Preimplantation genetic haplotyping (PGH) is a clinical method of preimplantation genetic diagnosis (PGD) used to determine the presence of single gene disorders in offspring. PGH provides a more feasible method of gene location than whole-genome association experiments, which are expensive and time-consuming. PGH differs from common PGD methods such as fluorescence ''in situ'' hybridization (FISH) and

polymerase chain reaction The polymerase chain reaction (PCR) is a method widely used to rapidly make millions to billions of copies (complete or partial) of a specific DNA sample, allowing scientists to take a very small sample of DNA and amplify it (or a part of it) t ...

(PCR) for two primary reasons. First, rather than focusing on the genetic makeup of an

embryo An embryo is an initial stage of development of a multicellular organism. In organisms that reproduce sexually, embryonic development is the part of the life cycle that begins just after fertilization of the female egg cell by the male spe ...

PGH compares the genome of affected and unaffected members of previous generations. This examination of generational variation then allows for a

haplotype A haplotype ( haploid genotype) is a group of alleles in an organism that are inherited together from a single parent. Many organisms contain genetic material ( DNA) which is inherited from two parents. Normally these organisms have their DNA or ...

of genetic markers statistically associated with the target disease to be identified, rather than searching merely for a mutation. PGH is often used to reinforce other methods of genetic testing, and is considered more accurate than certain more common PGD methods because it has been found to reduce risk of misdiagnoses. Studies have found that misdiagnoses due to allele dropout (ADO), one of the most common causes of interpretation error, can be almost eliminated through use of PGH.Coskun S, Qubbaj W. 2010. Preimplantation genetic diagnosis and selection. J. Reprod Stem Cell Biotechnol 1(1): 120-140. Further, in the case of mutation due to translocation, PGH is able to detect chromosome abnormality to its full extent by differentiating between embryos carrying balanced forms of a translocation versus those carrying the homologous normal

chromosome A chromosome is a long DNA molecule with part or all of the genetic material of an organism. In most chromosomes the very long thin DNA fibers are coated with packaging proteins; in eukaryotic cells the most important of these proteins are ...

s. This is an advantage because PGD methods such as FISH are able to reveal whether an embryo will express the phenotypic difference, but not whether an embryo may be a carrier.Altarescu G, Zeevi DA, Zeligson S, Perlberg S, Eldar-Geva T, Margalioth EJ, Levy-Lahad E, Renbaum P. Familial haplotyping and embryo analysis for Preimplantation Genetic Diagnosis (PGD) microarrays: a proof of principle study. J Assist Reprod Genet (2013) 30:1595–1603. In 2015, PGH was used in conjunction with a whole-genome amplification (WGA) process to not only diagnose disease but also distinguish meiotic segregation errors from mitotic ones. Studies are being continually performed in an attempt to utilize and improve PGD methods since their initial invention. It has become increasingly popular because it grants individuals the option of detecting embryo abnormalities before implantation, rather than during the beginning weeks of pregnancy. The latter often results in embryo abortion, presenting an ethical dilemma for many that can now be avoided.

Procedure

PGH uses information regarding family history in conjunction with the use of linked polymorphic markers such as

short tandem repeats A microsatellite is a tract of repetitive DNA in which certain DNA motifs (ranging in length from one to six or more base pairs) are repeated, typically 5–50 times. Microsatellites occur at thousands of locations within an organism's genome. ...

(STRs) and

single nucleotide polymorphisms In genetics, a single-nucleotide polymorphism (SNP ; plural SNPs ) is a germline substitution of a single nucleotide at a specific position in the genome. Although certain definitions require the substitution to be present in a sufficiently larg ...

(SNPs) to locate genes responsible for disease. Both STRs and SNPs are variations in gene nucleotides, and it is estimated that there are tens of millions of each type of variation in human DNA. High frequency of STRs or SNPs in alleles of affected individuals in comparison to their unaffected direct relatives indicates the origin of a disease causing mutation. They thus "mark" alleles as having a mutation without having to specifically identify the mutation. Because the number of potential STRs and SNPs is so high, a family pedigree helps to narrow the scope of alleles to analyze. Further, understanding how the gene of interest gets expressed over time helps ultimately determine which haplotype is responsible for the alleles linked to the mutation. A haplotype map is thus created, not only exhibiting genes the offspring will contain, but also the parental origin of the genes. Once the alleles that correlate with a mutation are characterized, PGH of the embryos is possible and only embryos carrying the low risk haplotypes are selected for transfer. PGH is performed

in vitro ''In vitro'' (meaning in glass, or ''in the glass'') studies are performed with microorganisms, cells, or biological molecules outside their normal biological context. Colloquially called "test-tube experiments", these studies in biology an ...

until this point, when the chosen embryos get placed in the uterus of a surrogate mother for further development.

Advantages

Once a panel of associated genetic markers has been established for a particular disease it can be used for all carriers of that disease. In contrast, since even a monogenic disease can be caused by many different mutations within the affected gene, conventional PGD methods based on finding a specific mutation would require mutation-specific tests. Thus, PGH widens the availability of PGD to cases where mutation-specific tests are unavailable. PGH also has an advantage over

fluorescence in situ hybridization Fluorescence ''in situ'' hybridization (FISH) is a molecular cytogenetic technique that uses fluorescent probes that bind to only particular parts of a nucleic acid sequence with a high degree of sequence complementarity. It was developed b ...

(FISH) in that FISH is not usually able to make the differentiation between embryos that possess the balanced form of a

chromosomal translocation In genetics, chromosome translocation is a phenomenon that results in unusual rearrangement of chromosomes. This includes balanced and unbalanced translocation, with two main types: reciprocal-, and Robertsonian translocation. Reciprocal translo ...

and those carrying the homologous normal chromosomes. This inability can be seriously harmful to the diagnosis made. PGH can make the distinction that FISH often cannot. PGH does this by using polymorphic markers that are better suited at recognizing translocations. These polymorphic markers are able to distinguish between embryos that carried normal, balanced, and unbalanced translocations. FISH also requires more cell fixation for analysis whereas PGH requires only transfer of cells into polymerase chain reaction tubes. The cell transfer is a simpler method and leaves less room for analysis failure.Shamash, J. et al. (2011). Preimplantation genetic haplotyping a new application for diagnosis of translocation carrier’s embryos – preliminary observations of two robertsonian trans-location carrier families. Journal of Assisted Reproduction and Genetics, 28(1), 77-83.

Uses

PGH has been used to screen for: *

Cystic fibrosis Cystic fibrosis (CF) is a rare genetic disorder that affects mostly the lungs, but also the pancreas, liver, kidneys, and intestine. Long-term issues include difficulty breathing and coughing up mucus as a result of frequent lung infections. O ...

* Duchenne muscular dystrophy *

Huntington's disease Huntington's disease (HD), also known as Huntington's chorea, is a neurodegenerative disease that is mostly inherited. The earliest symptoms are often subtle problems with mood or mental abilities. A general lack of coordination and an unst ...

Spinal muscular atrophy Spinal muscular atrophy (SMA) is a rare neuromuscular disorder that results in the loss of motor neurons and progressive muscle wasting. It is usually diagnosed in infancy or early childhood and if left untreated it is the most common genetic ...

Alport's syndrome Alport syndrome is a genetic disorder affecting around 1 in 5,000-10,000 children, characterized by glomerulonephritis, end-stage kidney disease, and hearing loss. Alport syndrome can also affect the eyes, though the changes do not usually affect v ...

* Von Hippel–Lindau disease *

Sickle-cell disease Sickle cell disease (SCD) is a group of blood disorders typically inherited from a person's parents. The most common type is known as sickle cell anaemia. It results in an abnormality in the oxygen-carrying protein haemoglobin found in red blo ...

Hydatidiform mole A molar pregnancy also known as a hydatidiform mole, is an abnormal form of pregnancy in which a non-viable fertilized egg implants in the uterus. A molar pregnancy is a type of gestational trophoblastic disease that used to be known as a ''hyda ...

History

While PGD was initially carried out to sex rabbits in 1968, human PGD only became available after the development of PCR on a single cell DNA in 1985. PGH was first developed in 2006 at

London London is the capital and largest city of England and the United Kingdom, with a population of just under 9 million. It stands on the River Thames in south-east England at the head of a estuary down to the North Sea, and has been a majo ...

Guy's Hospital Guy's Hospital is an NHS hospital in the borough of Southwark in central London. It is part of Guy's and St Thomas' NHS Foundation Trust and one of the institutions that comprise the King's Health Partners, an academic health science centre. ...

References

External links

* {{cite web , url = http://www.guysandstthomas.nhs.uk/news/newsarchive/newsarticles/pgd.aspx , title = Revolutionary new technology will allow more couples to benefit from preimplantation genetic diagnosis , access-date = 2006-09-26 , date = 2006-09-12 , work = PGH Press Release , publisher = 2006 Guy's and St Thomas' NHS Foundation Trust , archive-url = https://web.archive.org/web/20060930051138/http://www.guysandstthomas.nhs.uk/news/newsarchive/newsarticles/pgd.aspx , archive-date = 2006-09-30 Fertility medicine Medical genetics