SUMMARY Patterns of daily human activity are controlled by an intrinsic circadian clock that promotes ~24 hour rhythms in many behavioral and physiological processes. This system is altered in Delayed Sleep Phase Disorder (DSPD), a common form of insomnia where sleep episodes are shifted to later times misaligned with the societal norm. Here, we report a hereditary form of DSPD associated with a dominant coding variation in the core circadian clock gene CRY1, which creates a transcriptional inhibitor with enhanced affinity for circadian activator proteins Clock and Bmal1. This gain-of-function CRY1 variant causes reduced expression of key transcriptional targets and lengthens the period of circadian molecular rhythms, providing a mechanistic link to DSPD symptoms. The allele has a frequency of up to 0.6% and reverse phenotyping of unrelated families corroborates late and/or fragmented sleep patterns in carriers, suggesting that it affects sleep behavior in a sizeable portion of the human population.

Quadrupedal gait in humans, also known as Unertan syndrome, is a rare phenotype associated with dysarthric speech, mental retardation, and varying degrees of cerebrocerebellar hypoplasia. Four large consanguineous kindreds from Turkey manifest this phenotype. In two families (A and D), shared homozygosity among affected relatives mapped the trait to a 1.3-Mb region of chromosome 9p24. This genomic region includes the VLDLR gene, which encodes the very low-density lipoprotein receptor, a component of the reelin signaling pathway involved in neuroblast migration in the cerebral cortex and cerebellum. Sequence analysis of VLDLR revealed nonsense mutation R257X in family A and singlenucleotide deletion c2339delT in family D. Both these mutations are predicted to lead to truncated proteins lacking transmembrane and signaling domains. In two other families (B and C), the phenotype is not linked to chromosome 9p. Our data indicate that mutations in VLDLR impair cerebrocerebellar function, conferring in these families a dramatic influence on gait, and that hereditary disorders associated with quadrupedal gait in humans are genetically heterogeneous.genetics ͉ Unertan syndrome O bligatory bipedal locomotion and upright posture of modern humans are unique among living primates. Studies of fossil hominids have contributed significantly to modern understanding of the evolution of posture and locomotion (1-5), but little is known about the underlying molecular pathways for development of these traits. Evaluation of changes in brain activity during voluntary walking in normal subjects suggests that the cerebral cortices controlling motor functions, visual cortex, basal ganglia, and the cerebellum might be involved in bipedal locomotor activities (6). The cerebellum is particularly important for movement control and plays a critical role in balance and locomotion (7).Neurodevelopmental disorders associated with cerebellar hypoplasias are rare and often accompanied by additional neuropathology. These clinical phenotypes vary from predominantly cerebellar syndromes to sensorimotor neuropathology, ophthalmological disturbances, involuntary movements, seizures, cognitive dysfunction, skeletal abnormalities, and cutaneous disorders, among others (8). Quadrupedal locomotion was first reported when Tan (9, 10) described a large consanguineous family exhibiting Unertan syndrome, an autosomal recessive neurodevelopmental condition with cerebellar and cortical hypoplasia accompanied by mental retardation, primitive and dysarthric speech, and, most notably, quadrupedal locomotion. Subsequent homozygosity mapping indicated that the phenotype of this family was linked to chromosome 17p (11). Thereafter, three additional families from Turkey (12-14) and another from Brazil (15) with similar phenotypes have been described, and video recordings illustrating the quadrupedal gait have been made (10-12). Here, we report that VLDLR is the gene responsible for the syndrome in two of these four Turkish families and report additional gene mapping st...

The biological basis for the development of the cerebro-cerebellar structures required for posture and gait in humans is poorly understood. We investigated a large consanguineous family from Turkey exhibiting an extremely rare phenotype associated with quadrupedal locomotion, mental retardation, and cerebro-cerebellar hypoplasia, linked to a 7.1-Mb region of homozygosity on chromosome 17p13.1-13.3. Diffusion weighted imaging and fiber tractography of the patients' brains revealed morphological abnormalities in the cerebellum and corpus callosum, in particular atrophy of superior, middle, and inferior peduncles of the cerebellum. Structural magnetic resonance imaging showed additional morphometric abnormalities in several cortical areas, including the corpus callosum, precentral gyrus, and Brodmann areas BA6, BA44, and BA45. Targeted sequencing of the entire homozygous region in three affected individuals and two obligate carriers uncovered a private missense mutation, WDR81 p.P856L, which cosegregated with the condition in the extended family. The mutation lies in a highly conserved region of WDR81, flanked by an N-terminal BEACH domain and C-terminal WD40 beta-propeller domains. WDR81 is predicted to be a transmembrane protein. It is highly expressed in the cerebellum and corpus callosum, in particular in the Purkinje cell layer of the cerebellum. WDR81 represents the third gene, after VLDLR and CA8, implicated in quadrupedal locomotion in humans.

Essential tremor is one of the most frequent movement disorders of humans and can be associated with substantial disability. Some but not all persons with essential tremor develop signs of Parkinson disease, and the relationship between the conditions has not been clear. In a six-generation consanguineous Turkish kindred with both essential tremor and Parkinson disease, we carried out whole exome sequencing and pedigree analysis, identifying HTRA2 p.G399S as the allele likely responsible for both conditions. Essential tremor was present in persons either heterozygous or homozygous for this allele. Homozygosity was associated with earlier age at onset of tremor (P < 0.0001), more severe postural tremor (P < 0.0001), and more severe kinetic tremor (P = 0.0019). Homozygotes, but not heterozygotes, developed Parkinson signs in the middle age. Among population controls from the same Anatolian region as the family, frequency of HTRA2 p.G399S was 0.0027, slightly lower than other populations. HTRA2 encodes a mitochondrial serine protease. Loss of function of HtrA2 was previously shown to lead to parkinsonian features in motor neuron degeneration (mnd2) mice. HTRA2 p. G399S was previously shown to lead to mitochondrial dysfunction, altered mitochondrial morphology, and decreased protease activity, but epidemiologic studies of an association between HTRA2 and Parkinson disease yielded conflicting results. Our results suggest that in some families, HTRA2 p.G399S is responsible for hereditary essential tremor and that homozygotes for this allele develop Parkinson disease. This hypothesis has implications for understanding the pathogenesis of essential tremor and its relationship to Parkinson disease.gene identification | neurodegenerative disease | mitochondrial dysfunction | DNA sequencing | mutation

Cerebellar ataxia, mental retardation and dysequilibrium syndrome is a rare and heterogeneous condition. We investigated a consanguineous family from Turkey with four affected individuals exhibiting the condition. Homozygosity mapping revealed that several shared homozygous regions, including chromosome 13q12. Targeted next-generation sequencing of an affected individual followed by segregation analysis, population screening and prediction approaches revealed a novel missense variant, p.I376M, in ATP8A2. The mutation lies in a highly conserved C-terminal transmembrane region of E1 E2 ATPase domain. The ATP8A2 gene is mainly expressed in brain and development, in particular cerebellum. Interestingly, an unrelated individual has been identified, in whom mental retardation and severe hypotonia is associated with a de novo t(10;13) balanced translocation resulting with the disruption of ATP8A2. These findings suggest that ATP8A2 is involved in the development of the cerebrocerebellar structures required for posture and gait in humans.

Characterizing the patterns and rate of de novo mutations is crucial for our perception of evolution and genetic basis of human disease. Direct observation of de novo single nucleotide variation (SNV) rate in healthy individuals revealed a rate in a range of 0.82 -1.70 ×10 -8 base pair per generation. However, the developmental timing of the de novo mutations is unknown and thus, contribution of the early post-zygotic mutations to the human de novo SNV rate remained unknown. In an attempt to estimate the rate of de novo mutations regarding the developmental timing of mutagenesis, we sequenced the whole genomes of a healthy monozygotic twin pair and their parents with a total of 170 fold coverage. We identified the de novo SNVs through examination of the genotypes of each individual for each of the variants in a synchronous manner. Subsequent to the Sanger sequencing based validation, we conservatively characterized a total of 32 de novo SNVs. Of these 23 were shared by the twin pair, 8 were specific to twin I, and 1 was specific to twin II. We estimated the overall de novo SNV rate of 1.31 × 10 -8 for twin I and 1.01 × 10 -8 for twin II. The rate of the early post-zygotic de novo SNVs was calculated to be 0.34 × 10 -8 and 0.04 × 10 -8 for twin I and twin II, respectively. These data indicate the growing importance of genome mosaicism which might be resulted from de novo mutations of early post-zygotic origin in disease pathogenesis.

The construction of population-based variomes has contributed substantially to our understanding of the genetic basis of human inherited disease. Here, we investigated the genetic structure of Turkey from 3,362 unrelated subjects whose whole exomes (n = 2,589) or whole genomes (n = 773) were sequenced to generate a Turkish (TR) Variome that should serve to facilitate disease gene discovery in Turkey. Consistent with the history of present-day Turkey as a crossroads between Europe and Asia, we found extensive admixture between Balkan, Caucasus, Middle Eastern, and European populations with a closer genetic relationship of the TR population to Europeans than hitherto appreciated. We determined that 50% of TR individuals had high inbreeding coefficients (≥0.0156) with runs of homozygosity longer than 4 Mb being found exclusively in the TR population when compared to 1000 Genomes Project populations. We also found that 28% of exome and 49% of genome variants in the very rare range (allele frequency < 0.005) are unique to the modern TR population. We annotated these variants based on their functional consequences to establish a TR Variome containing alleles of potential medical relevance, a repository of homozygous loss-of-function variants and a TR reference panel for genotype imputation using high-quality haplotypes, to facilitate genome-wide association studies. In addition to providing information on the genetic structure of the modern TR population, these data provide an invaluable resource for future studies to identify variants that are associated with specific phenotypes as well as establishing the phenotypic consequences of mutations in specific genes.

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