Presentation of m.3243A>G (MT-TL1; tRNALeu) variant with focal neurology in infancy

INTRODUCTION

Mitochondrial tRNALeu (MT-TL1) mutation m.3243A>G constitutes the most commonly reported pathogenic mitochondrial genome (mtDNA) mutation. Characteristically, giving rise to a mitochondrial encephalopathy, lactic acidosis and stroke-like episodes (MELAS) phenotype [Hirano et al., 1992], a widening clinical spectrum is increasingly evident [van den Ouweland et al., 1992; Yamagata et al., 2002; Lev et al., 2004; Michaelides et al., 2008; de Laat et al., 2013; Nesbitt et al., 2013]. Typically, presenting from 2 years of age, infantile MELAS presentation is the exception, with early-onset epileptic encephalopathy predominant among the reported cases to date (Table I) [Sue et al., 1999; Shah et al., 2002; Kanaumi et al., 2006; Guevara-Campos et al., 2007; Selim and Mehaney, 2013]; indeed, focal neurological deficits without encephalopathy, suggestive of stroke-like episodes, have not been described before 12 months of age.

We report on a male infant with early-onset hemiparesis, without lactic acidosis or encephalopathy, and the m.3243A>G variant, born to a mother with broadly comparable heteroplasmy rates, herself manifesting adult-onset insulin dependent diabetes mellitus and sensorineural hearing loss. This child's features extend the phenotypic spectrum of disease and emphasizing the phenotypic variability of m.3243A>G-related disease. Potential contributing factors to the observed clinical heterogeneity are discussed.

CLINICAL REPORT

The index case was a male infant (birth weight 2480 g) born at 34 weeks gestation to nonconsanguineous Caucasian parents. His mother was diagnosed with and began treatment for diabetes mellitus not long prior to becoming pregnant; antenatal history was otherwise normal. Delivery by caesarean followed premature, prolonged rupture of membranes. An uncomplicated perinatal course ensued and early neurodevelopment proceeded within appropriate limits (corrected for prematurity).

At 9 months, a left-sided hemiparesis was recognized, including facial asymmetry in an upper-motor pattern; cranial nerve and motor examinations were otherwise normal. A prodromal encephalopathic episode was not identified.

Cerebral magnetic resonance imaging (MRI) at 12 months of age was normal, without abnormality to account for the manifest hemiparesis; electroencephalography demonstrated age appropriate background activity, with no epileptiform features. Electrocardiography and cardiac sonography were similarly normal. Serum lactate was normal on repeated, free-flowing venous samples.

At 16 months, neurological deficit persisted with mild growth arrest and appendicular spasticity; myotatic reflexes were pathologically brisk (asymmetrically greater on the left), and an ipsilateral positive Babinski response elicited. Head circumference tracked between the 50th and 75th centile (corrected age). Hemifacial paresis persisted and bilateral blepharoptosis was evident. Fundoscopy showed retinal pigmentation. Visual field testing was age-appropriate. Systemic examination remained normal.

A family history of maternal insulin-dependent diabetes mellitus and post-lingual sensorineural hearing impairment was identified; both parents have normal intellect, although the father had difficulties with attention in childhood, without a formal diagnosis. No additional history of neurological disease was noted. His 5-year-old brother remains medically well, with normal neurocognitive development.

Targeted mitochondrial DNA sequencing was performed for variants associated with neurological phenotypes. m.3243A>G was detected with a heteroplasmy rate of 75% in blood and 84% in urine epithelial cells. Subsequent cascade testing demonstrated heteroplasmy rates of 45% and 79% in maternal whole blood and urine epithelial cells. Fraternal urine epithelial cell heteroplasmy rate was 81%.

Chromosomal microarray (BlueGnome Cytochip oligo ISCA 8 × 60K array) demonstrated an additional finding of a paternally inherited 483 kb duplication of 7q36.3 (hg19 outer breakpoints: 157723016–158464183) in both the index case and his brother. There were five Reference Sequence (RefSeq) genes, two Mendelian Inheritance in Man (OMIM) genes, and no known disease-causing genes (see Supplemental Tables I and II). Search of the DECIPHER database, International Standards For Cytogenomic Arrays Consortium (ISCA) database, database of genomic variants (DGV), EMBASE, and PubMed raise the possibility that duplication of genes within this region may be a risk factor for developmental and behavioral issues; focal neurological deficit is not reported.

Whole exome sequencing was performed (supplemental method 1 and supplemental data 1), which did not find any additional findings in OMIM morbid or candidate genes which may explain the presentation.

Initiation of L-arginine (150 mg/kg/day), ubiquinone (10–30 mg/kg/day), alpha-lipoid acid (300 mg daily), levocarnitine (50 mg/kg/day), and vitamin B preparation was commenced.

DISCUSSION

To our knowledge, this is the first reported case of infantile onset focal neurological deficit in m.3243A>G-related disease in the absence of comorbid encephalopathy or classic neuroimaging features of MELAS. Disease onset before 12 months of age is rare, with manifest encephalopathy, developmental delay, and seizures (particularly infantile spasms), dominant among the seven cases reported (Table I). Significant clinical heterogeneity is evident among the proband and affected family members, the pathophysiological basis of which remains unclear. Risk factors for developing symptoms include heteroplasmy rate (the proportion of cellular mitochondria carrying a nucleotide variant), tissue distribution (the rate of heteroplasmy in each tissue) [Chinnery et al., 1997], age, and sex [Mancuso et al., 2013; Mancuso et al., 2014].

Although increased rates of heteroplasmy are associated with an increase in the likelihood of symptoms [Chinnery et al., 1997; Jeppesen et al., 2006], they do not predict the age of onset of symptoms [Kaufmann et al., 2011; Mancuso et al., 2014; Mancuso et al., 2013]. Male sex was recently reported as a risk factor for developing MELAS in carriers of m.3243A>G in an Italian case-series [Mancuso et al., 2013], raising the question of unidentified sex-related environmental or genetic factors for the development of symptoms [Kaufmann et al., 2011]. Indeed, the proband's mother has not developed classic MELAS and demonstrates a phenotype of insulin-dependent diabetes mellitus and sensorineural deafness—another of the recognized m.3243A>G presentations [Nesbitt et al., 2013]. Phenotypic discordance is additionally evident between the male proband and his normal brother, despite comparable rates of heteroplasmy, where assessed, however, the prospect that the proband's elder brother might still present with classical MELAS cannot be excluded as onset of stroke-like episodes is reported as late as 40 years of age [Hirano et al., 1992].

Cases 3 and 4 (Table I) are the first and second of a case series of three children presenting with infantile-encephalopathy attributed to m.3243A>G-related disease [Sue et al., 1999]. However, both of these children had heteroplasmy rates of only 4% in blood and 5% in muscle, despite severe CNS disease; while differing central nervous system heteroplasmy might explain this discordance, the potential for other modifying factors exists, with a growing interest in the relevance of oligogenic factors contributing to phenotypic heterogeneity of m.3243A>G-related disease [Mancuso et al., 2014; Tran Hornig-Do et al., 2014].

The identified duplication in this case includes five RefSeq genes (see Table II); two are miRNAs (MIR595 and MIR5707) with no function currently assigned [Maglott et al., 2011], one is a non-functional pseudo gene (THAP5P1) and the remaining two genes (PTPRN2 and NCAPG2) have no known role in mitochondrial function or biogenesis. A search of PubMed, EMBASE, and Cases Database identified reports of 7q36.3 duplications associated with an increased risk for schizophrenia [Aleksic et al., 2013], and for bilateral Duane retraction syndrome (type 3), bilateral hearing impairment, and cerebrovascular malformations [Abu-Amero et al., 2013].

A further search of genetics databases for duplications involving PTPRN2 and NCAPG2 identified six patients with autism, three of whom also had intellectual impairment, in the SFARI Gene database (see Supplemental Table I) [Basu et al., 2009]. Querying the ISCA database identified 21 cases of 7q36.3 duplication with most having developmental delay and some with physical anomalies (see Supplemental Table II). Focal neurology was not reported as a feature of any of these cases. Notably, the normal elder sibling of the reported proband, also carries the 7q36.3 duplication, mitigating the role of the 7q36.3 duplication in the patho-etiology of this case. Nevertheless, the possibility cannot be absolutely excluded in light of the differing phenotype displayed by the proband's mother, who does not carry the identified duplication—particularly if his elder brother proceeds to manifest classical MELAS at a future time-point.

CONCLUSION

We report on a patient with infantile hemiparesis, without comorbid lactic acidosis, encephalopathy or manifest seizures, in association with the m.3243A>G variant. This observation expands the phenotypic spectrum of disease and emphasizes the need to consider pathogenic mitochondrial variants in cases of cryptogenic neurological deficit in infancy.