Vol. 14, Spring 1997
Molecular Genetic Testing in Mainstream Medicine
Genetic Testing for Prader-Willi and Angelman Syndromes
- Introduction
The Prader-Willi and Angelman syndromes are distinct mental retardation disorders, but frequently discussed together because both are due to genetic abnormalities of chromosome 15. Genetic testing for these disorders has improved markedly over the last 15 years, reflecting our understanding of the molecular basis of these two conditions. This review will summarize the clinical findings, history of genetic testing and understanding of the basis of these conditions, and current recommendations for testing with a case example.
Both syndromes can be diagnosed by history and physical examination, but the laboratory tests are helpful for clinical confirmation, in very young patients where a phenotype has not evolved, in prenatal diagnosis, and in defining recurrence risks.
- Prader-Willi Syndrome (PWS)
- Mental retardation, hypogonadism, hypotonia, obesity, characteristic facial appearance
- Paternal 15q deletion, maternal 15q uniparental disomy (in 99 % of patients)
- Abnormal DNA methylation assay (99 % detection)
- Benefits of Dx: Pro-active recognition of appetite, weight, and cognitive behavioral problems
- Laboratory diagnosis in severe neonatal hypotonia
PWS is characterized by endocrine-type abnormalities associated with cognitive impairment and characteristic behavioral phenotype. Typical findings after age 2 years include obesity, short stature, small hands and feet, hypogonadism, hypotonia, hypopigmentation, and reduced facial expression with upslanting palpebral fissures, narrow bifrontal diameter, full cheeks, and downturned corners of the mouth. The history includes hypotonia and poor feeding in the newborn period, with later development of insatiable appetite, obesity, and oppositional behavior. A clinical scoring system with consensus diagnostic criteria has been developed by Holm et al. to assist in evaluation.
In 1981, a small chromosome deletion of the proximal long arm of chromosome 15 was discovered in about half of PWS patients. For many years this served as a useful confirmatory test for a clinically suspected PWS case. Recently, DNA and chromosomal molecular methods (FISH) have been utilized to more easily discover these deletions and to improve sensitivity in detection of smaller, submicroscopic deletions (microdeletions).
In 1989, researchers discovered that the chromosome showing the deletion in PWS was always paternally inherited (i.e. in the 15 chromosome contributed by the father with generally normal results in the father himself, exceptions to be described below). In non-deletion cases, patients were found to have inherited two maternal copies of the critical 15q11-13 region, a phenomenon called uniparental disomy (UPD). Over 99 % of PWS patients show either deletion or UPD. These two findings have led to an understanding that this chromosomal region is influenced by "imprinting". Imprinting refers to expression of a gene from only one parent's chromosome with silencing of the genes from the other parent's chromosome. In the case of PWS, the paternal region is active and maternal inactive, so that deletion or failure of inheritance of the paternal region causes the syndrome.
Recently, rare PWS patients have been studied who do not show any of the above changes, but who have a defect in the imprinting of the region. Recurrence risks for UPD and larger deletions are very low, but small deletions and imprinting defects can be familial, as discussed below.
- Angelman Syndrome (AS)
- Mental retardation, absent speech, seizures, inappropriate laughter, peculiar gait
- Maternal 15q deletion, paternal uniparental disomy (70 % of patients)
- Abnormal methylation assay (70 % detection)
- Benefits of Dx: Proper genetic counseling with secure diagnosis
This is a condition of severe mental retardation with absent speech, seizures, facial findings of prominent jaw with protruding tongue and wide mouth with widely spaced teeth, hypopigmentation, inappropriate laughter with a happy disposition, and characteristic gait with stiff legs and arms held flexed and perpendicular to the body (sometimes referred to as the "happy puppet" syndrome because of the latter two findings).
AS was not related to PWS until recognition in 1987 that similar deletions of chromosome 15q were also present in about half of AS patients. The critical factor, as in PWS, is that the deleted chromosome is parent-specific, in this case being the mother's 15 chromosome. The reasoning regarding imprinting is identical, but completely the opposite of that for PWS.
Recent work has indicated that the critical regions for PWS and AS are in fact separate within the small 15q region, but that larger deletions can encompass both regions. For AS, about 60 % of patients show deletion of the maternal chromosome, and 5-10% UPD for the paternal 15q region. The remainder cannot be identified by these laboratory tests. There have been several recurrences in AS families and there is a possibility of maternally inherited single gene or imprinting defects.
- Laboratory Testing for PWS/AS
Over the past few years, DNA testing for imprinting status of the 15q region has been developed. As abnormal imprinting is observed for PWS and AS patients with chromosome deletions, UPD, or the rare imprinting mutations, it is very effective in confirming a clinical diagnosis in over 99 % of PWS patients and 70 % of AS patients. The test is referred to as a DNA "methylation" assay (methylation of DNA is a probable mechanism of imprinting). The assay can determine whether there is abnormal maternal or paternal imprinting in the critical 15q region and therefore can differentiate PWS from AS. This is currently recommended as the initial confirmatory laboratory test for PWS and AS. However, it is critical to keep in mind that patients referred in for PWS or AS indications may have an unexpected chromosomal anomaly, so a high resolution karyotype is recommended. This could be done first to rule out a microscopically visible 15q deletion.
In addition, in patients suspected of PWS who have normal chromosomes and DNA methylation studies, a DNA test for Fragile X syndrome (FMR- 1 expansion) should be considered as patients with Fragile X syndrome can occasionally manifest a PWS-like phenotype.
Should a patient be found to have an abnormal result on the methylation assay, and no chromosomal deletion visible, FISH studies for a microdeletion should be ordered, as this is the most likely cause of the patient's abnormal methylation result. Should a deletion be found, the appropriate parent should be checked (the mother of an AS patient or the father of a PWS patient) for the deletion, as very small deletions can be transmitted to other offspring with a 50% risk. The parent carrying a small deletion in one of the two critical regions does not manifest the disorder because the deletion is carried on the chromosome that is normally inactive (e.g., the father of a PWS patient could carry a small PWS region deletion on the chromosome he inherited from his mother).
Parents cannot carry larger, visible deletions as they would manifest signs of one of these disorders. The likelihood of a parent carrying a microdeletion is low, but given the substantial recurrence risk, the follow-up parental testing is recommended.
The benefits of correct diagnosis in PWS and AS relate to specific treatment and counseling, especially for diet and behavior in PWS and for recurrence risk in AS. Increased parental knowledge, with availability of national and local support groups for these disorders is also highly beneficial.
- Illustrative Case Report
This patient was first evaluated at 7 weeks. He was born following a pregnancy with normal fetal activity but with late development of polyhydramnios and breech position (signs of possible fetal CNS dysfunction) requiring C-section delivery. Though born full term, the baby was lethargic, hypotonic, and a poor feeder requiring feedings by gavage. A karyotype was normal, and a brain CT scan was unremarkable.
Because of the significant and persistent hypotonia, DNA methylation studies for PWS were ordered. Many experts recommend a screen for PWS (and for myotonic dystrophy) in this clinical scenario. The methylation test indicated lack of an active, paternal 15q region, indicating a laboratory diagnosis of PWS. On repeat evaluation at one year of age, height was at the 75 %ile and weight at the 50 %ile. Milestones were minimally delayed, with sitting, pulling to stand, babbling, and waving. Testes were undescended and there were subtle facial findings of PWS. A FISH study showed deletion of the SNRPN gene, which is currently the best cytogenetic assay for PWS. The father has not yet been checked for a similar deletion. Parents will be vigilant for changes in appetite and weight gain.
Contributed by John P. Johnson, MD (MT)
The Genetic Drift Newsletter is not copyrighted. Readers are free to duplicate all or parts of its contents. The Genetic Drift Newsletter is published semiannually by the Mountain States Genetics Network for associates & those interested in Human Genetics. In accordance with accepted publication standards, we request acknowledgement in print of any article reproduced in another publication. The views expressed in the newsletter do not necessarily reflect local, state, or federal policy. For additional information, contact Carol Clericuzio, M.D., Editor, Department of Pediatrics, The University of New Mexico, Albuquerque, NM, 87131
Table of Contents:
Molecular Genetic Testing in Mainstream Medicine
Introduction
Venous Thrombosis and the Factor V (Leiden) Mutation
DNA Testing for Hereditary Hemochromatosis
APO E Genotype Testing for Broad Beta Disease (Type III Hyperlipoproteinemia)
Fetal Rh Testing for Maternal-Fetal Incompatibility
Type 1 (insulin-dependent) Diabetes Mellitus
Adult Onset Neurodegenerative Disorders: CAG Triplet Repeat Expansion Mutations
Genetic Testing for Prader-Willi and Angelman Syndromes
Clinical & Applied Molecular Genetics Labs - MoSt GeNe Region
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