Understanding Tyrosinemia Type 2: An Overview for Parents

Learning that your child has Tyrosinemia Type 2, also known as Richner-Hanhart syndrome, can feel overwhelming, especially because this condition is exceptionally rare. This page provides an overview of why this condition occurs, how it affects the body, and why early action is the best tool for your child’s long-term health.

What is Tyrosinemia Type 2?

Tyrosinemia Type 2 is a rare metabolic disorder that interferes with the body’s ability to break down tyrosine, an amino acid found in most protein-containing foods ^[1]. Because the body cannot process it properly, tyrosine builds up to high levels in the blood and tissues ^[2].

This condition is extremely rare. In one large study of over 2 million infants, the detection rate was approximately 0.46 per million births, or about 1 in 2.19 million ^[3]. Because it is so uncommon, many doctors may never see a case in their entire career, making it essential to work with specialists in metabolic genetics.

The Biological Mechanism

To understand this condition, it helps to look at the “instruction manual” for your child’s body: the genes.

The TAT Gene and Enzyme Deficiency

Tyrosinemia Type 2 is caused by mutations in the TAT gene ^[4]. This gene provides instructions for making an enzyme called hepatic cytosolic tyrosine aminotransferase (TAT). In a healthy body, this enzyme acts like a key that unlocks the process of breaking down tyrosine into other substances the body can use or get rid of ^[1].

In children with this condition, the TAT enzyme is either missing or does not work correctly. Without this functional “key,” tyrosine cannot enter the normal breakdown pathway. Instead, it accumulates in the blood—a state called hypertyrosinemia ^[2]. These high levels of tyrosine can eventually lead to the formation of crystals in the eyes and skin, and may increase the risk of affecting brain development ^[4].

Autosomal Recessive Inheritance

This condition is autosomal recessive. This means a child must inherit two changed copies of the TAT gene—one from each parent—to have the disorder ^[4]. Parents are typically “carriers,” meaning they have one changed gene and one working gene, and they usually do not show any symptoms of the condition themselves. When both parents are carriers, each pregnancy has a 25% chance of being affected, a 50% chance of being a carrier, and a 25% chance of inheriting neither familial variant. A genetic counselor can discuss testing for siblings and relatives.

Why Early Intervention is Critical

The primary goal of treatment is to lower the amount of tyrosine in the body. While some symptoms, such as those affecting the eyes and skin, often improve quickly once a specialized diet begins, neurological concerns require more urgent attention.

Preventing Neurological Complications

The brain is particularly sensitive to metabolic imbalances during early development. If tyrosine levels remain high for a long time, it can increase the risk of:

• Intellectual disability or cognitive impairment ^[4].
• Developmental delays in reaching milestones like walking or talking ^[5].
• Seizures, although these are less commonly reported ^[6].

Neurological involvement is a possible, variably reported complication, not an absolute certainty. Research indicates that while skin and eye issues often resolve with treatment, neurological symptoms that have already developed may be more difficult to reverse ^[4]. This is why diagnosing the condition early and strictly following a prescribed low-protein diet is the most effective way to reduce the risk of cognitive complications ^[6][3].

A Note on Newborn Screening

While many metabolic disorders are caught through routine newborn screening, Tyrosinemia Type 2 can sometimes be missed ^[5]. If a child shows signs like extreme light sensitivity (photophobia), painful skin thickening on the palms or soles (palmoplantar hyperkeratosis), or unexplained developmental delays, doctors should check tyrosine levels even if the initial newborn screen was normal ^[5][7].

Managing the Condition

Treatment focus is almost entirely on a specialized diet that limits tyrosine and phenylalanine (another amino acid the body turns into tyrosine) ^[4].

Unlike Tyrosinemia Type 1, which is treated with a medication called nitisinone, this medication is typically not used for Type 2 ^[8]. In fact, nitisinone can actually increase tyrosine levels, which would be harmful for a child with Type 2 ^[8]. Management instead relies on:

1. Protein Prescription: Restricting high-protein foods like meat, dairy, and eggs according to a dietitian’s individualized prescription ^[9].
2. Medical Formula: Providing a special tyrosine-free formula to ensure the child receives an amino-acid substitute necessary for growth ^[3].
3. Regular Monitoring: Frequent blood tests to ensure tyrosine levels stay within an individualized safe range ^[3].

By maintaining these levels, families can help prevent the crystals that cause pain and vision issues and provide a supportive environment for the child’s development.