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Phenylalanine hydroxylase (PAH) encodes the liver-secreted enzyme of the same name, a catalyst for the hydroxylation of tyrosine from phenylalanine, a rate-limiting step in the catabolism of the latter. This reaction only occurs in the presence of the cofactor tetrahydrobiopterin (BH4) as well as molecular oxygen and iron (1).
Mutations in the PAH gene are generally caused by a change of an amino acid, for example, the change of arginine to tryptophan (2, 3). The numerous possible mutations in this gene result in a lack of enzyme activity. Thus, because of its main function, the deficiency in the activity of PAH causes a marked intolerance of the consumption of phenylalanine, an essential amino acid. This causes phenylketonuria (PKU), non-phenylketonuria hyperphenylalaninemia (non-PKU HPA), mild hyperphenylalaninemia (MHP), and other variant PKU (4, 5, 6).
Defects in the PAH gene leads to the deficiency or the disruption of the production of the PAH enzyme; this is most commonly related to the resulting disorder, phenylketonuria. PKU is an autosomal, inborn, recessive disorder of phenylalanine metabolism (7). There are three common types of PKU. First, there is classical PKU, caused by the mutation of both alleles of the PAH gene in chromosome 12 which results in a severe deficiency or complete absence of the PAH enzyme, leading to toxic levels of unhydroxylated phenylalanine, typically over 10 times higher than normal concentrations (i.e. over 1000 Âµmol compared to the normal 100 Âµmol). Next, there is MHP, the mildest form of the PAH enzyme deficiency, with phenylalanine levels below 600 Âµmol but above normal. Thirdly, there is non-PKU HPA, caused by mutations in the PAH locus that hinder BH4 synthesis and regeneration. This relatively milder form of the disorder often results in heterozygous cases through a combination of mild and severe mutations (4, 7, 8).
Severe classical PKU, if left untreated, is commonly known to result in the impedance of postnatal cognitive development causing mental retardation and in metabolic abnormalities causing increased phenylalanine in in the blood circulation and phenylpyruvic acid in the urine. PKU has also been known to cause skin abnormalities, organ damage, different kinds of posture peculiarities, pregnancy problems (maternal PKU), an odor describe as "mousy", as well as other mental issues such as epilepsy, hyperactivity, and psychotic episodes (1,4,7,8). The most common negative effect associated with PKU, mental retardation, is caused by a neurotoxic effect of HPA. And while PKU is an inherited disorder, its negative effects could also be induced in the offspring of mothers with PKU, resulting not only in high fetus mortality rates but also in a high probability that the children are born with growth and mental retardations as well as malformations. This is known as PKU embryofetopathy or maternal PKU syndrome (8). Conversely, children born with non-PKU HPA and MHP have marked lower risks of being affect with the adverse effects of the disorder and can have normal development mentally and physically even with the absence of treatment (4,8).
Despite the severe potential effects of classical PKU, newborn screening for high levels of phenylalanine has helped early diagnosis of the disorder, which is then followed by rapid treatment. Dietary restrictions of phenylalanine has been used for early treatment of PKU which, while not necessarily lead to complete normalization of IQ, was shown to be predictive of overall IQ with the complete lack of treatment in classical PKU patients leading to severe and irreversible cognitive retardation.(1,8) Thus, primary screening of neonates and children as well as awareness of the disorder for the parents are essential (3, 6).
Results and Discussion:
PAH chromosomal map position and nearby genes:
The location of the PAH gene is at chromosome 12. Its long arm (q) is comprised of 13 exons with an approximate length of 90 kb.
Figure 1 Chromosome 12 (9)
Figure 1, above, is a representation of the entire chromosome 12 with both its short arm (p) and long arm (q) as it appears in the Ensembl website, albeit cropped to fit the page. This figure can be found by searching for the PAH gene and clicking on the "Location" link on the PAH listing. The website lists the location of the gene to be at "Chromosome 12: 103,232,104-103,311,381 reverse strand."(2) Though the website does not explicitly state where in chromosome 12 PAH is located, one can infer additional details from the provided images. For example, confusion can ensue from the fact that the indicated location in the image in the Ensembl website is on the long arm on q23.2, while previous sources have stated that it is located on q22-24.2. However, from the code in the location and the additional images, one can infer that these are the transcribed portions of the gene, two of which are illustrated in the site. Furthermore, one can see that the PAH gene is flanked by the genes insulin-like growth factor 1 (IGF1), or somatomedin C, and achaete-scute complex homolog 1 (ASCL1). To obtain the information, though, one needs to explore the interactive image (see Figure 2 below) and go to the individual pages of the neighbor genes.
Figure 2 Detailed view of region near PAH (9)
The NCBI website, however, while very extensive in details, and containing multiple transcripts pertaining to the PAH gene, can be somewhat confusing with regard to the Map Viewer. Going through the home page and directly searching for the desired gene results in a very large and confusing map, with the details of the gene and its neighboring gene beyond the page to right. For a beginner who is not quite sure what to look for, the NCBI Map Viewer can be very overwhelming. Focusing on the table and not the map, however, one can see that the PAH gene is located in Chromosome 12, in the long arm q22-q24.2; this information is under the heading "Cyto" (for cytogenic) and stated as "12q22-q24.2" (10). Again, this might not be immediately clear to a beginner. Furthermore, the different master map options (Morbid, Gene_cyto, etc.) individually show different arrangements of the symbols, not all of which seem to be genes. Thus, it is very hard to decipher which genes are actually near PAH, although zooming in on the "Genes on Sequence" and "Phenotype" maps do reveal the proximity of IGF1 and ASCL1. In all, for a beginner, the Ensembl website proved to be much easier to use to answer the first question.
The intron/exon structure of the PAH gene:
It was very difficult to find an illustration of the structure of the PAH gene in the NCBI website. However, the information page for the gene stated that the gene spans 90 kb with the entire sequence and its adjacent regions a total of 171 kb. Furthermore, it states that the gene contains 13 exons, which consequently means that it has 12 introns (number of introns is one less than the number of exons) (1). After some searching, however, beginning with clicking the available links for PAH in the Map Viewer table, the link "sv" led to a page with the title "Homo sapiens chromosome 12 genomic contig, GRCh37 reference primary assembly." Searching for the gene gives the following (zoomed-in and cropped) structure:
Figure 3 Structure of PAH gene (11)
Though not obvious from the first glance, later we will see that the bottom sequence actually represents the structure of the PAH, with the vertical green lines representing the 13 exons. After further searching, the following (rotated) PAH structure showing the 13 exons and 12 introns can be found in the Map Viewer under "ensRNA":
Figure 4 Another illustration of the structure of PAH gene (11)
Finding those, however, takes previous explicit knowledge and some work to track down the specific illustrations. In contrast, finding the number of exons and introns and an illustration of the structure of the PAH gene in the Ensembl website was very straightforward. The following illustration can be found in the same page as Figure 1:
Figure 5 Ensembl illustration of PAH gene structure
This strand, one of the transcripts available in the Ensembl page, clearly shows the 13 exons in a DNA sequence. Comparing this structure to Figures 3 and 4, the numbers and the arrangements of the exons and introns are exactly the same. However, relative to all the tedious searching needed to find the same answers in the NCBI website, the information needed for the question was instantly available from the Ensembl site, and the interface was very easy to understand.
Common PAH mutations:
Mutations in general can refer to abnormalities in function or structure of the concerned enzyme in the gene phenotype. As previously discussed, however, such as the causes of PKU and HPA, the human PAH gene has displayed allelic differences and pathogenic transformations throughout its structure. The common types of mutations and their occurrence according to a previous study are: missense mutations with 62% of the alleles, small or large deletions with 13%, splicing defects with 11%, silent polymorphisms with 6%, nonsense mutations with 5%, and insertions with 2% of the PAH alleles. (6)
Table1 PAH mutation statistics
# of Mutation(s)
Total mutations: 547
Most reported Mutation
(Association): p.R408W (214)
Missense, as can be seen above, is the most common cause of mutation in the PAH gene, the molecular mechanism of this is the improper folding of the protein structure, causing aggregation or degradation. As mentioned earlier, the mutations of PAH are commonly caused by single changes in the amino acid. One of the missense mutations, for example, occurs in E1 nucleotide 1 with the change of ATG to GTG. However, there is also missense mutation in region E3 with sequence 187.000 in nucleotide 187; this is called ACC/CCC;CAC/AAC. The second most common type of mutation is deletion. An example of deletion mutation is in regions E2-12 with sequence 168.001 in nucleotide 168. This is called GAG/GAA;G/A and has been noted to have occurred in Palestinians Arabs. (2, 3, 12) Other examples can be seen in Appendix (I).
As mentioned earlier, there are three common variations of PKU: classical PKU, MHP, and non-PKU HPA. These variations which are basically different degrees of severity of the disorder are caused by the different kinds of mutations that cause varying PAH activity as well as allelic variations. The latter effect at the locus of the gene determines the metabolic phenotype of the enzyme deficiency. In general, however, the mutations in the PAH gene are localized in a main part of the gene instead of being randomly distributed, as they occur either within or without the active site. What is interesting to note is that the PAH gene in intron 12 involves the single base change of guanine to adenine in the canonical 5-prime splice donor site where the first identified PKU mutation occurred. (3)
Two out of the 6 links given by the Gene Gateway page were no longer working, one was solely dedicated to SNP, one was a link to a database that had links to other databases, and the last two were already explored thoroughly in previous parts of this assignment. The data presented in this section were mostly from the entire site dedicated to PAH gene mutations, the Phenylalanine Hydoxylase Locus Knowledgebase (5). This site, also a database, was arrived at after searching through the Locus Specific Mutation Databases which in turn arrived at from Human Genome Variation Society: Variation Databases and Related Sites. While the OMIM site did give some details about previous studies related to PAH gene mutations, they were more of a history of the mutations and examples of the studies. Finding the needed information was difficult because one needed to go through link after link and website after website, sometimes even arriving at the same website numerous times through different pathways and still not obtaining any results. The PAHdb was by far, the only site that showed any data regarding the common mutations.
Single nucleotide polymorphisms (SNPs) of the PAH gene:
To date, 1220 SNPs for the PAH gene have been discovered, although GeneCards (2) states only 1097 from the NCBI website. In general, the SNPs involve the changing of a single base, as shown in Appendices I and II. Examples are the three found on exon 3, each of which has a single change of base, name cytocine, thiamine, and adeninine(13).
Examples of these PAH gene SNPs are the rs63749677, rs63749676, rs63581460 and rs63499960; some of these are tabulated in Appendix (II). These SNPs are not randomly distributed as out of the 13 exons, they are seen in exons 1-7 and 12. Searching the NCBI website, however, resulted in 55 entries of SNPs with the following format:
rs79931499 [Homo sapiens]
The above entry, an example of the results from the query in the NCBI SNP website, shows essential information about the SNP as well as options one can view. Compared to the other related links, which did not yield any useful information other than linking back to this site, the NCBI site dedicated purely to SNPs was simple and the information was easy to retrieve. Due to the very large number of SNPs, however, it would be difficult to evaluate all of them.
Designing PCR primers:
The given instructions and the program given in the website were rather straightforward, so the designing of the primer was the easiest part of the activity. The mRNA sequence was easily downloadable and the program was user-friendly (14). Being able to design primers this way was very fast and easy. The resulting primers are in Appendix (III).