Understanding the Landscape of X-linked Variants Causing Intellectual Disability in Females Through Extreme X Chromosome Inactivation Skewing.

Intellectual disability (ID) affects 30% more males than females. This sex bias can be attributed to the enrichment of genes on the X chromosome playing essential roles in the central nervous system and their hemizygous state on males. Moreover, as a result of X chromosome inactivation (XCI), most genes on one of the X chromosomes in female somatic cells are epigenetically silenced, so that females carrying X-linked variants are not expected to be so severely affected as males. Consequently, the knowledge about X-linked ID (XLID) in females is still scarce. Herein, we used extreme XCI skewing (≥ 90%) to predict X-linked variants in females with idiopathic ID. XCI profiles from 53 probands were estimated from blood and buccal mucosa through a methylation-sensitive AR/RP2 assay. DNA samples with extreme XCI skewing were then submitted to array-comparative genomic hybridization and whole-exome sequencing. Seven females (13.2%) exhibited extreme XCI skewing, a percentage significantly higher than expected for healthy females in our population. XLID-potentially related variants were identified in five patients with extreme XCI skewing, including one pathogenic rstructural rearrangement [der(X) chromosome from a t(X;2)] and four single nucleotide variants in NLGN4X, HDAC8, TAF1, and USP9X genes, two of which affecting XCI escape genes. XCI skewing showed to be an outstanding approach for the characterization of molecular mechanisms underlying XLID in females. Beyond expanding the spectrum of variants/phenotypes associated with ID, our results pointed to compensatory biological pathways underlying XCI and uncover new insights into the involvement of escape genes on XLID, impacting genetic counseling.

Skewed X-Chromosome Inactivation and Compensatory Upregulation of Escape Genes Precludes Major Clinical Symptoms in a Female With a Large Xq Deletion.

In mammalian females, X-chromosome inactivation (XCI) acts as a dosage compensation mechanism that equalizes X-linked genes expression between homo- and heterogametic sexes. However, approximately 12-23% of X-linked genes escape from XCI, being bi-allelic expressed. Herein, we report on genetic and functional data from an asymptomatic female of a Fragile X syndrome family, who harbors a large deletion on the X-chromosome. Array-CGH uncovered that the de novo, terminal, paternally originated 32 Mb deletion on Xq25-q28 spans 598 RefSeq genes, including escape and variable escape genes. Androgen receptor (AR) and retinitis pigmentosa 2 (RP2) methylation assays showed extreme skewed XCI ratios from both peripheral blood and buccal mucosa, silencing the abnormal X-chromosome. Surprisingly, transcriptome-wide analysis revealed that escape and variable escape genes spanning the deletion are mostly upregulated on the active X-chromosome, precluding major clinical/cognitive phenotypes in the female. Metaphase high count, hemizygosity concordance for microsatellite markers, and monoallelic expression of genes within the deletion suggest the absence of mosaicism in both blood and buccal mucosa. Taken together, our data suggest that an additional protective gene-by-gene mechanism occurs at the transcriptional level in the active X-chromosome to counterbalance detrimental phenotype effects of large Xq deletions.

CHCHD2 mutational screening in Brazilian patients with familial Parkinson's disease.

Robust evidence on the involvement of genetic factors in the etiology of Parkinson's disease (PD) expands our knowledge about monogenic causes that contribute for this important neurodegenerative disorder. Mutations in the CHCHD2 gene have been linked to autosomal dominant forms of PD, although there is still lack of evidence for CHCHD2 variants leading to the disease in mixed populations as those from South America. To assess the contribution of CHCHD2 as a causal factor for familial PD in Brazil, one of the most heterogeneous populations in the world, we conducted the first molecular analysis of the CHCHD2 gene in a cohort of 122 index cases from Brazilian families with autosomal dominant forms of PD. Genomic DNA was isolated from peripheral blood and the 4 exons of the CHCHD2 gene, and their intron-exon boundaries were analyzed by bidirectional Sanger sequencing. No pathogenic or risk variants were found, suggesting that genetic variants of CHCHD2 are not a common cause of familial PD in Brazilian patients.

Clinical profiles associated with LRRK2 and GBA mutations in Brazilians with Parkinson's disease.

BACKGROUND: Parkinson's disease (PD) is a neurodegenerative disorder characterized by remarkable phenotypic variability. Accumulated evidence points that the manifestation of PD clinical signs might be differentially modified by genetic factors, as mutations in LRRK2 and GBA genes. In this sense, the clarification of the genotype-phenotype correlations in PD has important implications in predicting prognosis and can contribute to the development of specific therapeutic approaches. METHODS: Here, we conducted the first comparative analysis of motor and non-motor features in 17 LRRK2 and 22 GBA mutation carriers and 93 non-carriers unrelated PD patients from Brazil, a highly admixed population. RESULTS: Significant differences were found between the three groups. LRRK2 PD patients presented more occurrence of familiar history. Resting tremor was observed in a lower frequency in GBA mutation carries. In contrast, gait freezing and dysautonomia was present in lower frequencies in LRRK2 carriers. Besides that, LRRK2 and GBA mutation carriers showed a higher incidence of depressive symptoms and a younger age at onset, when compared to non-carriers. CONCLUSION: Our results suggest that specific mutations in GBA and LRRK2 influence the clinical signs of the disease, with significant implications for handling of specific patient groups.