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Individual differences |
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1p36 Deletion Syndrome
- Mental retardation affects 2-3% of the population and chromosome abnormalities account for about 20% of cases
- Improved techniques in cytogenetics (FISH) have helped to identify submicroscopic deletions at the cytogenetic level
- Syndrome is characterized by distinct facial features, mental retardation, and delayed growth
- This condition is likely a contiguous gene deletion syndrome
- There is no uniform region of deletion but rather a spectrum of different deletion sizes with a common minimal region of deletion overlap
- Genotype/phenotype comparisons can help researchers to start to define a region in which to search for candidate genes for specific features
Etiology of 1p36 deletion syndrome (monosomy 1p36)Edit
- Caused by deletion of the most distal (telomeric) light band of the short arm of chromosome 1
- Most clinical manifestations are probably caused by the absence of one copy of a dose-sensitive gene
- Patients with a 1p36 deletion have different sized pieces of the chromosome missing and may result in phenotype variability
- Mechanism causing chromosome breakage is unknown
- Severity of associated disorders varies, but the physical features are very similar
- Degree of MR and ability to acquire complex speech is somewhat dependent on deletion size
- Most deletions affect the chromosome inherited from the mother (68%), but there doesn't seem to be differences in the clinical manifestations based on whether the deletion is on the paternal or maternal chromosome.
- Those with paternally derived deletions tend to have larger deletions than those with maternally derived deletions
- Prevalence of this deletion is estimated to be 1 in 10,000 to as high as 1 in 5,000 (making it the most common terminal deletion)
- Most cases result from sporadic deletions and are not inherited
- 1p36 deletion syndrome is usually diagnosed through recognition of the symptoms and characteristics as well as lab testing.
- This is done using high-resolution chromosome analysis or FISH with a chromosome 1-specific subtelomeric probe.
- The deletion often doesn't show up clearly with standard cytogenetic banding techniques.
- Children with 1p36 deletion syndrome are all unique individuals, but do have some common characteristics.
- Distinct facial features including:
- Large anterior fontanel (~100% of patients)
- Small and pointed chin (~80%)
- Flat nasal bridge (~65%)
- Clinodactyly and/or short fifth finger (~64%)
- Low-set ears, ear asymmetry (~59%)
- Thickened ear helices (~53%)
- Small head
- Deep-set eyes (~50%)
- Most patients have mental retardation
- Most patients have delayed growth (~85%) and/or difficulty gaining weight
- Some have difficulty with sucking and swallowing while others may not grow well even though they eat well.
- Some older children may become obese.
- Most young children with 1p36 deletion syndrome have delayed development.
- They sit up, walk and talk later than typical children.
- Speech is severely affected with many patients learning only a few words
- Other medical problems:
- Hypotony (which may explain delayed motor skills) (seen in ~92%)
- Hearing loss (~56%)
- Seizures (~72%)
- KCNAB2 is a voltage-gated K+ channel beta-subunit gene that has been localized to 1p36 and thought to be associated with the epilepsy phenotype of 1p36
- Eye/vision problems (~75%)
- Increased risk for neuroblastoma
- Heart defects (which have included cardiomyopathy, ductus arteriosus, tetralogy of Fallot, VSD)
- Orofacial clefting abnormalities
- SKI, a proto-oncogene located at distal 1p36.3 has been found to be deleted in a number of patients tested and may contribute to facial clefting seen in this syndrome
- SKI overexpression has suggested that the gene is involved in neural tube development and muscle differentiations.
Recurrence Risk for next pregnancyEdit
- The majority of patients with 1p36 deletion syndrome are isolated cases resulting from a de novo deletion. There have been reports of patients with 1p36 deletion syndrome whose parents have a balanced translocation.
- Parents should be tested for chromosomal rearrangements because about 6% of parents with an affected child have a balanced translocation
- Translocation increases the chances of having another child with a 1p36 deletion.
- If no translocation is found the chances of having another child with a 1p36 deletion are very small
- In 1999 a case was reported in which a 1p36 deletion in a fetus was detected when an amniocentesis was performed due to detection of multiple malformations on ultrasound
- Another case was picked up prenatally because of the presence of elevated maternal serum alpha-fetoprotein (no abnormal sign seen on ultrasound)
- Another was diagnosed prenatally because the mother was known to have a balanced translocation between the short arms of chromosome 1 and 20 that was picked up after their first child had inherited an unbalanced product. (no abnormal sign seen on ultrasound)
- Seizures can usually be controlled with medication
- Medical interventions and feeding therapy may be needed to manage feeding issues
- Early Intervention for developmental delay
- Surgery may be necessary to correct heart defects, cleft lip, cleft palate if present
- Physical examinations should involve palpating the abdomen for lumps which might be neuroblastomas
- There are no clear indications on whether they should continue to be followed by a cardiologist or whether the cardiomyopathy is limited to a congenital form.
- Initially parents may experience denial about how severe the developmental delay and mental retardation will be
- Blaming of self or partner may occur
- Stress of having a child with developmental delays and mental retardation
- Stress associated with having a child with serious medical problems especially if they have heart malformations or seizures
- Time commitment involved with early intervention services (OT, PT, speech therapy)
- Colmenares, C., et al. Loss of the SKI proto-oncogene in individuals affected with 1p36 deletion syndrome is predicted by strain-dependent defects in Ski -/- mice. Nature Genetics 30(1), 106-109, Jan 2002.
- Faivre, L. et al. Prenatal Detection of a 1p36 Deletion in a Fetus with Multiple Malformations and a Review of the Literature. Prenatal Diagnosis. 19:49-53, 1999.
- Heilstedt, H.A. et al. Loss of the Potassium Channel B-subunit gene, KCNAB2, Is Associated with Epilepsy in Patients with 1p36 Deletion Syndrome. Epilepsia. 42(9): 1103-1111, 2001.
- Shaffer, L.G. and Heilstedt, H.A. Terminal Deletion of 1p36. The Lancet Supplement 358: 89, 2001.
- Shapira, Stuart, et al. Chromosome 1p36 Deletions: The Clinical Phenotype and Molecular Characterization of a Common Newly Delineated Syndrome. American Journal of Human Genetics 61: 642-650, 1997.
- Slavotinek, Anne, Lisa G. Shaffer, Stuart K Sharpira. Monosomy 1p36. Journal of Medical Genetics 36: 657-663, 1999.
The information for this outline was last updated in June of 2002.
Material obtained under GFDL Licence from http://en.wikibooks.org/wiki/Handbook_of_Genetic_Counseling