# Mixed Segmental Uniparental Disomy of Chromosome 15q11-q1 Coexists with Homozygous Variant in GNB5 Gene in Child with Prader–Willi and Lodder–Merla Syndrome

## TL;DR

A child with two rare genetic disorders had mixed UPD of chromosome 15 and a homozygous GNB5 gene variant, highlighting the need for thorough genetic testing.

## Contribution

This case report identifies a novel comorbidity of Prader–Willi and Lodder–Merla syndromes linked to UPD and a GNB5 variant.

## Key findings

- A child with Prader–Willi syndrome also had Lodder–Merla syndrome due to a novel GNB5 gene variant.
- Mixed segmental UPD of chromosome 15q11-q1 coexisted with a homozygous GNB5 pathogenic variant.
- Comprehensive genetic testing is essential in severe or unusual PWS cases to detect additional genetic conditions.

## Abstract

Background: Uniparental disomy (UPD) refers to the condition in which both chromosomes (or part of chromosome) of a pair are inherited from the same parent. There are two types of UPD: uniparental isodisomy (both chromosomes inherited from one parent are identical copies) and uniparental heterodisomy (two different chromosomes are inherited from one parent). UPD presents two primary developmental risks: recessive trait inheritance or an imprinting disorder. These risks may coexist, leading to an ultra-rare comorbidity. Managing the comorbidities associated with rare diseases presents unique clinical challenges. Results: The existence of such phenomena is evidenced by our case report of a boy who was ultimately diagnosed with two rare diseases: Prader–Willi syndrome (PWS), due to the maternal uniparental disomy of chromosome 15 (UPD), and autosomal recessive Lodder–Merla type 1 syndrome, linked to a novel pathogenic variant in the G protein subunit β 5 (GNB5) gene, as detailed in this paper. Conclusions: An unusual or severe phenotype in a patient diagnosed with PWS should invariably prompt the consideration of a comorbid genetic disease attributable to genes located in the PWS critical region of chromosome 15q, or elsewhere on chromosome 15. In cases of epileptic encephalopathy with cardiac arrhythmia, prompt consultation with a cardiologist and comprehensive genetic testing are essential to reduce the risks associated with untreated arrhythmia and ensure the provision of appropriate and safe anti-epileptic therapy. The presented case provides further support for the hypothesis that uniparental disomy may serve as an underlying cause of Lodder–Merla syndrome. This underscores the significance of comprehensive genetic testing, encompassing parental testing and familial cascade testing (in selected cases where there is consanguinity, or the likelihood of close common ancestral background between partners) to establish the recurrence risk.