RAC1 Missense Mutations in Developmental Disorders with Diverse Phenotypes.

RAC1 is a widely studied Rho GTPase, a class of molecules that modulate numerous cellular functions essential for normal development. RAC1 is highly conserved across species and is under strict mutational constraint. We report seven individuals with distinct de novo missense RAC1 mutations and varying degrees of developmental delay, brain malformations, and additional phenotypes. Four individuals, each harboring one of c.53G>A (p.Cys18Tyr), c.116A>G (p.Asn39Ser), c.218C>T (p.Pro73Leu), and c.470G>A (p.Cys157Tyr) variants, were microcephalic, with head circumferences between -2.5 to -5 SD. In contrast, two individuals with c.151G>A (p.Val51Met) and c.151G>C (p.Val51Leu) alleles were macrocephalic with head circumferences of +4.16 and +4.5 SD. One individual harboring a c.190T>G (p.Tyr64Asp) allele had head circumference in the normal range. Collectively, we observed an extraordinary spread of ∼10 SD of head circumferences orchestrated by distinct mutations in the same gene. In silico modeling, mouse fibroblasts spreading assays, and in vivo overexpression assays using zebrafish as a surrogate model demonstrated that the p.Cys18Tyr and p.Asn39Ser RAC1 variants function as dominant-negative alleles and result in microcephaly, reduced neuronal proliferation, and cerebellar abnormalities in vivo. Conversely, the p.Tyr64Asp substitution is constitutively active. The remaining mutations are probably weakly dominant negative or their effects are context dependent. These findings highlight the importance of RAC1 in neuronal development. Along with TRIO and HACE1, a sub-category of rare developmental disorders is emerging with RAC1 as the central player. We show that ultra-rare disorders caused by private, non-recurrent missense mutations that result in varying phenotypes are challenging to dissect, but can be delineated through focused international collaboration.

TREX tetramer disruption alters RNA processing necessary for corticogenesis in THOC6 Intellectual Disability Syndrome.

THOC6 variants are the genetic basis of autosomal recessive THOC6 Intellectual Disability Syndrome (TIDS). THOC6 is critical for mammalian Transcription Export complex (TREX) tetramer formation, which is composed of four six-subunit THO monomers. The TREX tetramer facilitates mammalian RNA processing, in addition to the nuclear mRNA export functions of the TREX dimer conserved through yeast. Human and mouse TIDS model systems revealed novel THOC6-dependent, species-specific TREX tetramer functions. Germline biallelic Thoc6 loss-of-function (LOF) variants result in mouse embryonic lethality. Biallelic THOC6 LOF variants reduce the binding affinity of ALYREF to THOC5 without affecting the protein expression of TREX members, implicating impaired TREX tetramer formation. Defects in RNA nuclear export functions were not detected in biallelic THOC6 LOF human neural cells. Instead, mis-splicing was detected in human and mouse neural tissue, revealing novel THOC6-mediated TREX coordination of mRNA processing. We demonstrate that THOC6 is required for key signaling pathways known to regulate the transition from proliferative to neurogenic divisions during human corticogenesis. Together, these findings implicate altered RNA processing in the developmental biology of TIDS neuropathology.

Mechanisms of mRNA processing defects in inherited THOC6 intellectual disability syndrome.

THOC6 is the genetic basis of autosomal recessive THOC6 Intellectual Disability Syndrome (TIDS). THOC6 facilitates the formation of the Transcription Export complex (TREX) tetramer, composed of four THO monomers. The TREX tetramer supports mammalian mRNA processing that is distinct from yeast TREX dimer functions. Human and mouse TIDS model systems allow novel THOC6-dependent TREX tetramer functions to be investigated. Biallelic loss-of-functon(LOF) THOC6 variants do not influence the expression and localization of TREX members in human cells, but our data suggests reduced binding affinity of ALYREF. Impairment of TREX nuclear export functions were not detected in cells with biallelic THOC6 LOF. Instead, mRNA mis-splicing was observed in human and mouse neural tissue, revealing novel insights into THOC6-mediated TREX coordination of mRNA processing. We demonstrate that THOC6 is required for regulation of key signaling pathways in human corticogenesis that dictate the transition from proliferative to neurogenic divisions that may inform TIDS neuropathology.

Children's acceptance of others with disability: the influence of a disability-simulation program.

Elementary school is a transitional and impressionable period for any child. In inclusive classroom settings, children with disabilities may experience the challenges of acceptance and integration into peer relationships. The experience of children with disabilities in such classrooms can be positive if their classmates are accepting and open. One way to encourage these attitudes is by increasing awareness through disability-simulation. This study evaluates an existing school program, whose goal is to increase acceptance of others with disability. We surveyed students to assess the influence of the program. An adapted acceptance scale was distributed to students before and after participation in the program. Our hypothesis was that student acceptance would increase after participating in the program. Additionally, we hypothesized that physical, visual, and auditory disabilities would be easier for students to understand, making acceptance scores for these disabilities higher than those for a learning disability. We report changes in acceptance using group means. Our results reveal that post-survey responses showed higher acceptance than pre-survey responses, especially for auditory disabilities. Other significant findings include differences in attitudes between the two schools and differences based on prior experience with someone with a disability. By evaluating the effectiveness of this type of program (whose efficacy has been controversial in the literature), this study can provide teachers, administrators, genetic counselors, and parents with a better idea of how to address children's attitudes towards peers. In our study, the effectiveness of promoting understanding through disability-simulation has proven its worth to improve children's acceptance.

47, XY, +der(Y),t(X;Y)(p21.1;p11.2): a unique case of XY sex reversal.

Translocations involving the short arms of the X and Y chromosomes are rare and can result in a functional disomy of the short arm of the X chromosome, including the dosage-sensitive sex reversal (DSS) locus. A result of such imbalance may be sex reversal with multiple congenital anomalies. We present the clinical and cytogenetic evaluation of a newborn infant with DSS and additional clinical findings of minor facial anomalies, left abdominal mass, 5th finger clinodactyly, and mild hypotonia. The external genitalia appeared to be normal female. The infant had bilateral corneal opacities and findings suggestive of anterior segment dysgenesis. Ultrasonography showed a small uterus with undetectable ovaries, and a left multicystic dysplastic kidney. High-resolution chromosome analysis identified the presence of a derivative Y chromosome, 47,XY, +der(Y)t(X;Y)(p21.1;p11.2), which was confirmed by fluorescence in situ hybridization studies. Array CGH showed a 35.1 Mb copy number gain of chromosome region Xp22.33-p21.1 and a 52.2 Mb copy number gain of Yp11.2-qter, in addition to the intact X and Y chromosomes. Previously reported patients with XY sex reversal have not had DSS with corneal opacities, dysgenesis of the anterior segment of the eye, and unilateral multicystic dysplastic kidney. These findings represent a new form of XY sex reversal due to an Xp duplication.

The genomic landscape of balanced cytogenetic abnormalities associated with human congenital anomalies.

Despite the clinical significance of balanced chromosomal abnormalities (BCAs), their characterization has largely been restricted to cytogenetic resolution. We explored the landscape of BCAs at nucleotide resolution in 273 subjects with a spectrum of congenital anomalies. Whole-genome sequencing revised 93% of karyotypes and demonstrated complexity that was cryptic to karyotyping in 21% of BCAs, highlighting the limitations of conventional cytogenetic approaches. At least 33.9% of BCAs resulted in gene disruption that likely contributed to the developmental phenotype, 5.2% were associated with pathogenic genomic imbalances, and 7.3% disrupted topologically associated domains (TADs) encompassing known syndromic loci. Remarkably, BCA breakpoints in eight subjects altered a single TAD encompassing MEF2C, a known driver of 5q14.3 microdeletion syndrome, resulting in decreased MEF2C expression. We propose that sequence-level resolution dramatically improves prediction of clinical outcomes for balanced rearrangements and provides insight into new pathogenic mechanisms, such as altered regulation due to changes in chromosome topology.