Mulchandani S et al (2016) report eight cases of
maternal UPD of chromosome 20 with normal karyotypes. All individuals had intra-uterine growth restriction (IUGR)
and failure to thrive. Six of the cases required chronic gastric tube feedings. The authors postulate an imprinting
disorder due to maternal UPD of chromosome 20.
Pseudohypoparathyroidism type Ib (PHP-Ib), characterised by resistance to parathyroid hormone (PTH), has been
identified in patients with paternal UPD of chromosome 20. PHP-Ib is caused by imprinting defects of GNAS, the gene
that codes for the alpha subunit of guanine nucleotide binding protein (G(s)α). G(s)α is an important signaling protein
for the action of PTH (Bastepe M et al 2011; Fernández-Rebollo E et al 2010).
Geelen J et al (2013) report a case of severe
combined immunodeficiency caused by adenosine deaminase deficiency (ADA-SCID) due to paternal UPD of
chromosome 20. This revealed homozygosity for a 5bp deletion in the ADA gene.