Introduction
Behavior is commonly characterized as a response to stimuli, regardless of whether internal or external, that changes an organism’s response to its habitat. Animals run, stay still, or counterstrike to predators; in response to external and internal stimuli birds construct complex and distinguished nests; plants show positive phototropism; and humans behave in both simple and complex ways depending on their keenness and culture. Behavior depends on the expression of the genotype of an organism, which takes happens inside a progressive system of environmental settings. Gene expression is dependent on cell, tissue, organism and eventually, population and habitat interactions.
It has been demonstrated that genetic regulation of behavior in humans is more difficult to characterize than in other organisms, partially because it is difficult to research the types of responses are considered the most fascinating facets of human nature, including intellect, speech, personality, and emotional aspects. In researching such behaviors two issues arise. The first one is that it is difficult to objectively describe and quantitatively estimate these characteristics. Second, environmental factors influence behavior and a vast spectrum of individual variation are seen in the responses to these factors. In each case, in limiting, shaping or facilitating the final phenotype, the environment is very critical.
Does human have a genetic component? The response to that question is disputable to some extent in light of the fact that moral and lawful concerns present challenges in formulating controlled studies of human behavior. However, the evidence for a genetic component of behavior is ample, considering these limitations. Three approaches may be used to study human behavioural genetics: examination of single genes with behavioral elements, the use of animal models, and genomic approaches.
Single Gene Behavior: Huntington Disease
In the United States and around the world, neurodegenerative disorders influence millions of people. These diseases are associated with the gradual accumulation, with behavioural implications, of midfielder proteins in intra- and extracellular spaces, leading to brain cell death. Some of these such as Inherited Alzheimer’s disease (AD),Parkinson’s disease, amyotrophic lateral disease (ALD), and Huntington's disease (HD) may result from mutations in single genes.
Huntington disease (HD) is an autosomal dominant, neurodegenerative condition. HD is caused by repeat expansion of a DNA trinucleotide (triplet) equal to or greater than 40 CAG repeats within the Huntington gene (HTT, OMIM 613004). In the general population, repeat figures range from 6 to 35. There is no expression of HD as there are fewer than 27 repeats, and the gene is stable upon transmission. The formation of HD is often not consistent with repeat lengths in the range of 27 to 35; however, there is a probability of expansion upon transmission, giving rise to genetic anticipation phenomenon. Transmission expansion is most likely within this spectrum at longer repeat lengths, and is often likely to occur during male transmission. CAG repeats in the 36 to 39 range are of incomplete penetrance with varying manifestations of disease.
HD is a rare condition with an occurrence of around 10 to 12 patients per 100,000 people of European descent. The number of HTT repeats is inversely correlated with the onset of the disease, so the higher the number, the sooner the onset. Disease onset is described as the manifestation of major motor or neurological symptoms and occurs on average about the age of 40. Although roughly 50 to 70 percent of the difference in age of onset accounts for the number of HTT repeats, other influential variables continue to be defined; these are likely to be environmental elements or modifiers of genes. While repeat length does not account for all the variation in age of onset, certain generalisations are made possible by the close relationship.
Those with later onset are more likely, sometimes in the middle range, to have less repeats. Repeat lengths lying between 40 and 49 are associated with classic adult onset between the ages of 30 and 50. Usually, repetition lengths greater than 50 are correlated with the onset between the ages of 20 and 30. If the onset of disease occurs before the age of 21, it is referred to as juvenile HD (JHD), which comprises around 5 percent of all cases of HD. Since the was found in 1993, it is possible to screen individuals who are at risk of HD (each offspring of a parent with HD has a 50 percent chance of inheriting the mutation due to autosomal dominance).
Transgenic Mouse model of Huntington Disease
In order to study the normal function of Huntington, model the disease process and study it’s effects at the molecular level, and to develop drugs that delay or stop the degeneration and death of brain cells, several animal HD models have been developed. Danilo Tagle and his colleagues constructed transgenic mice carrying a human HD gene with 16, 48, or 89 copies of CAG repeat to study the link between CAG repeat length and disease progression. The HD gene was positioned adjacent to a promoter and an enhancer to ensure high levels of expression in the vector used for gene transfer.
To assess the age of onset and development of abnormal behavioral phenotypes, mice carrying the transgene were tracked from birth to death. Animals carrying 48- or 89- repeat HD genes displayed behavioral abnormalities as early as 8 weeks, and by 20 weeks, compared to control animals and mice carrying human HD genes with 16 CAG repeats, these mice showed behavioral and motor coordination abnormalities. Brains of wild- type mice and transgenic animals carrying mutant alleles with 16,48, and 89 repeat copies of CAG were examined for changes in structure at different ages. In mice carrying 48 and 89 repeats, degenerating neurons and cell death were apparent, but no changes were seen in the brains of wild type mice or those carrying a 16 - repeat transgene. In particular brain regions in transgenic mice, behavioral changes and degeneration are parallel to the development of HD in humans.
Mechanisms of Huntington Disease
The mechanism by which mutant forms of the Htt protein (mHtt) induce HD is still unclear, although the HD gene was mapped, isolated and characterized over 25 years ago. Mutant Huntington (mHtt) is particularly susceptible to aggregation and one of the most striking hallmarks of HD is the development of cytoplasmic aggregates and nuclear inclusions throughout the brain. Polyglutamine inclusions include highly organized amyloid fibres with high β- sheet content and poor solubility of detergent; they also sequester many other proteins, including important factors for transcription and quality control of protein, indicating that their existence is deleterious to cellular function and leads to a complex loss-of-function phenotype. As the most toxic species, several lines of evidence include small oligomeric types of mHtt and indicate that the development of large inclusions may reflect an alternative coping strategy in which mHtt is separated into a structure less prevalent. Aggregate formation is a complex multi- step process in which, before inclusions are formed, mHtt monomers assemble into a variety of intermediate oligomeric species. The amino acid sequences flanking the polyglutamine stretch, post translational mHtt modifications, and levels of molecular chaperones influence this process. As mHtt monomers, oligomers, and broad inclusions can co-exist and disrupt multiple cellular pathways and affect disease progression, the spectrum of oligomeric confirmations adopted by mHtt has made it difficult to understand the pathogenic function of each species. In addition, to facilitate polyglutamine aggregation, extracellular polyglutamine aggregates may be internalised by cells. This poses the interesting possibility of mHTT spreading during disease development between cells and regions.
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HD onset is associated with decrease in mRNA levels for genes encoding certain neurotransmitter receptors in affected people. Researchers used cDNA array to screen nearly 6000 mRNAs from striatal cells in a transgenic HD mouse model. From a small set of genes involved in signalling pathways that are important to striatal cell activity, they find lower levels of mRNA. In another transgenic HD mouse model, similar results were observed and are consistent with findings from HD patients, suggesting aberrant gene expression as one of HD's underlying causes. The mutant type of Htt interacts with many transcription factors and binds to them and can modify gene expression patterns by rendering these proteins unavailable. Some proteins that interact with Htt have histone modifying activity in the brains of the transgenic HD mice and HD patients and are contained in Htt protein aggregates.
Schizophrenia
Schizophrenia is a complex, persistent mental health condition characterized by a variety of symptoms, including delusions, hallucinations, disorganized speech or actions and damaged cognitive skills. For many patients and their families, the early onset of the disease, along with its chronic progression, renders it a debilitating condition. Disability is also caused by both negative symptoms (characterized by loss or deficits) and cognitive symptoms, such as concentration disorder, working memory or executive function. Moreover, due to positive signs, such as suspiciousness, delusions, and hallucinations, relapses may occur. Schizophrenia's intrinsic heterogeneity has resulted in a lack of consensus on the criteria for diagnosis, epidemiology and pathophysiology of the condition.
People affected by the disease are unable to lead normal lives and are impaired by their symptoms on a periodic basis. With families of schizophrenics having a much higher prevalence of the disease than the general population, it is evidently a family illness. In addition, the closer the genetic or biological link to an affected individual is, greater the likelihood of a person having the condition. A genetic link to schizophrenia was helped by twin research. Whether a characteristic is shared by both twins, they are assumed to be consistent with that characteristic; if one expresses the characteristic, but the other does not, they are inconsistent with that characteristic. Concordance in monozygotic twins is higher in trials of monozygotic and dizygotic twins for schizophrenia than in dizygotic twins. While these findings indicate that a genetic aspect exists, the exact genetic basis of schizophrenia is not disclosed.
The quest for the role of genes in schizophrenia parallels the advancement of technologies that enable genes associated with complex traits to be searched for in new ways. Linkage studies with RFLP markers identified chromosome regions that were segregated in families with affected members starting in the late 1980s. By 2000, on chromosome 6,8,and 13, a small number of candidates genes had been identified. The role of some of the encoded proteins in the transmission of nerve impulses supports the idea that, although there is some debate about their significance in the disease process, these genes may play a role in schizophrenia. In order to study schizophrenia, new experimental methods developed during and after the Human Genome Project have used haplotype mapping and high throughput sequencing approaches. Genome wide association studies (GWAS) are the most significant of these, utilising some of the more than 10 million single nucleotides in the human genome.
GWAS evidence on schizophrenia suggests that no single gene or allele contributes significantly to this disorder. Instead, the findings point to hundreds of genes being involved, each contributing only a small amount to schizophrenia. GWAS is based on the premise that common gene variants lead to disease risk (the common disease / common variant hypothesis, abbreviated as CDCV) (present in more than 5 percent of the population).
As a way of detecting these variants, large-scale all-genome resequencing is suggested to classify allelic variants associated with gene-phenotype associations. Others also argue that other mechanisms could be important in schizophrenia and other psychiatric disorders, such as gene-gene interactions, protein-protein interactions, epigenetic alterations, or copy number variations (CNVs). A recent research has shown that in schizophrenia cases, de novo CNVs representing both deletions and duplications were three to four times more common than in normal individuals. Many genes in these CNVs are involved in the development of the brain, offering clues to schizophrenia 's cellular basis.
Autism
Autism spectrum disorder (ASD) is a common heterogeneous neurodevelopmental condition that has three main characteristics: Language delay, social interaction and communication impairment, and repetitive actions or interests. Furthermore, in addition to these main symptoms, autistic people often have comorbidities such as intellectual disability, gastrointestinal disorders, epilepsy, immune disorders, and sleeplessness.
In children with autism, the recurrence risk of omnipresent developmental disability in siblings is 2% to 8%; and it increases to 12% to 20% if one takes into account siblings with impairment in one or two of the three autism-impaired domains, respectively. In addition, some twin studies have indicated that shared genes, as opposed to shared environments, better explain this aggregation within families. Interestingly, even though the findings are heterogeneous (heritability 40 percent to 80 percent), the variation of autistic characteristics in the general population has been shown to be strongly heritable, at a comparable degree of genetic impact to autism itself. These findings have contributed to an immense research initiative to try to unravel the genetic factors that underlie the condition. Two recent twin studies, however, have offered interesting results. One research revealed that monozygotic twins had higher concordance rates for ASDs, attention deficit hyperactivity disorder ( ADHD), developmental coordination disorder, and tic disorder with discrepancies between monozygotic and dizygotic twins in cross-disorder results, increasing the issue of the specificity of the genetic factors influencing them. The high heritability model of autism was recently challenged by another study, estimating the heritability of autism to be 55 percent. This study generated considerable debate, the main criticisms regarding the very large odds ratio confidence interval (9 percent to 81 percent) and the low participation rate. This research, however, is the first population-based twin autism study that has used contemporary criteria for autism diagnosis.
The independent heritability in each of the autistic symptomatology domains is still a topic of discussion. While some claim that multiple autism symptoms have distinct hereditary origins to a substantial degree, others claim that clear evidence exists in support of the theory that the domains of symptoms reflect associated behavioural manifestations of a single underlying quantitative neurodevelopmental disorder.
Transmission in simplex and multiplex families: in subjects from simplex families (one person affected), the prevalence of de novo chromosomal rearrangements is greater than in subjects from multiplex families, according to two reports, which is consistent with the high rate of prominent de novo mutations found in simplex families. This is also in line with the findings of studies that have shown that only in multiplex families can family accumulation of subclinical autistic traits occur, indicating differential mechanisms.
Expression profiles show that schizophrenia and ASD are disorders that represent opposite sides of the same coin. ASD is associated with increased activity in these developmental pathways leading to cellular overgrowth and increased brain and head size, while schizophrenia is characterized by a reduced activity in these pathways, resulting in developmental undergrowth and reduced brain and head size. Mapping of other genes in these chromosomal regions into functional networks should advance our knowledge of normal brain development and the nature of the defects in gene regulation associated with both schizophrenia and autism.
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