On this page
PNP
purine nucleoside phosphorylase
PNP is located on the long (q) arm of chromosome 14, at band 14q11.2. Arm ratio per GRCh38 - banding schematic.
Explore chromosome 14 in the library →Available at Jeen Health
Clinical tests that include this
Overview
The PNP gene encodes purine nucleoside phosphorylase, an enzyme found throughout the body that plays a critical role in clearing cellular waste. Although present in many tissues, this enzyme is especially active in lymphocytes, immune cells that identify and destroy infectious agents and foreign pathogens. These immune cells originate in lymphoid organs - the thymus, a gland situated posterior to the sternum, and in lymphatic nodules dispersed across multiple body sites - before circulating through the bloodstream where they contribute to immune surveillance.
Purine nucleoside phosphorylase functions as a metabolic housekeeping enzyme, catalysing the conversion of two nucleoside metabolites. Specifically, one substrate called deoxyinosine undergoes transformation into a molecule known as hypoxanthine, and a second substrate called deoxyguanosine is converted into guanine. These waste products accumulate when DNA is degraded, and their efficient removal is particularly important for maintaining normal lymphocyte development and function. When PNP function is disrupted by genetic variants, the resulting accumulation of toxic metabolites can severely compromise immune defences.
What the gene does
Purine nucleoside phosphorylase catalyses the breakdown of specific purine nucleosides. Deoxyinosine undergoes conversion to hypoxanthine, and deoxyguanosine is transformed into guanine through the enzyme's action. This enzymatic activity is part of the purine salvage pathway, which recycles components from degraded nucleic acids. By clearing these intermediates, the enzyme prevents the build-up of molecules that would otherwise interfere with cellular processes.
The enzyme's activity is particularly crucial in lymphocytes, where rapid cell division and DNA turnover generate substantial amounts of nucleoside waste. T lymphocytes (T cells), which mature in the thymus and coordinate immune responses, appear especially vulnerable to disruption of this metabolic pathway. When purine nucleoside phosphorylase cannot effectively process deoxyguanosine, toxic derivatives accumulate and selectively impair T-cell development and survival. This selective vulnerability explains why PNP deficiency primarily manifests as T-cell immunodeficiency, even though the enzyme is expressed widely across tissues. The enzyme operates within broader nucleotide metabolism networks that maintain the balance of purine pools essential for DNA replication and cellular energy metabolism.
Video: Genetics 101
Chromosome location
The PNP gene is located on the long arm of chromosome 14 at position 11.2 (14q11.2). This chromosomal region contains multiple genes involved in immune function and metabolic processes. The gene comprises eight exons that together encode the 289-amino-acid purine nucleoside phosphorylase protein. Chromosome 14 is a medium-sized human chromosome. The 14q11.2 region lies within the proximal portion of the long arm, a chromosomal segment that houses several clinically significant loci associated with immune regulation and developmental pathways. Genetic variants within this chromosomal territory can be identified through standard cytogenetic and molecular diagnostic techniques employed in NHS genomic testing pathways. The precise chromosomal position facilitates targeted sequencing approaches when PNP deficiency is suspected on clinical grounds, enabling accurate carrier detection and prenatal diagnosis in at-risk families.
Protein structure
Domain architecture has not been experimentally characterised in detail for this protein. The 289-amino-acid purine nucleoside phosphorylase functions as a trimer, with three identical subunits assembling to form the active enzyme. The protein's catalytic activity depends on the correct folding and assembly of these subunits, which create the active site pockets where nucleoside substrates bind and undergo phosphorolysis.
Key variants
More than 35 pathogenic variants in the PNP gene have been documented in individuals with purine nucleoside phosphorylase deficiency. The condition follows autosomal recessive inheritance, requiring pathogenic changes in both gene copies for disease to manifest. Pathogenic changes include missense variants that alter critical amino acids in the enzyme's structure, nonsense variants that introduce premature stop codons, and deletions or insertions that disrupt the reading frame. The functional consequence of these variants is reduced or absent enzyme activity, leading to accumulation of toxic purine metabolites.
No pathogenic or likely-pathogenic ClinVar variants recorded yet for this gene.
Associated conditions
Pathogenic variants in PNP cause purine nucleoside phosphorylase deficiency, a rare autosomal recessive disorder characterised by severe combined immunodeficiency. The condition is an immune system disorder in which the body cannot effectively defend against foreign invaders such as bacteria and viruses. Affected individuals experience recurrent and severe infections that may become life-threatening due to impaired T-cell function, whilst B-cell responses may be relatively preserved initially. Neurological complications frequently occur in individuals with this condition and may include developmental delay, movement disorders, or muscle weakness. The immune deficiency typically manifests in infancy or early childhood, and without intervention such as haematopoietic stem cell transplantation, the condition carries a poor prognosis. The severity and specific features can vary depending on the residual enzyme activity retained by particular variant combinations.
No disease links recorded for this gene in our reference set.
Inheritance pattern
Conditions caused by pathogenic PNP variants typically follow autosomal recessive inheritance.
When both parents are carriers, each child has a 25% chance of being affected, 50% of being a carrier, and 25% of being unaffected.
UK clinical status
PNP appears on several NHS Genomic Medicine Service gene panels, reflecting its clinical importance in diagnosing immune and metabolic disorders. The gene holds green (high-evidence) classification on panels including Primary immunodeficiency or monogenic inflammatory bowel disease (R15), Severe combined immunodeficiency with PNP deficiency (R234), and Likely inborn error of metabolism (R98). It is also included on the Undiagnosed metabolic disorders panel and featured in COVID-19 research initiatives. This multi-panel presence indicates that PNP testing is available through NHS genomic testing pathways for patients presenting with recurrent infections, immunodeficiency, or unexplained metabolic abnormalities.
Sources: NHS GMS PanelApp · Genomics England PanelApp · NHS National Genomic Test Directory
Frequently asked questions
What is purine nucleoside phosphorylase deficiency?
Purine nucleoside phosphorylase deficiency is a rare inherited immune disorder caused by pathogenic variants in both copies of the PNP gene. Affected individuals cannot effectively break down certain waste products from DNA metabolism, leading to toxic accumulation that impairs T-cell function. This results in severe combined immunodeficiency with recurrent life-threatening infections and, in many cases, neurological complications.
How is PNP deficiency inherited?
PNP deficiency follows an autosomal recessive inheritance pattern. This means an affected individual inherits one altered copy of the PNP gene from each parent. Parents who each carry one pathogenic variant typically have no symptoms themselves but have a 25% chance with each pregnancy of having an affected child.
Can PNP deficiency be detected through carrier screening?
Yes, carrier screening can identify individuals who carry one pathogenic PNP variant. Whilst carriers do not develop immunodeficiency, identifying carrier status is valuable for reproductive planning, particularly when both prospective parents carry variants in the same gene. Genetic counselling can help families understand recurrence risks and available options.
References
- Nyhan WL. Disorders of purine and pyrimidine metabolism. Molecular genetics and metabolism. 2005. PMID: 16176880