High resolution measurement of DUF1220 domain copy number from whole genome sequence data

Human-specific genetics: new tools to explore the molecular and cellular basis of human evolution

Our ancestors acquired morphological, cognitive and metabolic modifications that enabled humans to colonize diverse habitats, develop extraordinary technologies and reshape the biosphere. Understanding the genetic, developmental and molecular bases for these changes will provide insights into how we became human. Connecting human-specific genetic changes to species differences has been challenging owing to an abundance of low-effect size genetic changes, limited descriptions of phenotypic differences across development at the level of cell types and lack of experimental models. Emerging approaches for single-cell sequencing, genetic manipulation and stem cell culture now support descriptive and functional studies in defined cell types with a human or ape genetic background. In this Review, we describe how the sequencing of genomes from modern and archaic hominins, great apes and other primates is revealing human-specific genetic changes and how new molecular and cellular approaches - including cell atlases and organoids - are enabling exploration of the candidate causal factors that underlie human-specific traits.

Tandem NBPF 3mer HORs (Olduvai triplets) in Neanderthal and two novel HOR tandem arrays in human chromosome 1 T2T-CHM13 assembly

It is known that the ~ 1.6 kb Neuroblastoma BreakPoint Family (NBPF) repeats are human specific and contributing to cognitive capabilities, with increasing frequency in higher order repeat 3mer HORs (Olduvai triplets). From chimpanzee to modern human there is a discontinuous jump from 0 to ~ 50 tandemly organized 3mer HORs. Here we investigate the structure of NBPF 3mer HORs in the Neanderthal genome assembly of Pääbo et al., comparing it to the results obtained for human hg38.p14 chromosome 1. Our findings reveal corresponding NBPF 3mer HOR arrays in Neanderthals with slightly different monomer structures and numbers of HOR copies compared to humans. Additionally, we compute the NBPF 3mer HOR pattern for the complete telomere-to-telomere human genome assembly (T2T-CHM13) by Miga et al., identifying two novel tandem arrays of NBPF 3mer HOR repeats with 5 and 9 NBPF 3mer HOR copies. We hypothesize that these arrays correspond to novel NBPF genes (here referred to as NBPFA1 and NBPFA2). Further improving the quality of the Neanderthal genome using T2T-CHM13 as a reference would be of great interest in determining the presence of such distant novel NBPF genes in the Neanderthal genome and enhancing our understanding of human evolution.

Tandemly repeated NBPF HOR copies (Olduvai triplets): Possible impact on human brain evolution

Previously it was found that the neuroblastoma breakpoint family (NBPF) gene repeat units of ∼1.6 kb have an important role in human brain evolution and function. The higher order organization of these repeat units has been discovered by both methods, the higher order repeat (HOR)-searching method and the HLS searching method. Using the HOR searching method with global repeat map algorithm, here we identified the tandemly organized NBPF HORs in the human and nonhuman primate NCBI reference genomes. We identified 50 tandemly organized canonical 3mer NBPF HOR copies (Olduvai triplets), but none in nonhuman primates chimpanzee, gorilla, orangutan, and Rhesus macaque. This discontinuous jump in tandemly organized HOR copy number is in sharp contrast to the known gradual increase in the number of Olduvai domains (NBPF monomers) from nonhuman primates to human, especially from ∼138 in chimpanzee to ∼300 in human genome. Using the same global repeat map algorithm method we have also determined the 3mer tandems of canonical 3mer HOR copies in 20 randomly chosen human genomes (10 male and 10 female). In all cases, we found the same 3mer HOR copy numbers as in the case of the reference human genome, with no mutation. On the other hand, some point mutations with respect to reference genome are found for some NBPF monomers which are not tandemly organized in canonical HORs.

Human-Specific Genes, Cortical Progenitor Cells, and Microcephaly

Over the past few years, human-specific genes have received increasing attention as potential major contributors responsible for the 3-fold difference in brain size between human and chimpanzee. Accordingly, mutations affecting these genes may lead to a reduction in human brain size and therefore, may cause or contribute to microcephaly. In this review, we will concentrate, within the brain, on the cerebral cortex, the seat of our higher cognitive abilities, and focus on the human-specific gene ARHGAP11B and on the gene family comprising the three human-specific genes NOTCH2NLA, -B, and -C. These genes are thought to have significantly contributed to the expansion of the cerebral cortex during human evolution. We will summarize the evolution of these genes, as well as their expression and functional role during human cortical development, and discuss their potential relevance for microcephaly. Furthermore, we will give an overview of other human-specific genes that are expressed during fetal human cortical development. We will discuss the potential involvement of these genes in microcephaly and how these genes could be studied functionally to identify a possible role in microcephaly.

Leveraging large genomic datasets to illuminate the pathobiology of autism spectrum disorders

"Big data" approaches in the form of large-scale human genomic studies have led to striking advances in autism spectrum disorder (ASD) genetics. Similar to many other psychiatric syndromes, advances in genotyping technology, allowing for inexpensive genome-wide assays, has confirmed the contribution of polygenic inheritance involving common alleles of small effect, a handful of which have now been definitively identified. However, the past decade of gene discovery in ASD has been most notable for the application, in large family-based cohorts, of high-density microarray studies of submicroscopic chromosomal structure as well as high-throughput DNA sequencing-leading to the identification of an increasingly long list of risk regions and genes disrupted by rare, de novo germline mutations of large effect. This genomic architecture offers particular advantages for the illumination of biological mechanisms but also presents distinctive challenges. While the tremendous locus heterogeneity and functional pleiotropy associated with the more than 100 identified ASD-risk genes and regions is daunting, a growing armamentarium of comprehensive, large, foundational -omics databases, across species and capturing developmental trajectories, are increasingly contributing to a deeper understanding of ASD pathology.

Disorders Associated With Diverse, Recurrent Deletions and Duplications at 1q21.1

The subchromosomal region 1q21.1 is one of the hotspots in the human genome for deletions and reciprocal duplications, owing to the existence of hundreds of segmental duplications. Recurrent deletions and duplications in this region are thought to be causative in patients with variable clinical manifestations. Based on the genomic locations, deletions and duplications at the 1q21.1 locus have been associated with distinguishable syndromes: chromosome 1q21.1 deletion syndrome, chromosome 1q21.1 duplication syndrome, and thrombocytopenia-absent radius (TAR) syndrome, which is partially due to deletions at the proximal 1q21.1 region. We report here diverse, recurrent deletions and duplications at the 1q21.1 locus in 36 patients from a cohort of 5,200 individuals. Among the 36 patients, 18 patients carry 1q21.1 deletions, nine individuals have reciprocal duplications at 1q21.1, two patients share an identical short deletion, and the remaining seven possess variable sizes of duplications at the proximal 1q21.1 region. Furthermore, we provide cytogenetic characterization and detailed clinical features for each patient. Notably, duplications at the proximal 1q21.1 region have not been associated with a defined disorder in publications. However, recurrent duplications at the proximal 1q21.1 region among the seven patients strongly suggested that the variants are likely pathogenic. The common phenotypical features of those disorders are also summarized to facilitate clinical diagnoses and genetic counseling.

The Driver of Extreme Human-Specific Olduvai Repeat Expansion Remains Highly Active in the Human Genome

Sequences encoding Olduvai protein domains (formerly DUF1220) show the greatest human lineage-specific increase in copy number of any coding region in the genome and have been associated, in a dosage-dependent manner, with brain size, cognitive aptitude, autism, and schizophrenia. Tandem intragenic duplications of a three-domain block, termed the Olduvai triplet, in four NBPF genes in the chromosomal 1q21.1-0.2 region, are primarily responsible for the striking human-specific copy number increase. Interestingly, most of the Olduvai triplets are adjacent to, and transcriptionally coregulated with, three human-specific NOTCH2NL genes that have been shown to promote cortical neurogenesis. Until now, the underlying genomic events that drove the Olduvai hyperamplification in humans have remained unexplained. Here, we show that the presence or absence of an alternative first exon of the Olduvai triplet perfectly discriminates between amplified (58/58) and unamplified (0/12) triplets. We provide sequence and breakpoint analyses that suggest the alternative exon was produced by an nonallelic homologous recombination-based mechanism involving the duplicative transposition of an existing Olduvai exon found in the CON3 domain, which typically occurs at the C-terminal end of NBPF genes. We also provide suggestive in vitro evidence that the alternative exon may promote instability through a putative G-quadraplex (pG4)-based mechanism. Lastly, we use single-molecule optical mapping to characterize the intragenic structural variation observed in NBPF genes in 154 unrelated individuals and 52 related individuals from 16 families and show that the presence of pG4-containing Olduvai triplets is strongly correlated with high levels of Olduvai copy number variation. These results suggest that the same driver of genomic instability that allowed the evolutionarily recent, rapid, and extreme human-specific Olduvai expansion remains highly active in the human genome.

A Third Linear Association Between Olduvai (DUF1220) Copy Number and Severity of the Classic Symptoms of Inherited Autism

OBJECTIVE

The authors previously reported that the copy number of sequences encoding an Olduvai protein domain subtype (CON1) shows a linear association with the severity of social deficits and communication impairment in individuals with autism. In this study, using an improved measurement method, the authors replicated this association in an independent population.

METHOD

The authors obtained whole genome sequence (WGS) data and phenotype data on 215 individuals from the Autism Speaks MSSNG project. They derived copy number from WGS data using a modified sequence read-depth technique. A linear mixed-effects model was used to test the association between Olduvai CON1 copy number and symptom severity as measured by the Autism Diagnostic Interview-Revised. The authors then combined data from previous studies (N=524) for final analyses.

RESULTS

A significant linear association was observed between CON1 copy number and social diagnostic score (SDS) (β=0.24) and communicative diagnostic score (CDS) (β=0.23). Using the combined data, the authors present strong significant associations of CON1 dosage with SDS (β=0.18) and CDS (β=0.13). The authors also implicate Olduvai subtypes found in two genes, NBPF1 and NBPF14 (R²=6.2%). Associations were preferentially found in multiplex versus simplex families.

CONCLUSIONS

The finding of a third dose-dependent association between Olduvai sequences and autism severity, preferentially in multiplex families, provides strong evidence that this highly duplicated and underexamined protein domain family plays an important role in inherited autism.

Construction, expression, and characterization of AG11-843 and AG11-1581

AG1, a member of the DUF1220 protein family, exhibits the most extreme human lineage-specific copy number expansion of any protein-coding sequence in the genome. These variations in copy number have been linked to both brain evolution among primates and brain size in humans. Unfortunately, our current understanding of the structure and function of these proteins is limited because current cloning and expression techniques fail to consistently produce recombinant protein for in vitro studies. The present work describes a method for amino acid and DNA sequence optimization and synthesis, recombinant protein expression and analysis of two AG1 fragments, AG1^1-843 and AG1^1-1581. It was first necessary to modify the nucleotide sequence, while holding the GC content at 52.9%. The genes were then sectionally synthesized by overlap PCR. The resulting segments were cloned into the pET-15 b-sumo expression vector and subsequently transformed into BL21 (DE3) cells. After inducing their expression, the AG1^1-843 and AG1^1-1581 proteins were isolated and purified. Furthermore, using dynamic light scattering and gel filtration analysis, AG1^1-843 and AG1^1-1581 were shown to be present in tetrameric and dimeric forms in solution. To our knowledge, this is the first study to synthesize and express fragments of the DUF1220 protein family for in vitro analysis. Taken together, the proven utility and versatility of this method indicate that it can be used as an effective technique to construct and express other proteins with complicated sequences, thus providing the means to study their function and structure in vitro.

Human Accelerated Regions and Other Human-Specific Sequence Variations in the Context of Evolution and Their Relevance for Brain Development

The review discusses, in a format of a timeline, the studies of different types of genetic variants, present in Homo sapiens, but absent in all other primate, mammalian, or vertebrate species, tested so far. The main characteristic of these variants is that they are found in regions of high evolutionary conservation. These sequence variations include single nucleotide substitutions (called human accelerated regions), deletions, and segmental duplications. The rationale for finding such variations in the human genome is that they could be responsible for traits, specific to our species, of which the human brain is the most remarkable. As became obvious, the vast majority of human-specific single nucleotide substitutions are found in noncoding, likely regulatory regions. A number of genes, associated with these human-specific alleles, often through novel enhancer activity, were in fact shown to be implicated in human-specific development of certain brain areas, including the prefrontal cortex. Human-specific deletions may remove regulatory sequences, such as enhancers. Segmental duplications, because of their large size, create new coding sequences, like new functional paralogs. Further functional study of these variants will shed light on evolution of our species, as well as on the etiology of neurodevelopmental disorders.