Understanding Hereditary Orotic Aciduria (HOA)

Receiving a diagnosis of Hereditary Orotic Aciduria (HOA), also known as UMPS deficiency, can feel overwhelming, especially given how rare the condition is. It is natural to feel a sense of isolation when a disease is described as having only about 20 cases in historical older medical literature ^[1]. However, it is important to know that the landscape of this condition is changing. With the expansion of newborn screening programs in specific regions, doctors are beginning to identify more children with HOA before they ever show symptoms ^[2].

This page explains how the condition works, why it happens, and what is occurring inside your child’s body.

The Role of the UMPS Gene

Your child’s body relies on a specific gene called UMPS to provide instructions for making an enzyme (a protein that triggers chemical reactions) called uridine 5’-monophosphate synthase ^[3].

This enzyme is a “bifunctional” worker, meaning it has two distinct jobs that happen at the very end of a process called de novo pyrimidine synthesis ^[3][4]. Pyrimidines are essential building blocks that the body uses to create DNA and RNA. Without enough pyrimidines, cells—especially those that divide quickly, like those in the bone marrow—cannot function or multiply correctly ^[5].

Why Orotic Acid Builds Up

Think of the pyrimidine production line as a conveyor belt. In a child with HOA, the UMPS enzyme at the end of the belt is missing or not working correctly.

• The Blockage: The production line works fine up until the very last steps, where a substance called orotic acid (or orotate) is produced ^[3].
• The Accumulation: Because the UMPS enzyme isn’t there to convert orotic acid into the next building block (UMP), the orotic acid has nowhere to go ^[5]. It backs up in the system and spills over into the urine, which is why the condition is called “orotic aciduria” ^[3].
• The Shortage: The most critical problem is not the orotic acid itself, but the lack of the final product (UMP). This impaired availability prevents the body from making enough DNA and RNA to support healthy growth and blood cell production ^[5][1].

Inheritance: How HOA Happens

HOA is an autosomal recessive condition. This means that for a child to have the disease, they must inherit two pathogenic (disease-causing) variants of the UMPS gene—one from each parent ^[5][6].

• Recurrence Risk: If both parents are carriers, each pregnancy has a 25% chance of the child having HOA, a 50% chance the child will be a carrier, and a 25% chance of inheriting neither pathogenic variant.
• Carriers: Parents typically carry only one pathogenic variant and one healthy copy. Carriers usually have no symptoms, though some may have slightly higher levels of orotic acid in their urine. However, urine results alone do not determine carrier status; formal genetic testing and counseling are required ^[5][6].
• Biallelic Disease: When a child inherits two pathogenic variants (referred to as biallelic), they cannot produce enough functional UMPS enzyme to meet their body’s needs ^[5].

Rare but Increasingly Recognized

Historically, HOA was only diagnosed when a child became very ill with severe anemia or growth delays. This is why older texts suggest there are only a few dozen cases worldwide ^[1].

Today, some regions have added orotic acid testing to newborn screening. For example, a screening program in Israel identified 10 affected newborns out of roughly 1.5 million screened ^[2]. All 10 of those infants were asymptomatic at the time of their diagnosis, demonstrating that early detection allows for treatment before a lack of pyrimidines causes physical harm ^[2].

Impact on the Body

When left untreated, the lack of pyrimidines can lead to several health challenges:

• Megaloblastic Anemia: A condition where the bone marrow produces unusually large, structurally abnormal, and immature red blood cells ^[5]. These cells reduce the blood’s total oxygen-carrying capacity, leading to extreme fatigue.
• Immune System Issues: A low count of white blood cells (specifically neutropenia) can make it harder for the body to fight off infections ^[5][1].
• Growth and Development: Because DNA synthesis is slowed, children may experience failure to thrive (poor weight gain and growth) and delays in reaching developmental milestones ^[5][1].

Fortunately, because the primary problem is a shortage of a specific building block, modern management focuses on providing the missing pyrimidine (often through a medication called uridine triacetate) so the body can bypass the blocked step ^[5][1][7].