Dr. Conant is a combined child psychiatry resident at the University of Massachusetts, Worcester, MA.
Copy number variants (CNVs) are a common finding in autism. Perhaps the most common CNV in autism are proximal duplications of the long arm of chromosome 15 (15q) due to the presence of low copy repeats, which make this region susceptible to chromosomal rearrangement.[1, 2] There are five common breakpoints (BP1–BP5) in the 15q11.2-q13 region, which are at the boundaries of both deletions and duplications of chromosomal regions. Deletions result in either Angelman syndrome (AS) or Prader-Willi syndrome (PWS), depending on the parent-of-origin of the affected allele containing the PWS/AS critical region (PWACR) located between BP2-BP3. Conversely, the reciprocal duplication of the region between BP2 and BP3 results in interstitial duplications (int dup(15)) that confer an increased risk of autism. In addition, U-type crossover recombinations can result in a supernumerary isodicentric chromosome (idic(15)) most often due to recombination between BP4 and BP5, resulting in two extra copies of the BP2–BP3 region. Maternal duplications of this region are the most commonly identified genetic cause of autism, with prevalence estimates in autism ranging from 0.5% to 3%.
Multiple genes from this region are implicated in the pathogenesis of autism spectrum disorders, epilepsy, and schizophrenia,[2, 6-8] and numerous studies implicate UBE3A as the causative gene resulting in impaired cognitive function in AS. These results suggest that altered function of UBE3A through underexpression (AS) or overexpression (idic(15) or int dup(15)) impairs neurocognitive development.
The 15q11.2-q13 region is subject to genomic imprinting, by which one parental allele is silenced by a methylation dependent process. Parent-of-origin gene dosing appears to influence phenotype in idic(15); maternally inherited or derived interstitial duplications are associated with increased autism risk.[1, 10-13] Idic(15) chromosomes that include the BP2–BP3 region are virtually all maternally derived. Large maternally derived partial hexosomy for 15q11.2-q13 in two boys with intractable epilepsy further suggests that increased dosage of maternally expressed genes negatively impacts the idic(15) phenotype.
It is likely that the maternally expressed UBE3A gene is a major contributing factor to the autism phenotype in both int dup(15) and idic(15), since paternal duplications of this region, which do not over-express UBE3A, often do not result in an autistic spectrum disorder (ASD) phenotype.[3, 10] The most likely candidates involved in seizure phenotypes in the 15q syndrome are a cluster of biallelically expressed γ-aminobutyric acid (GABA)A-receptor subunit genes: GABRB3, GABRA5, and GABRG3, which are not subject to imprinting and are located within the same BP2–BP3 critical region. Two single nucleotide polymorphisms in the GABRG3 gene are associated with autism in large studies, and GABRB3 is reduced in postmortem brain samples of individuals with ASD.[15, 16] GABRB3 transcript and protein levels, however, were not increased in cortical brain samples from individuals with 15q duplications. GABRB3 knockout mice have severe neurologic manifestations, including seizures.[18, 19] Studies of the PWACR in idic(15) suggest an effect of both dose-dependent gene expression of the GABAA-receptor subunits and the size of the duplication.[6, 20-23]
The X-linked gene, MeCP2, encodes the methyl CpG binding protein 2 (MeCP2), which binds to sites in the imprinting control region (PWACR) as well as intermittent binding along the 15q11.2-q13 region, perhaps isolating functional domains of imprinted regions for expression. Some studies showed that MeCP2 deficiency is associated with impaired expression of both UBE3A and GABRB3,[15, 16, 25] thus linking Rett, Angelman, and Dup15q syndromes at the molecular level.
Seizures in AS are frequent (>80%) and well characterized,[26, 27] but seizure data in individuals with Dup15q are limited,[28-30] with lifetime prevalence estimates of 26.5–50%.[6, 8, 31, 32] Infantile spasms are associated with maternally inherited duplications of 15q11-q13, suggesting a role for GABAergic synapses and postsynaptic density in the etiology of Dup15q seizure semiology.
Idic(15) is characterized by developmental delay, severe epilepsy, moderate-to-severe intellectual disability, early central hypotonia, autistic behavior, absent or poor language (e.g., marked echolalia), and minor dysmorphic features (e.g., down-slanting palpebral fissures, epicanthal folds, low-set ears, “coarse” facial features, and hypopigmented areas of the skin).[29, 30] Metabolic workup and intracranial magnetic resonance imaging (MRI) are typically normal and characterization of seizures is limited in prior studies, which describe early onset of treatment-refractory epilepsy evolving into a Lennox-Gastaut syndrome (LGS) phenotype.[23, 29] Among 10 individuals with idic15, 7 presented with epilepsy and the most severely affected had the smallest duplication marker in the PWACR. Seizures were characterized by infantile spasms, with four evolving into epilepsy with multiple seizure types and one remaining seizure-free following spasms responsive to steroid treatment.
Int dup(15) typically has a much milder phenotype than idic(15). ASD occurs in int dup(15),[34, 35] and the largest study of 20 children reported developmental delay but no subjects with clinical seizures. Recently, Urraca et al. found that maternal duplication resulted in an autism phenotype in 10 of 14 int dup(15) individuals. None of these individuals had clinical seizures, but excessive beta activity on electroencephalography (EEG) was found in 10 of 14 individuals with int dup(15) regardless of the parental origin of the duplicated chromosome.
This 15q duplication syndrome cohort provides a rare opportunity to study epilepsy in a population of individuals who share a common duplication. To gain a better understanding of the character and effective treatments for epilepsies associated with Dup15q beyond prior case-based reports, we performed a detailed electronic survey assessing seizures and their treatments for 95 family members of Dup15q individuals. To our knowledge, this is the largest study to date assessing seizures and response to antiepileptic treatments in an international population of individuals with 15q duplication syndromes.
- Top of page
To our knowledge, this is the largest study assessing seizures and their treatments in duplication 15q syndrome. The main goal was to assess seizure phenotypes and effective treatments in a large population of individuals with duplications of 15q. This was an important objective as there is minimal data in the literature, especially regarding newer AED treatments. A questionnaire-based study was used to recruit as many subjects as possible. This design, however, has inherent flaws in that the results depend on responses from family members and not medical records. Given the complexity of molecular diagnoses in this region, family members were required to send us a genetic report prior to completing the survey, which helped ensure accurate classification of molecular subtypes for data analysis.
For those with int dup(15), only 3 (25%) of 12 had seizures with one of the three reporting only a single seizure. This seizure rate is similar to that reported in the autistic spectrum population in general, and much lower than seizure rates reported in those with idic(15).[6, 8, 31, 32] This, however, may be artificially inflated by response bias, since parents of children with seizures may be more likely to respond to a seizure survey than parents of children with no seizures. It is also possible that families of children with more severe epilepsy might have been more likely to respond. This milder clinical phenotype in int dup(15) has been reported in previous studies.[13, 35] Those with idic(15) had more severe epilepsy phenotypes, with mean EChess scores more than twice as high as those with int dup(15) (11.3 vs. 5.3). The rate of seizures in those with idic(15) (63%) was higher than reported previously in smaller cohorts,[5, 8, 30, 31] but this may reflect response bias for families of individuals with seizures.
A striking finding was the high prevalence of infantile spasms in children with idic(15) who had seizures (42%). This high prevalence is similar to other genetic disorders such as tuberous sclerosis complex. The prevalence of spasms may be underestimated as epileptic spasms beyond infancy are often misdiagnosed as atonic or myoclonic seizures. One of the most common causes of epileptic spasms is cortical dysplasia, and children with idic(15) have a significantly higher burden of microdysgenesis than those with idiopathic ASD. The other most common seizure types in idic(15) were generalized tonic–clonic (60%), followed by atonic, myoclonic, focal-onset, and tonic (38–40%). This is consistent with prior studies reporting an LGS phenotype,[23, 31] and LGS is associated with focal lesions such as cerebral dysgenesis.
Active seizures (nonspasms), as defined by those with seizures in the past year, increased in nearly every age group from <3 years (14%) to 18 years (68%). Active seizures were most prevalent in those 15–17 years old (67%) and 18+ years (68%), but there were relatively few respondents for those 15+ years; therefore, these rates may have been affected by response bias. Spasms typically resolved before 3 years of age, as the average age of onset was 6–7 months with an average duration of nearly 1 year.
Parental reporting indicates that epilepsy in idic(15) is refractory to medication, as only 24% had a >90% seizure reduction from the first medication, sustained over a period of at least 1 year. This rate of response is higher than reported in AS (15%), but much lower than reported in the general adult population (47%). The epilepsy phenotype in idic(15) often begins as infantile spasms, evolving into an LGS-type picture with multiple generalized seizures types (tonic–clonic, tonic, myoclonic, atonic, and absence) along with focal-onset seizures. This type of epilepsy typically responds well to broad spectrum AED and can be exacerbated by medications used to treat focal seizures such as CBZ and OXC. Seizures in those with idic(15) responded well, as expected, to broad-spectrum AED such as RUF, VPA, ZNS, and LTG. They also, however, responded well to CBZ (44% reported that it worked best) and OXC (33% reported that it worked best), with only 7% (1/14) of those on CBZ reporting seizure exacerbation. Because these medications typically worsen generalized epilepsy, this may suggest that epilepsy in idic(15) is more of a multifocal epilepsy with secondary generalization than a true generalized epilepsy, even though the prevalence of focal seizures (40%) is similar to that reported in other LGS-type epilepsies, including AS, in which seizures are exacerbated by CBZ and OXC.
Generalized epilepsies typically respond very well to benzodiazepines, but those with idic(15) did not respond well to benzodiazepines, with only 1 (4.5%) of 22 who tried CLB, CLZ, or lorazepam (LZP) reporting that it was the medication that worked best. For those taking CLZ or LZP who provided enough information (N = 7), none had >50% seizure reduction and 4(57%) of 7 had seizure exacerbation. CLB, an atypical benzodiazepine, was better tolerated with none reporting seizure exacerbation and two (50%) of four reporting >90% seizure reduction, although the other two (50%) of four reported sedation as an intolerable adverse effect. For those with idic(15) and spasms, ACTH/prednisone was superior to VGB, with 9 (75%) of 12 of those using ACTH/prednisone reporting a >90% reduction in spasms as opposed to 2 (29%) of 7 of those using VGB. Because VGB is also a GABAergic medication, it appears that AEDs that are GABA agonists are relatively ineffective and, in some cases, may exacerbate seizures in this population. This may be because GABA receptor subunits are duplicated in 15q duplication syndrome in contrast to AS in which these genes are deleted and benzodiazepines have been shown to be very effective.
- Top of page
Epilepsy is common in idic(15) with a high prevalence of infantile spasms and is typically refractory to medication. Although the epilepsy phenotype in idic(15) appears to be most consistent with an LGS-type of epilepsy and responds well to broad spectrum AEDs, the seizures also respond well to CBZ, which typically exacerbates LGS, suggesting that these epilepsies may be more multifocal than generalized. These seizures did not respond well to typical benzodiazepines, with lower response rates and relatively higher rates of seizure exacerbation, although CLB, an atypical benzodiazepine, was relatively effective and had no reported seizure exacerbations.
Although this is the largest study to date assessing epilepsy in this population, larger studies—ideally prospective studies—would be necessary to make more definitive determinations regarding the treatment of seizures in this population. This would overcome the major limitation of the current study which, with a questionnaire response rate of only about 25%, could have been subject to considerable responder bias.