A novel familial MECP2 mutation in a young boy: Clinical and molecular findings

Case report.

The patient is a 6-year-old boy, the second child of unrelated parents. His mother is affected by anxiety and depression, whereas his 7-year-old sister is healthy. The boy was born at term after an uneventful pregnancy and delivery. Birth weight was 3.2 kg; head circumference at birth is unknown. Motor and language development were delayed. From the first years of life, he showed hyperactivity, irritability, and relational difficulties. At age 4 years, he developed seizures characterized by brief loss of contact and uni-bilateral myoclonic jerks involving the upper and lower limbs. He received monotherapy with sodium valproate, levetiracetam, and then topiramate for brief periods, with no benefits for the seizures. At our first observation, at age 6 years, seizures occurred with daily frequency. On examination, the patient showed defects in various areas of social interaction. Spoken language was slightly slurred and poorly used for communication. Language comprehension was very poor. General examination revealed a head circumference of 52.2 cm (slightly above the 50th percentile), adiposity, and some nonspecific distinctive facial features: frontal bossing, low-set ears, and irregular placement of teeth. Neurologic examination showed generalized hypotonia, lower limb weakness, decreased muscle stretch reflexes in the lower limbs, truncal and locomotor ataxia, clumsiness, dysmetria, and intention tremor. Pain and touch were unimpaired. The patient was not toilet trained. Ophthalmologic examination showed bilateral ptosis and hypermetropic astigmatism.

A dynamic EEG was recorded, whereas a polygraphic EEG with EMG signal acquisition was not obtained because of the patient's psychomotor restlessness. The awake EEG showed diffuse and symmetric polyrhythmic background activity with dominant activity at 12 to 13 Hz. There were multiple anomalies consisting of multifocal isolated or clustered spikes, multispikes, or spike-wave complexes, mainly on the right frontal and left occipital regions; these anomalies were activated during sleep. The video-EEG allowed us to note that uni-bilateral myoclonic jerks involving the upper and lower limbs occurred sometimes in association with the paroxysms described above. EMG, motor, and sensor conduction velocities in the arms and legs and somatosensory evoked potentials by median nerve stimulation were normal. Visual evoked potentials by flash revealed normal latency bilaterally and decreased amplitude in the left eye. EKG, echocardiography, and abdominal ultrasound were normal. Radiography of the rachis showed dorsolumbar scoliosis.

Brain MRI was normal.

Results of routine metabolic and immunologic evaluations, including serum immunoglobulin, ammonia, serum copper, and serum ceruloplasmin, were normal.

Cognitive assessment revealed a moderate grade of mental retardation on the Stanford Binet Scale (III revision): full-scale IQ, 45. On the Childhood Autism Rating Scale, a total score of 35.5 was obtained, compatible with a moderate degree of autism. Vineland Adaptive Behavior Scales revealed an adaptive level corresponding to less than 2 years. The Psychoeducational Profile revealed a developmental age of 24 months. This neuropsychological and behavioral profile does not clearly fulfill the criteria for an autistic disorder, because of the presence of minimal emotional reciprocity, so a diagnosis of pervasive developmental disorder not otherwise specified and moderate mental retardation was made, according to the Diagnostic and Statistical Manual of Mental Disorders, Fourth Edition, Test Revised.¹ Therapy with sodium valproate led to a slight reduction in the frequency of seizures.

Karyotype analysis on peripheral blood leukocytes revealed a 46,XY karyotype. MECP2 mutation analysis with direct sequencing in the patient revealed a c.964>T (p.P322S) mutation. His mother, a 40-year-old homemaker, carries the same mutation in heterozygous form (figure, A). On examination, she had a head circumference of 53.5 cm (in the low normal range), imbalanced gait, clumsiness, slight intention tremor, scanning speech, and decreased deep tendon reflexes. Although a formal cognitive assessment was not performed because of the patient's poor cooperation, she was noted to function in the low normal intelligence range. Moreover, she exhibited anxious depressive symptoms. The X chromosome inactivation pattern was examined in the propositus and his mother by PCR analysis of a polymorphic CAG repeat in the androgen receptor gene (AR). The mother was heterozygous for the AR polymorphism (figure, B, mother, HpaII(−)). After digestion with HpaII (figure, B, mother, HpaII(±)), the relative intensity of the two allelic peaks was roughly the same as in the undigested sample, suggesting random X inactivation. The boy had only one allelic peak, which disappeared after digestion with HpaII. Both maternal grandparents were tested, and neither carried the mutation (figure, A, grandfather). Molecular typing of three highly polymorphic chromosome X loci demonstrated that the mutation arose on the chromosome inherited from the maternal grandfather (figure, C).

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Figure. (A) Sequence tracing of the MECP2 964C>T mutation in the proband (T hemizygous), his mother (C/T heterozygous), and his normal grandfather (C homozygous). (B) X chromosome inactivation analysis. HpaII(−), before digestion with HpaII; HpaII(+), after digestion with HpaII for 5 hours at 37°C. (C) Pedigree of the family. Affected subjects are indicated by filled symbols. Loci are ordered according to their chromosomal position. The haplotype transmitted from the maternal grandfather to the mother and then to the propositus is boxed.

Discussion.

Rett syndrome is a neurodevelopmental disorder occurring almost exclusively in females. It is characterized by regression of psychomotor development, typical hand movements, deceleration of head growth, breathing abnormalities, epilepsy, and ataxia. This syndrome is caused by mutations in the gene encoding methyl-CpG-binding protein 2 (MECP2), located at chromosome Xq28.²

Although Rett syndrome was formerly considered to be lethal in males, in the past years, there has been a growing number of reports of MECP2 mutations in males. Males with MECP2 mutations may present with a Rett phenotype when they have X-chromosome aneuploidy or somatic mosaicism for the mutation.³ Various other neurologic phenotypes apart from Rett have been described in association with the MECP2 mutation in chromosomally normal males. The clinical spectrum seen in these males ranges from severe neonatal encephalopathy, Angelman syndrome, mental retardation, and childhood-onset schizophrenia to a combination of mental retardation, psychosis, and macroorchidism (the so-called PPM-X syndrome).^4,5 Other neurologic signs frequently associated with MECP2 mutations in males are tremor, choreoathetosis, dystonia, ataxia, spasticity, hypotonia, and motor stereotypes.³

We report a case of a young boy carrying a familial 964C>T (P322S) mutation on the MECP2 gene. The clinical picture of our patient is dominated by mental retardation with autistic features and epilepsy of myoclonic type, associated with other neurologic signs including diffuse hypotonia, lower limb weakness, tremor, ataxia, and dysmetria. His mother, who is a carrier of the same mutation, showed slight cognitive impairment, gait disturbance, speech difficulties, and slight tremor. Molecular typing of maternal grandparents demonstrated that the mutation appeared de novo in the mother, strengthening the hypothesis that it is a causative mutation rather than a polymorphism.

Currently, more than 100 different mutations, including missense and nonsense mutations, deletions, and insertions have been described in the MECP2 gene. The correlation between type of mutation and disease severity is not always apparent. Some reports seem to indicate that patients with methyl-CpG-binding domain mutations (both missense and truncating) show a more severe clinical presentation than patients with missense and nonsense mutations further downstream in the gene.⁶ Because the structure and function of the MECP2 protein have not yet been fully clarified and no functional assays are available, it may be difficult to differentiate between disease-causing mutations and rare polymorphisms. In fact, several variations initially identified as mutations in familial studies have since been reclassified as polymorphisms.⁷

The P322S mutation found in our patient has not previously been reported. Two missense mutations, P322A and P322L, have been described in Rett females.^2,8–10 Consequently, additional substitutions involving proline 322 may be considered as good candidates for bona fide MECP2 mutations. Because the P322S mutation caused a relatively mild phenotype in the affected boy and only moderate cognitive impairment in his mother in the absence of skewed X-inactivation, it can be classified as the mildest MECP2 mutation described so far.