Beckwith-Wiedemann syndrome (BWS) is a disorder characterized by prenatal and /or postnatal overgrowth, neonatal hypoglycemia, congenital malformations, and an increased risk for embryonal tumors. Physical findings are variable and can include abdominal wall defects, macroglossia, and hemihyperplasia. The predisposition for tumor development is associated with specific tumor types such as adrenal carcinoma, nephroblastoma (Wilms tumor), hepatoblastoma, and rhabdomyocsarcoma. In infancy, BWS has a mortality rate of approximately 20%.
Current data suggest that the etiology of BWS is due to the dysregulation of imprinted genes in the 11p15 region of chromosome 11, including H19 (maternally expressed), LIT1 (official symbol KCNQ1OT1; paternally expressed). Expression of these genes is controlled by 2 imprinting centers (IC).
Approximately 85% of BWS cases appear to be sporadic, while 15% of cases are associated with an autosomal dominant inheritance pattern. When a family history is present, the etiology is often due to inherited point mutation in the CDKN1C or an unknown cause. The etiology of sporadic cases includes:
●Hypomethylation of imprinting center 2 (IC2) (LIT1) approximately 50-60%
●Paternal uniparental of imprinting disomy of chromosome 11: approximately 10-20%
●Hypermethylation of imprinting center 1 (IC1) (H19): approximately 2-7%
●Unknown: approximately 10-20%
●Point mutation in CDKN1C: approximately 5-10%
●Cytogenetic abnormality: approximately 1-2%
●Differentially methylated region 1 (DMR1) or DMR2 microdeletion: rare
The clinical presentation of BWS is dependent on which gene in the 11p15 region is involved. The risk for cancer has been shown to be significantly higher in patients with abnormal methylation of IC1 (H19) vs IC2 (LIT1). In patients with abnormal methylation of IC2 (LIT1), abdominal wall defects and overgrowth are seen at a higher frequency.
Russell-Silver syndrome (RSS) is a rare genetic condition with incidence of approximately 1 in 100,000. RSS is characterized by pre- and postnatal growth retardation with normal head circumference, characteristic facies, fifth finger clinodactyly, and asymmetry of the face, body, and/or limbs. Less commonly observed clinical features include café au lait spots, genitourinary anomalies, motor, speech, cognitive delays, and hypoglycemia. Although clinical diagnostic criteria have been developed, it has been demonstrated that many patients with molecularly confirmed RSS do not meet strict clinical diagnostic criteria for RSS. Therefore, most groups recommend a relatively low threshold for considering molecular testing in suspected cases of RSS.
RSS is a genetically heterogeneous condition that is associated with genetic and epigenetic alterations at chromosome 7 and the chromosome 11p15.5 region. The majority of cases of RSS are sporadic, although familial cases have been reported. The etiology of sporadic cases of RSS includes:
●Hypomethylation of IC1 (H19): approximately 30-50%
●Maternal uniparental disomy (UPD) of chromosome 7: approx. 5-10%
●11p15.5 duplications: rare
●Chromosome 7 duplications: rare
Note: This test does not detect chromosome 7 UPD.
The clinical phenotype of RSS has been associated with the specific underlying molecular etiology. Patients with hypomethylation of IC1 (H19)are more likely to exhibit “classic” RSS phenotype (ie, severe intrauterine growth retardation, postnatal growth retardation, and asymmetry), while patients with maternal UPD7 often show a milder clinical phenotype. Despite these general genotype-phenotype correlations, many exceptions have been reported.
Methylation abnormalities of IC1 (H19) and IC2 (LIT1) can be detected by methylation-sensitive multiple ligation-dependant probe amplification. While testing can determine methylation status, it does not identify the mechanism responsible for the methylation defect (such as paternal uniparental disomy or cytogenetic abnormalities). Hypomethylation of IC2 (LIT1) is hypothesized to silence the expression of a number of maternally expressed genes, including CDKN1C. Hypermethylation of IC1 is hypothesized to silence the expression of H19, while also resulting in overexpression of IGF2. Absence of CDKN1C and H19 expression, in addition to overexpression of IGF2, is postulated to contribute to the clinical phenotype of BWS. Hypomethylation os IC1 is hypothesized to result in overexpression of H19 and underexpression of IGF2, which is thought to contribute to the clinical phenotype of RSS.