) | 0% (0/3) | 0% (0/5) | 0% (0/9) | 2.86% (1/35) |
| Delayed myelination | 100% (1/1) | 0% (0/2) | 14.29% (1/7) | 100% (1/1) | 57.14% (4/7) | 33.33% (1/3) | 0% (0/5) | 11.11% (1/9) | 21.71% (9/35) |
| Enlarged CSF or subarachnoid spaces | 100% (1/1) | 0% (0/2) | 0% (0/7) | 0% (0/1) | 14.29% (1/7) | 0% (0/3) | 0% (0/5) | 0% (0/9) | 5.71% (2/35) |
| Abnormal EEG | 100% (1/1) | 0% (0/2) | 0% (0/7) | 0% (0/1) | 14.29% (1/7) | 0% (0/3) | 0% (0/5) | 11.11% (1/9) | 8.57% (3/35) |
| Diffuse brain atrophy | 0%(0/1) | 0% (0/2) | 0% (0/7) | 0% (0/1) | 28.57% (2/7) | 33.33% (1/3) | 0% (0/5) | 11.11% (1/9) | 14.29% (5/35) |
| Corpus callosum abnormality | 0%(0/1) | 0% (0/2) | 14.29% (1/7) | 0% (0/1) | 14.29% (1/7) | 0% (0/3) | 0% (0/5)0 | 0% (0/9) | 5.71% (2/35) |
| General neurologic issue | 0%(0/1) | 0% (0/2) | 28.57% (2/7) | 0% (0/1) | 0% (0/7) | 0% (0/3) | 60% (3/5 | 0% (0/9) | 14.29% (5/35) |
| Hypoplasia of basal ganglia | 0%(0/1) | 0% (0/2) | 0% (0/7) | 100% (1/1) | 0% (0/7) | 0% (0/3) | 0% (0/5) | 0% (0/9) | 2.86% (1/35) |
| Cortical atrophy | 0%(0/1) | 0% (0/2) | 14.29% (1/7) | 100% (1/1) | 0% (0/7) | 0% (0/3) | 20% (1/5) | 11.11% (1/9) | 11.43% (4/35) |
| Encephalopathy | 0%(0/1) | 0% (0/2) | 42.86% (3/7) | 0% (0/1) | 0% (0/7) | 0% (0/3) | 0% (0/5) | 0% (0/9) | 8.57% (3/35) |
| Heart defect | 0%(0/1) | 0% (0/2) | 0% (0/7) | 0% (0/1) | 0% (0/7) | 0% (0/3) | 20% (1/5) | 0% (0/9) | 2.86% (1/35) |
| Ophthalmology | |||||||||
| Delayed visual maturation | 100% (1/1) | 0% (0/2) | 0% (0/7) | 0% (0/1) | 0% (0/7) | 0% (0/3) | 0% (0/5) | 0% (0/9) | 2.86% (1/35) |
| Cortical visual impairment | 100% (1/1) | 0% (0/2) | 0% (0/7) | 0% (0/1) | 0% (0/7) | 0% (0/3) | 0% (0/5) | 0% (0/9) | 2.86% (1/35) |
| Hyperopic astigmatism | 100% (1/1) | 0% (0/2) | 0% (0/7) | 0% (0/1) | 0% (0/7) | 0% (0/3) | 0% (0/5) | 0% (0/9) | 2.86% (1/35) |
| Strabismus | 0%(0/1) | 50% (1/2) | 14.29% (1/7) | 0% (0/1) | 28.57% (2/7) | 0% (0/3) | 20% (1/5) | 44.44% (4/9) | 21.71% (9/35) |
| Other diagnosis | 100% (1/1) | 0% (0/2) | 0% (0/7) | 0% (0/1) | 0% (0/7) | 0% (0/3) | 0% (0/5) | 0% (0/9) | 2.86% (1/35) |
One of the aspects to consider when evaluating these patients is methemoglobin reductase levels. All of the cases compiled for this report either reported an actual methemoglobin reductase level, the percentage of activity of the normal enzyme activity, or were described as “reduced” or “deficient.” Out of the 35 cases, there were five cases that had methemoglobin reductase activity levels that were within or slightly higher than the normal range (6.6–13.3 U/g Hb). The five cases with normal or slightly elevated methemoglobin reductase activity levels included one heterozygous missense–nonsense mutation, two homozygous missense mutations, a homozygous nonsense mutation, and a nine nucleotide deletion. The remaining cases had severely reduced levels/activity of methemoglobin reductase. We attempted to determine whether there was a correlation between the severity of the mutation and the level of methemoglobin reductase. This analysis showed that 9 of 30 cases with abnormal methemoglobin reductase activity contained homozygous splice site variant, 11 of 30 cases with either a homozygous missense variant, nonsense variant, or deletion, 6 of 30 cases were heterozygous for a missense variant and either a deletion, termination, or splice site variant, 2 of 30 cases were missense heterozygous, and 2 of 30 cases had two different nonsense variants or a heterozygous nonsense and deletion. There seems to be a wide variety of variants reported in these cases, which suggests that a particular variant type does not necessarily lead to decreased methemoglobin levels and activity.
This report introduces the first genetically confirmed case of RCM type II in the United States and details a previously unreported combination of causative genetic changes in the CYB5R3 gene. Although the condition may not be the ideal condition for inclusion on newborn screening, we would recommend that the CYB5R3 gene be included on common neurologic testing such as epilepsy, microcephaly, or leukodystrophy panels. To our knowledge, this gene is not readily available for analysis in these types of multigene panels that are often ordered prior to more large‐scale analyses such as whole‐exome sequencing. Earlier detection and recognition of the condition may shorten the often prolonged diagnostic odyssey for the relatively nonspecific signs and symptoms that are seen in RCM II.
Authorship
SSV, TP, and JW: involved in patient management. MTC and JJ: provided molecular analysis of samples. EM, TP, JW, MTC, JJ, and SSV: provided editing and review of the manuscript. EM, MTC, JJ, and SSV: wrote the manuscript.
Conflict of Interest
None declared
Acknowledgments
Dr. Schrier Vergano is a member of the medical advisory board for Ambry Genetics but has not received any compensation in relationship to this work. Jane Juusola and Megan T. Cho are employees of GeneDx, which performed the panel testing and exome sequencing analysis in the proband. The authors would like to acknowledge the contributions of Elizabeth Chisholm, MS, CGC, for her work with this patient. The authors would also like to acknowledge and thank the family of the proband for their willingness to participate in this report and for their tireless advocacy for their child.
Clinical Case Reports 2018; 6(1): 170–178
Disclaimer: The views expressed in this article are those of the authors and do not reflect the official policy or position of the Department of the Navy, Department of Defense, nor the US Government.
This work was prepared as part of the authors’ official duties. Title 17 U.S.C. 105 provides that “Copyright protection under this title is not available for any work of the United States Government.” Title 17 U.S.C. 101 defines a US Government work as a work prepared by a military service member or employee of the US Government as part of that person's official duties.
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