Sibling recurrence in intellectual disability of unknown cause

Family History of Mental and Neurological Disorders and Risk of Autism

IMPORTANCE

Familial aggregation of mental and neurological disorders is often observed in autism spectrum disorders (ASD), but reports have generally focused on single disorders and are limited to first-degree relatives.

OBJECTIVES

To examine family history of mental and neurological disorders among first- to fourth-degree relatives and risk of ASD with and without intellectual disability (ID) in index persons.

DESIGN, SETTING, AND PARTICIPANTS

In this population-based cohort study, 567 436 index persons were identified from the Stockholm Youth Cohort, an ongoing longitudinal register-linkage cohort study of the total population aged 0 to 17 years residing in Stockholm County, Sweden. Index persons were nonadopted singleton births born between 1984 and 2009 who were at least 2 years of age at the end of follow-up on December 31, 2011, had resided in Stockholm County for at least 2 years since birth, and could be linked to both biological parents. Data analysis was conducted from May 2017 to December 2018.

EXPOSURE

Mental and neurological diagnoses of relatives of the index persons.

MAIN OUTCOMES AND MEASURES

Diagnosis of ASD, with or without co-occurring ID, in the index persons.

RESULTS

The cohort included 567 436 index persons (291 191 [51.3%] male; mean [SD] age at the end of follow-up, 14.3 [7.5] years). The prevalence of ASD with and without ID was 0.4% and 1.5%, respectively. Positive family history of mental and neurological disorders was associated with higher odds of ASD in index persons; 6895 (63.1%) of index persons with ASD had a parent with history of mental and/or neurological disorders, compared with 252 454 (45.4%) of index persons without ASD. Family history of multiple disorders was associated with higher odds of ASD in index persons, including history of ASD (odds ratio among first-degree relatives for ASD with and without ID: 14.2, 9.0), intellectual disability (7.6, 2.3), attention-deficit/hyperactivity disorder (3.3, 4.7), obsessive compulsive disorder (1.9, 2.1), schizophrenia and other nonaffective psychotic disorders (2.1, 1.8), depression (1.4, 2.0), bipolar disorder (1.4, 2.2), personality disorder (2.1, 2.6), cerebral palsy (2.2, 1.5), and epilepsy (2.0, 1.3). The more closely related the affected family member was, the higher the odds was of ASD for the index person. ASD without intellectual disability was associated with more disorders compared to ASD with intellectual disability. ASD with intellectual disability exhibited a weaker familial association with other mental disorder diagnoses but a stronger familial association with some neurological diagnoses as compared to ASD without intellectual disability.

CONCLUSIONS AND RELEVANCE

This study suggests that family history of mental and neurological disorders is associated with increased risk of ASD. The familial component of ASD etiology may differ by presence or absence of co-occurring ID.

Reanalysis of exome sequencing data of intellectual disability samples: Yields and benefits

Recently, with the advancement in next generation sequencing (NGS) along with the improvement of bioinformatics tools, whole exome sequencing (WES) has become the most efficient diagnostic test for patients with intellectual disability (ID). This study aims to estimate the yield of a reanalysis of ID negative exome cases after data reannotation. Total of 50 data files of exome sequencing, representing 50 samples were collected. The inclusion criteria include ID phenotype, and previous analysis indicated a negative result (no abnormality detected). These files were pre-processed and reannotated using ANNOVAR tool. Prioritized variants in the 50 cases studied were classified into three groups, (1) disease-causative variants (2) possible disease-causing variants and (3) variants in novel genes. Reanalysis resulted in the identification of pathogenic/likely pathogenic variants in six cases (12%). Thirteen cases (26%) were classified as having possible disease-causing variants. Candidate genes requiring future functional studies were detected in seven cases (14%). Improvement in bioinformatics tools, update in the genetic databases and literature, and patients' clinical phenotype update were the main reasons for identification of these variants in this study.

Identification of pathogenic gene variants in small families with intellectually disabled siblings by exome sequencing

BACKGROUND

Intellectual disability (ID) is a common neurodevelopmental disorder affecting 1-3% of the general population. Mutations in more than 10% of all human genes are considered to be involved in this disorder, although the majority of these genes are still unknown.

OBJECTIVES

We investigated 19 small non-consanguineous families with two to five affected siblings in order to identify pathogenic gene variants in known, novel and potential ID candidate genes. Non-consanguineous families have been largely ignored in gene identification studies as small family size precludes prior mapping of the genetic defect.

METHODS AND RESULTS

Using exome sequencing, we identified pathogenic mutations in three genes, DDHD2, SLC6A8, and SLC9A6, of which the latter two have previously been implicated in X-linked ID phenotypes. In addition, we identified potentially pathogenic mutations in BCORL1 on the X-chromosome and in MCM3AP, PTPRT, SYNE1, and ZNF528 on autosomes.

CONCLUSIONS

We show that potentially pathogenic gene variants can be identified in small, non-consanguineous families with as few as two affected siblings, thus emphasising their value in the identification of syndromic and non-syndromic ID genes.

Exome sequencing and the genetics of intellectual disability

Exome sequencing has greatly impacted the speed at which new disease genes are identified. In the last year alone, six studies have used exome sequencing to identify new genes involved in intellectual disability, a genetically heterogeneous condition affecting 1-3% of the population. These studies encompass the full gamut of modes of inheritance and phenotypic presentation, including syndromic and non-syndromic conditions, sporadic and familial cases, and dominant and recessive inheritance patterns. Because different disease presentations require different approaches to gene discovery, studies of intellectual disability provide a nearly comprehensive showcase of strategies for exome-driven gene discovery. Despite these successes, the etiology of ~60% of cases of intellectual disability remains unknown. The application of exome sequencing to the clinical diagnosis of intellectual disability in the near future will ultimately reduce the number of idiopathic cases and provide a rich source of sequence variation for the identification of new intellectual disability genes.