A population-based study of the recurrence of developmental disabilities--Metropolitan Atlanta Developmental Disabilities Surveillance Program, 1991-94

Neurodevelopmental Disabilities in Canadian Children: Prevalence Data from the National Longitudinal Study of Children and Youth

Aim Using population surveys of chronic health conditions, the present study aimed to examine changing trends in the prevalence of neurodevelopmental disabilities (NDD) with age and determine population-based estimates of prevalence and census-based estimates of absolute numbers of affected children.

Methods We analyzed data from three cycles (1994–1999) of Canada's National Longitudinal Survey of Children and Youth (NLSCY) (Statistics Canada Survey).

Results Cross-sectional prevalence rates for chronic NDD in children from birth to 15 years across cycle 1 to 3 of the NLSCY show an increasing trend over the years from 1994 to 1999. Population-based estimates were also calculated from census data. Weighted prevalence rates for four conditions in children aged birth to 15 years increased across the three cycles, except for cerebral palsy. Prevalence estimates in cycle 3 were: epilepsy 5.26/1,000 (95% confidence interval [CI]: 5.01, 5.52), cerebral palsy 2.81/1,000 (95% CI: 2.62, 2.99), intellectual disability 4.77/1,000 (95% CI: 4.53, 5.02), and learning disability 57.06/1,000 (95% CI, 56.36, 57.76). A male gender preponderance was noted for each NDD using logistic regression.

Interpretation Prevalence rates of NDD in Canadian children show an incremental trend across three cycles in four conditions covered in the survey. The changing trends over the three cycles are discussed.

Familial risk of cerebral palsy: population based cohort study

OBJECTIVE

To investigate risks of recurrence of cerebral palsy in family members with various degrees of relatedness to elucidate patterns of hereditability.

DESIGN

Population based cohort study.

SETTING

Data from the Medical Birth Registry of Norway, linked to the Norwegian social insurance scheme to identify cases of cerebral palsy and to databases of Statistics Norway to identify relatives.

PARTICIPANTS

2,036,741 Norwegians born during 1967-2002, 3649 of whom had a diagnosis of cerebral palsy; 22,558 pairs of twins, 1,851,144 pairs of first degree relatives, 1,699,856 pairs of second degree relatives, and 5,165,968 pairs of third degree relatives were identified.

MAIN OUTCOME MEASURE

Cerebral palsy.

RESULTS

If one twin had cerebral palsy, the relative risk of recurrence of cerebral palsy was 15.6 (95% confidence interval 9.8 to 25) in the other twin. In families with an affected singleton child, risk was increased 9.2 (6.4 to 13)-fold in a subsequent full sibling and 3.0 (1.1 to 8.6)-fold in a half sibling. Affected parents were also at increased risk of having an affected child (6.5 (1.6 to 26)-fold). No evidence was found of differential transmission through mothers or fathers, although the study had limited power to detect such differences. For people with an affected first cousin, only weak evidence existed for an increased risk (1.5 (0.9 to 2.7)-fold). Risks in siblings or cousins were independent of sex of the index case. After exclusion of preterm births (an important risk factor for cerebral palsy), familial risks remained and were often stronger.

CONCLUSIONS

People born into families in which someone already has cerebral palsy are themselves at elevated risk, depending on their degree of relatedness. Elevated risk may extend even to third degree relatives (first cousins). The patterns of risk suggest multifactorial inheritance, in which multiple genes interact with each other and with environmental factors. These data offer additional evidence that the underlying causes of cerebral palsy extend beyond the clinical management of delivery.

Range of genetic mutations associated with severe non-syndromic sporadic intellectual disability: an exome sequencing study

BACKGROUND

The genetic cause of intellectual disability in most patients is unclear because of the absence of morphological clues, information about the position of such genes, and suitable screening methods. Our aim was to identify de-novo variants in individuals with sporadic non-syndromic intellectual disability.

METHODS

In this study, we enrolled children with intellectual disability and their parents from ten centres in Germany and Switzerland. We compared exome sequences between patients and their parents to identify de-novo variants. 20 children and their parents from the KORA Augsburg Diabetes Family Study were investigated as controls.

FINDINGS

We enrolled 51 participants from the German Mental Retardation Network. 45 (88%) participants in the case group and 14 (70%) in the control group had de-novo variants. We identified 87 de-novo variants in the case group, with an exomic mutation rate of 1·71 per individual per generation. In the control group we identified 24 de-novo variants, which is 1·2 events per individual per generation. More participants in the case group had loss-of-function variants than in the control group (20/51 vs 2/20; p=0·022), suggesting their contribution to disease development. 16 patients carried de-novo variants in known intellectual disability genes with three recurrently mutated genes (STXBP1, SYNGAP1, and SCN2A). We deemed at least six loss-of-function mutations in six novel genes to be disease causing. We also identified several missense alterations with potential pathogenicity.

INTERPRETATION

After exclusion of copy-number variants, de-novo point mutations and small indels are associated with severe, sporadic non-syndromic intellectual disability, accounting for 45-55% of patients with high locus heterogeneity. Autosomal recessive inheritance seems to contribute little in the outbred population investigated. The large number of de-novo variants in known intellectual disability genes is only partially attributable to known non-specific phenotypes. Several patients did not meet the expected syndromic manifestation, suggesting a strong bias in present clinical syndrome descriptions.

FUNDING

German Ministry of Education and Research, European Commission 7th Framework Program, and Swiss National Science Foundation.

Candidate genes and risk for CP: a population-based study

Studies suggest that genetic polymorphisms may increase an individual's susceptibility to CP. Most findings have yet to be corroborated in an independent cohort. This case-control study is nested within all 334,333 infants ≥36 wk gestation born at Kaiser Permanente Medical Care Program, 1991-2002. We included only non-Hispanic whites who had a neonatal blood sample available. Case patients (n = 138) were identified from medical records to have spastic or dyskinetic CP. Controls (n = 165) were randomly selected from the population. We genotyped polymorphisms previously associated with CP: inducible NOS (iNOS)-231, apolipoprotein E (apoE) ε2 and ε4 alleles, TNF-α-308, IL-8 -251, lymphotoxin 60, endothelial NOS -922, endothelial protein C receptor 219, mannose-binding lectin 54 and 52, factor V Leiden, methyltetrahydrofolate reductase 1298 and 667, prothrombin 20210, and platelet activator inhibitor 11053. Similar to previous reports, the iNOS-231 T allele (25.7 versus 18.9%, p = 0.04) and the apoE ε4 allele (19.3 versus 13.2%, p = 0.04) were more common in patients with CP than in controls. However, there was no statistically significant association between any genetic polymorphism and CP after correction for multiple comparisons.

Copy number and SNP arrays in clinical diagnostics

The ability of chromosome microarray analysis (CMA) to detect submicroscopic genetic abnormalities has revolutionized the clinical diagnostic approach to individuals with intellectual disability, neurobehavioral phenotypes, and congenital malformations. The recognition of the underlying copy number variant (CNV) in respective individuals may allow not only for better counseling and anticipatory guidance but also for more specific therapeutic interventions in some cases. The use of CMA technology in prenatal diagnosis is emerging and promises higher sensitivity for several highly penetrant, clinically severe microdeletion and microduplication syndromes. Genetic counseling complements the diagnostic testing with CMA, given the presence of CNVs of uncertain clinical significance, incomplete penetrance, and variable expressivity in some cases. While oligonucleotide arrays with high-density exonic coverage remain the gold standard for the detection of CNVs, single-nucleotide polymorphism (SNP) arrays allow for detection of consanguinity and most cases of uniparental disomy and provide a higher sensitivity to detect low-level mosaic aneuploidies.

Auditory function and hearing loss in children and adults with Williams syndrome: cochlear impairment in individuals with otherwise normal hearing

Hearing loss is common in school-age individuals with Williams syndrome (WS) and extensive in adults. Prior studies with relatively small sample sizes suggest that hearing loss in WS has an early onset and may be progressive, yet the auditory phenotype and the scope of the hearing loss have not been adequately characterized. We used standard audiometric tools: Otoscopy, tympanometry, air-conduction (bone conduction when available) behavioral testing, and distortion product otoacoustic emissions (DPOAEs) to measure hearing sensitivity and outer hair cell function. We tested 81 individuals with WS aged 5.33-59.50 years. Sixty-three percent of the school-age and 92% of the adult participants had mild to moderately-severe hearing loss. The hearing loss in at least 50% was sensorineural. DPOAE testing corroborated behavioral results. Strikingly, 12 of 14 participants with hearing within normal limits bilaterally had 4,000-Hz DPOAE input/output (DPOAE IO) functions indicative of outer hair cell damage and impaired cochlear compression. Our results indicate that hearing loss is very common in WS. Furthermore, individuals with WS who have "normal" hearing as defined by behavioral thresholds may actually have sub-clinical impairments or undetected cochlear pathology. Our findings suggest outer hair cell dysfunction in otherwise normal hearing individuals. The DPOAE IO in this same group revealed growth functions typically seen in groups with noise-induced damage. Given this pattern of findings, individuals with WS may be at increased risk of noise-induced hearing loss. Recommendations regarding audiological testing for individuals with WS and accommodations for these individuals in both academic and nonacademic settings are provided.

U.S. Maternally linked birth records may be biased for Hispanics and other population groups

PURPOSE

We sought to advance understanding of linkage error in U.S. maternally linked datasets and how the error might affect results of studies based on the linked data.

METHODS

North Carolina birth and fetal death records for 1988-1997 were maternally linked (n=1,030,029). The maternal set probability, defined as the probability that all records assigned to the same maternal set do in fact represent events to the same woman, was used to assess differential maternal linkage error across race/ethnic groups.

RESULTS

Maternal set probabilities were lower for records specifying Asian or Hispanic race/ethnicity, suggesting greater maternal linkage error for these sets. The lower probabilities for Hispanics were concentrated in women of Mexican origin who were not born in the United States.

CONCLUSIONS

Differential linkage error may be a source of bias in studies that use U.S. maternally linked datasets to make comparisons between Hispanics and other groups or among Hispanic subgroups. Methods to quantify and adjust for this potential bias are needed.

How physicians use array comparative genomic hybridization results to guide patient management in children with developmental delay

PURPOSE

Array comparative genomic hybridization is an emerging test used clinically to identify the etiology of children with developmental delay, yet little data are available regarding how physicians use these results. This pilot study evaluated how positive test results were used to influence patient management.

METHODS

We surveyed 14 physicians of 48 patients who had copy number changes detected by microarray technology.

RESULTS

Of 48 patients, 34 (70.8%) had 65 management changes after receiving the test result (with individual patients having 1-3 changes). Most commonly, physicians provided patients' families with a recurrence risk for affected subsequent pregnancies (35% of patients). Patients avoided other forms of testing (35%) and had improved access to services (25%). In 27% of patients, physicians altered medical management by referring patients to a specialist or recommending medical screening. Patients with known syndromes had multiple changes, but patients with novel copy number changes also had recommendations made based on the array result.

CONCLUSIONS

Overall, physicians reported making changes in management among most patients with positive test results, in ways similar to abnormalities detected by conventional cytogenetics. Our study demonstrates that this testing, in our clinical setting, is affecting management of children with developmental delay.

High familial risks for cerebral palsy implicate partial heritable aetiology

Cerebral palsy is the commonest cause of severe childhood disability, the aetiology of which is largely unknown. Data on familial aggregation of cerebral palsy are very limited. We defined familial risks for siblings who were hospitalised because of cerebral palsy in Sweden. A nationwide database for neurological diseases was constructed by linking the Multigeneration Register to the Hospital Discharge Register for the years 1987-2001. Standardised hospitalisation ratios (SHRs) were calculated for affected singletons and twins by comparing them with siblings who had no cerebral palsy. A total of 3997 patients were recorded with cerebral palsy. Familial cerebral palsy was uncommon, and it accounted for 1.6% of all cerebral palsy cases. However, for parents who had had one affected child the risk of recurrence in another child was considerably increased. Parents of one affected child had a 4.8-fold risk of having a second affected child, and where the siblings were twins, the risk was 29-fold. These familial risks were particularly high in some clinical subgroups: 17-25 in singletons and 37-155 in twins, including hemiplegia, diplegia and quadriplegia. The remarkably high familial risks are difficult to explain without some contribution of heritable factors. The lack of discordant pairs may suggest that heritable factors are disorder type-specific. Affected concordant sibling pairs should be subjected to molecular studies aiming at identifying the susceptibility gene.

Diagnostic yield of various genetic approaches in patients with unexplained developmental delay or mental retardation

The underlying cause of mental retardation remains unknown in up to 80% of patients. As chromosomal aberrations are the most common known cause of mental retardation, several new methods based on FISH, PCR, and array techniques have been developed over recent years to increase detection rate of subtle aneusomies initially of the gene rich subtelomeric regions, but nowadays also genome wide. As the reported detection rates vary widely between different reports and in order to compare the diagnostic yield of various investigations, we analyzed the diagnostic yield of conventional karyotyping, subtelomeric screening, molecular karyotyping, X-inactivation studies, and dysmorphological evaluation with targeted laboratory testing in unselected patients referred for developmental delay or mental retardation to our cytogenetic laboratory (n = 600) and to our genetic clinic (n = 570). In the cytogenetic group, 15% of patients showed a disease-related aberration, while various targeted analyses after dysmorphological investigation led to a diagnosis in about 20% in the genetic clinic group. When adding the patients with a cytogenetic aberration to the patient group seen in genetic clinic, an etiological diagnosis was established in about 40% of the combined study group. A conventional cytogenetic diagnosis was present in 16% of combined patients and a microdeletion syndrome was diagnosed in 5.3%, while subtelomeric screening revealed only 1.3% of causes. Molecular karyotyping with a 10 K SNP array in addition revealed 5% of underlying causes, but 29% of all diagnoses would have been detectable by molecular karyotyping. In those patients without a clear diagnosis, 5.6% of mothers of affected boys showed significant (>95%) skewing of X-inactivation suggesting X-linked mental retardation. The most common diagnoses with a frequency of more than 0.5% were Down syndrome (9.2%), common microdeletion 22q11.2 (2.4%), Williams-Beuren syndrome (1.3%), Fragile-X syndrome (1.2%), Cohen syndrome (0.7%), and monosomy 1p36.3 (0.6%). From our data, we suggest the following diagnostic procedure in patients with unexplained developmental delay or mental retardation: (1) Clinical/dysmorphological investigation with respective targeted analyses; (2) In the remaining patients without an etiological diagnosis, we suggest conventional karyotyping, X-inactivation screening in mothers of boys, and molecular karyotyping, if available. If molecular karyotyping is not available, subtelomeric screening should be performed.

The genetics of mental retardation

Genetic abnormalities frequently give rise to a mental retardation phenotype. Recent advances in resolution of comparative genomic hybridization and genomic sequence annotation has identified new syndromes at chromosome 3q29 and 9q34. The finding of a significant number of copy number polymorphisms in the genome in the normal population, means that assigning pathogenicity to deletions and duplications in patients with mental retardation can be difficult but has been identified for duplications of MECP2 and L1CAM. Novel autosomal genes that cause mental retardation have been identified recently including CC2D1A identified by homozygosity mapping. Several new genes and pathways have been identified in the field of X-linked mental retardation but many more still await identification. Analysis of families where only a single male is affected reveals that the chance of this being due to a single X-linked gene abnormality is significantly less than would be expected if the excess of males in the population is entirely due to X-linked disease. Recent identification of novel X-linked mental retardation genes has identified components of the post-synaptic density and multiple zinc finger transcription factors as disease causing suggesting new mechanisms of disease causation. The first therapeutic treatments of animal models of mental retardation have been reported, a Drosophila model of Fragile X syndrome has been treated with lithium or metabotropic glutamate receptor (mGluR) antagonists and a mouse model of NF1 has been treated with the HMG-CoA reductase inhibitor lavastatin, which improves the learning and memory skills in these models.

X linked mental retardation: a clinical guide

Mental retardation is more common in males than females in the population, assumed to be due to mutations on the X chromosome. The prevalence of the 24 genes identified to date is low and less common than expansions in FMR1, which cause Fragile X syndrome. Systematic screening of all other X linked genes in X linked families with mental retardation is currently not feasible in a clinical setting. The phenotypes of genes causing syndromic and non-syndromic mental retardation (NLGN3, NLGN4, RPS6KA3(RSK2), OPHN1, ATRX, SLC6A8, ARX, SYN1, AGTR2, MECP2, PQBP1, SMCX, and SLC16A2) are first discussed, as these may be the focus of more targeted mutation analysis. Secondly, the relative prevalence of genes causing only non-syndromic mental retardation (IL1RAPL1, TM4SF2, ZNF41, FTSJ1, DLG3, FACL4, PAK3, ARHGEF6, FMR2, and GDI) is summarised. Thirdly, the problem of recurrence risk where a molecular genetics diagnosis has not been made and what proportion of the male excess of mental retardation is due to monogenic disorders of the X chromosome are discussed.