Barkur S. Shastry

The genetic make-up of an individual not only determines disease susceptibility but also response to drug treatment. It is because of this reason tremendous progress has recently been made in cataloging human sequence variations with an intention such that a high-density map will provide necessary tools to develop genetic-based diagnostic and therapeutic options. The availability of the human DNA sequence, its variation between individuals and the functional understanding of genetic determinants between individuals may enable pharmaceutical companies to discover safer and effective drugs. This will provide physicians with better tools to select the most appropriate treatment for each patient and avoid drugs that cause adverse effects. When a personalized medication becomes a more realistic and beneficial treatment, every newborn child in the neonatal unit will probably be genotyped as a routine procedure for improved treatment. This newly developed toxicogenomic or pharmacogenomic field is rapidly advancing and may change the practice of medicine in the future, providing personalized medicine for each individual.

In two randomly selected human genomes, 99.9% of the DNA sequence is identical. The remaining 0.1% of DNA contains sequence variations. The most common type of such variation is called a single-nucleotide polymorphism, or SNP. SNPs are highly abundant, stable, and distributed throughout the genome. These variations are associated with diversity in the population, individuality, susceptibility to diseases, and individual response to medicine. Recently, it has been suggested that SNPs can be used for homogeneity testing and pharmacogenetic studies and to identify and map complex, common diseases such as high blood pressure, diabetes, and heart disease. Consistent with this proposal is the identification of the patterns of SNPs in conditions such as diabetes, schizophrenia, and blood-pressure homeostasis. Although these studies have provided insight into the nature of human sequence variation, it is not known at present whether these variations are truly significant toxicologically and pharmacologically. Moreover, it is possible that most complex, common disorders are caused by the combined effects of multigenes and nongenetic environmental factors (multifactorial). Therefore, it is likely that sequence variation alone is not sufficient to predict the risk of disease susceptibility, particularly in homeostatic organisms like humans. Nevertheless, these variants may provide a starting point for further inquiry.

Schizophrenia is a complex neurodevelopmental disorder characterized by mental dysfunction across multiple domains of the brain. It affects 1% of world's general population and the nature of neurobiological lesions in the schizophrenic brain are not known. Although the exact etiology of the disorder is not understood, twin, family and adoption studies have provided consistent evidence that genetic factors play a major role in the pathogenesis. A genome-wide genetic linkage screen identified loci on chromosomes 1, 2, 4, 5, 6, 7, 8, 9, 10, 11, 13, 15, 18, 22 and the X with positive lod scores, thus excluding a single major locus for schizophrenia. Association studies have generated disappointing results in identifying the susceptible DNA sequence variants and the anticipation hypothesis on trinucleotide repeat expansion provided equivocal results or lack of enthusiasm. Although there are no biological markers at present, the recent finding that human endogenous retrovirus is activated in cerebrospinal fluid as well as in the postmortem schizophrenic brain may change our understanding of the etiopathogenesis of this disease. Meanwhile, treatment with newly developed anti-psychotic drugs combined with educational and cognitive rehabilitation procedure may help the patients to cope with the illness.

Parkinson's disease is a multifactorial disease. In early-onset cases, genetic predisposition may play a role, but typical Parkinson's disease is most likely determined by genetic and environmental factors. Since age is a consistent risk factor, an age-dependent cumulative insult mechanism may be responsible for the selective degeneration of nigrostriatal neurons. Although there is no cure, several types of treatments are currently available to substantially reduce the clinical symptoms of Parkinson's disease. An understanding of the age-dependent complex pattern of genetic variation (SNP genotyping) together with the genetic expression profile (using chip technology) of the degenerating neurons and the effect of environmental factors is a prerequisite to deciphering the mechanism of pathogenesis. This understanding may lead to predicting drug response and ultimately to developing preventive medicine at the individual level in the future.

Retinopathy of prematurity (ROP) is a leading cause of blindness in premature children. It is a multifactorial disorder which causes fibrovascular tissue changes that affect the retina in low birth-weight and short gestational age infants. To determine the prevalence of Norrie disease (ND) gene mutations, clinical examination and molecular genetic analyses were performed in 100 pre-term babies of different ethnic backgrounds who developed advanced ROP. The leukocyte DNA was extracted, amplified by the polymerase chain reaction (PCR), and analyzed by single-strand conformation polymorphism (SSCP), G/T and C/A scanning, and by DNA sequencing. All three exons, including splice sites and the 3′-untranslated region, were screened. Of the 100 patients analyzed, 2 patients with advanced ROP showed a mobility shift in the DNA. In 1 patient, this mobility shift was caused by the insertion of an additional 12-bp CT repeat in exon 1, and in the second patient, there was a 14-bp deletion in the same exon of the ND gene, as evidenced by direct sequencing of the amplified products. Similar analyses of exons 2 and 3 and the 3′-untranslated region failed to detect additional mutations in the gene. None of the 130 normal, unrelated controls revealed similar changes. Taking into account the above results, as well as those of other studies, it appears that the ND gene mutations can account for 3% of cases of advanced ROP. Although the ND gene is not frequently involved in advanced ROP, the present large-scale study further supports the hypothesis that genetic influences may play an important role in the development of severe ROP in some premature infants.

X-Linked retinitis pigmentosa is a most severe and heterogeneous disorder of the retina. Recently, genes (RP2 and RPGR) from two X-linked loci have been positionally cloned and mutations have been identified in many families. To further evaluate allelic and non-allelic heterogeneity and the genotype--phenotype relationships, and to determine the prevalence of mutations in the gene, we have analyzed one previously unreported X-linked retinitis pigmentosa family, using a combination of haplotype analysis and DNA sequencing. Our extensive analysis of the RP2 gene failed to detect any disease--causing or polymorphic mutations. In the case of the RP3 gene, the alleles of the dinucleotide repeat marker did not segregate with the disease. Although we cannot completely exclude the possibility of the RP2 and RP3 genes as candidate genes, the above results suggest that structural and functional changes associated with the RP2 gene are not responsible for the phenotype in the family analyzed. Further identification of the X-linked genes may facilitate the elucidation of the molecular basis of the disorder in the family analyzed.

Juvenile retinoschisis (RS) and Norrie disease (ND) are X-linked recessive retinal disorders. Both disorders, in the majority of cases, are monogenic and are caused by mutations in the RS and ND genes, respectively. Here we report the identification of a family in which mutations in both the RS and ND genes are segregating with RS pathology. Although the mutations identified in this report were not functionally characterized with regard to their pathogenicity, it is likely that both of them are involved in RS pathology in the family analyzed. This suggests the complexity and digenic nature of monogenic human disorders in some cases. If this proves to be a widespread problem, it will complicate the strategies used to identify the genes involved in diseases and to develop methods for intervention.