Diagnosis & Differentiating from PKU

The path to a diagnosis of DNAJC12 deficiency often begins with a phone call after a newborn screening test. Because this condition is so rare and was only recently discovered, the diagnostic journey can sometimes be confusing, with the condition masquerading as more common disorders like PKU (Phenylketonuria).

Getting to the right answer requires a step-by-step process to distinguish between several different causes of high phenylalanine.

Why It “Looks Like” PKU

When a baby is born, a small prick of blood is taken to screen for various conditions. Most of these screens look for high levels of an amino acid called phenylalanine (a condition called hyperphenylalaninemia or HPA).

In standard PKU, the high levels are caused by a “broken machine” (the PAH enzyme) ^[1]. In DNAJC12 deficiency, the PAH enzyme itself is intact, but the “mechanic” (the DNAJC12 protein) is missing ^[2]. Because both problems lead to the same result—elevated phenylalanine in the blood—the newborn screen flags both conditions the exact same way ^[3].

The BH4 “Loading Test” Trap

One of the first things a metabolic specialist may do is a BH4 loading test (or sapropterin challenge). They give a dose of BH4 to see if blood phenylalanine levels drop.

In many children with DNAJC12 deficiency, the levels do drop—sometimes by 30% or more ^[4][5]. In the past, this “pass” was taken as proof that the child had either a BH4 synthesis deficiency or BH4-responsive PKU. However, we now know that DNAJC12-deficient patients can respond to this test ^[5]. Relying on this test alone without further biochemical or genetic evaluation can lead to an incorrect diagnosis.

The Diagnostic Checklist

To be certain of a DNAJC12 diagnosis, your medical team must look at several pieces of evidence. No single test establishes the diagnosis.

1. Evaluating Other Causes

Doctors will check for indicators of more common disorders. In DNAJC12 deficiency, these tests typically show normal patterns:

• Urine Pterins: These are chemicals related to BH4. They are generally normal in DNAJC12 deficiency but show specific abnormal patterns in BH4 disorders ^[1][6].
• DHPR Activity: This is an enzyme test often done via a blood spot. It is normal in DNAJC12 deficiency ^[1].

2. Genetic Testing (The Key Evidence)

A diagnosis generally depends on finding two disease-causing variants (pathogenic or likely pathogenic) in the DNAJC12 gene ^[4]. Note that a “variant of uncertain significance” (VUS) does not confirm the disorder on its own.

• Important Note on Deletions: Standard genetic tests (like a “whole exome”) can sometimes miss large chunks of missing DNA called deletions depending on the laboratory platform ^[7]. At least one case was missed initially because the child had a 6.9-kb deletion that standard analysis didn’t detect ^[7]. Parents should explicitly ask if “copy-number analysis” (deletion and duplication testing) was performed.

3. CSF Neurotransmitter Testing (Selected Cases)

Because DNAJC12 affects brain chemicals, your doctor may recommend a lumbar puncture (spinal tap) to check the cerebrospinal fluid (CSF). This is an invasive procedure and may be considered selectively, rather than routinely for every patient. It looks for indirect metabolites:

• HVA (Homovanillic Acid): A metabolite of dopamine ^[1].
• 5-HIAA (5-Hydroxyindoleacetic Acid): A metabolite of serotonin ^[1].

In children with DNAJC12 deficiency, these markers are typically low, though results must be interpreted using age-specific reference ranges ^[8][9].

Typical Diagnostic Patterns

(Note: These are typical patterns that require specialist interpretation, not absolute diagnostic rules. Other secondary causes of HPA exist).

• Classic PKU: High Phenylalanine | Normal Urine Pterins/DHPR | Pathogenic PAH variants
• BH4 Disorders: High Phenylalanine | Abnormal Urine Pterins/DHPR (depends on disorder) | Positive BH4 response often seen | Pathogenic variants in PTS, GCH1, etc.
• DNAJC12 Deficiency: High Phenylalanine | Normal Urine Pterins/DHPR | Positive BH4 response often seen | Low CSF HVA/5-HIAA | Pathogenic DNAJC12 variants