This study describes a fetus with noncompaction cardiomyopathy, a short anteroposterior diameter of the corpus callosum and relative macrocephaly that carried a nonsense mutation in exon 4 of the NONO gene (NM_007363.5:c.214C > T; p.Gln72Ter). This de novo mutation has not been previously reported as pathogenic or benign and has not been detected in the general population (dbSNP150, 1000 Genomes Project, gnomAD). A literature review revealed that phenotypes, including noncompaction cardiomyopathy, abnormalities of the corpus callosum and macrocephaly, are consistent features of MRXS34. This mutation turned the 72nd Gln into a stop codon. The loss of protein from this allele was expected to occur through nonsense-mediated mRNA decay. Therefore, we classified this mutation in NONO as pathogenic according to the American College of Medical Genetics and Genomics guidelines[8].
Loss-of-function variants in NONO might predispose males to CHD and LVNC. The gene had a haploinsufficiency index of 4.11 according to DECIPHER, and the probability of loss-of-function intolerance (pLI) was 0.99 according to GnomAD. We report a fetus with a de novo mutation, c.214C > T, p.Gln72Ter, which was classified as pathogenic and predicted to undergo nonsense-mediated mRNA decay, ultimately leading to loss of function in NONO. In addition, noncompaction cardiomyopathy involving both the left and right ventricles was the only cardiac phenotype in this patient. LVNC was the most common cardiac phenotype. We also found that LVNC is diagnosed early in almost all individuals (Table 1): six individuals who were pregnant, including the fetus we reported[6], four individuals who were in the neonatal period[3, 5], and three individuals who were in infancy[2, 4, 9], indicating that LVNC is an early clue for diagnosing MRXS34 and emphasizing the importance of fetal echocardiography for identifying cardiac structural anomalies in NONO-related fetuses.
Table 1
Craniocerebral and cardiomyopathy of fetuses with NONO mutations.
Study, patient identifier, age | NONO variant (NM_001145408.2), inheritance | age at diagnosis | Brain anomalies | Macrocephaly (relative or + 2 SD) | Cardiomyopathy |
Sewani et al ., S3, 2019 | c.457C > T, p.(Arg153*), de novo | prenatal (16 weeks gestation) | ND | ND | Cardiomegaly |
Sun et al., S(A1), 2020 | c.246_249del, p.Pro83Thrfs*7 | prenatal | ND | ND | LVNC |
Sun et al., S(A2), 2020 | c.246_249del, p.Pro83Thrfs*7 | prenatal | ND | ND | LVNC |
Sun et al., S(A3), 2020 | c.246_249del, p.Pro83Thrfs*7 | prenatal | ND | ND | LVNC |
Sun et al., S(B1), 2020 | c.471del, p.Gln157Hisfs*18 | prenatal | dysplasia of the corpus callosum | ND | LVNC |
Sun et al., S(B2), 2020 | c.471del, p.Gln157Hisfs*18 | prenatal | ND | ND | LVNC |
Sun et al.,S1, 2020 | c.154 + 9A > G, p.Asn52Serfs*3 | prenatal (26 week gestation) | ND | ND | ND |
This study,S1,2022 | C.214C > T, p.Gln75Ter | prenatal (30 week gestation) | Short anteroposterior diameter of corpus callosum, mild ventriculomegaly | relative macrocephaly | Myocardial thickening, ventricular noncompaction of myocardium |
Abbreviations: LVNC, left ventricular noncompaction; ND, not determined/not reported; S, subject. |
In addition to congenital heart defects and LVNC, minor cerebral structural anomalies and intellectual developmental disorders are other symptoms in male patients with loss-of-function mutations in NONO. Brain MRI of male patients with NONO mutations may indicate abnormalities of the corpus callosum, the most common phenotype. However, based on a review of the literature by Roessler et al.[7], among nine MRXS34 patients for whom antenatal ultrasound data were available, eight were diagnosed with corpus callosum agenesis (CCA) postnatally. However, prenatal diagnosis of CCA was made in only four patients out of the night patients with postnatal diagnosis of CCA. This means that the CCA is missed prenatally in nearly half of patients, which is limited by the use of prenatal ultrasound and the proficiency of sonographers. For evaluating brain development, prenatal ultrasound can only assess intracranial structures and morphology[10] but cannot detect functional abnormalities, including mental retardation, psychomotor development delay, and speech impediment, which are characterized phenotypes of MRXS34[1]. Indeed, prenatal cranial MRI provides clearer visualization of intracranial structures and improves the diagnosis of neurological abnormalities such as CCA[11]. MRI revealed that the corpus callosum was abnormal in our patient. Therefore, this study emphasizes the role of prenatal MRI for the identification of CCA in patients with MRXS34.
In addition, macrocephaly or relative macrocephaly (relative or + 2 SD) is another common feature. Macrocephaly or relative macrocephaly was observed in 10 (91%) of the 11 male patients. One of the males with macrocephaly was reported to have undergone multiple prenatal ultrasound examinations during pregnancy[5]. This suggests that the proportion of mothers with this phenotype appearing before childbirth is relatively small. However, in regard to fetal macrocephaly in the prenatal setting, in addition to MRXS34, Beckwith-Wiedemann syndrome, which is a rare overgrowth syndrome manifesting as macrocephaly, needs to be excluded[12]. Surprisingly, prenatal intrauterine growth restriction (IUGR) with or without short long bones was found in 6 MRXS34 patients (75%)[7], stressing the significance of biological measurements for prenatal diagnosis of MRXS34. Moreover, IUGR, which is caused by multiple factors, such as placental and maternal factors, as well as genetic factors, such as aneuploidy, pathogenic copy number variation, and monogenic genetic diseases, is not uncommon in prenatal diagnosis[13]. Moreover, supravalvular stenosis and intrauterine growth restriction are features of 7q11.23 microdeletion syndrome[14]. It is very interesting to investigate the contribution of phenotypic associations such as LNVC or CCA with FGR to the prenatal diagnosis of MRXS34.
This study has several limitations. The fetus is most likely to have MRXS34 according to its NONO mutation and the phenotypes of noncompaction cardiomyopathy, relative macrocephaly and short anteroposterior diameter of the corpus callosum, but we cannot confirm whether the mild ventriculomegaly revealed by prenatal ultrasound diagnosis and MRI is related to MRXS34. In addition, we cannot determine microsturctural abnormalities using prenatal ultrasound and fetal MRI, and some phenotypes, especially intellectual developmental disorders, cannot be evaluated before delivery. Therefore, even if the fetus has the brain phenotype of relative acrocephaly, an abnormal corpus callosum and mild ventriculomegaly, we cannot determine whether the fetus would have an intellectual developmental disorder of MRXS34 after birth. The fetus was identified as having a de novo mutation in NONO, but maternal germline mosaicism cannot be excluded.