Acute Promyelocytic Leukemia (APL)

In most cases, the exact causes of acute promyelocytic leukemia cannot be clearly defined. It is known, however, that acute promyelocytic leukemia is linked to a PML-RARA translocation. This genetic change involves an exchange of DNA between chromosomes 15 and 17. The translocation creates a fusion protein. This protein blocks the normal maturation of promyelocytes and triggers their uncontrolled growth. APL is therefore a form of leukemia with a well understood disease mechanism. This specific translocation is central to the development of APL. However, it is still only partly understood why this translocation arises in a single bone marrow cell.

Compared with other forms of leukemia, such as AML or ALL, risk factors in APL are less clearly defined. There is some evidence that exposure to ionizing radiation or certain chemical substances, such as benzene, can generally increase the risk of myeloid leukemias, including APL. In rare cases, therapy-related APL can also occur. This may happen after certain chemotherapy drugs, such as topoisomerase II inhibitors. Even so, APL is usually a form of leukemia that develops spontaneously. In most cases, there is no clearly identifiable external trigger.

Genetic or familial predispositions play an important role in some forms of leukemia. In most people with APL, however, no previous illnesses or risk factors can be found. It is important to understand that APL is not caused by lifestyle, diet, or environmental factors alone. In the end, it develops from a single, randomly acquired genetic change in a bone marrow cell. This spontaneous mutation is usually enough to disturb cell maturation.

At the beginning of APL, many people develop general symptoms. These may include severe tiredness, weakness, dizziness, or noticeably pale skin. Paleness can be a sign of developing anemia. At the same time, fever or increased susceptibility to infections may also occur. This happens because the existing white blood cells are immature and do not function properly.

The symptoms of acute promyelocytic leukemia usually appear suddenly. They can become much worse within a few days. This is not only caused by immature promyelocytes crowding out healthy blood formation in the bone marrow. It is also caused by early and serious problems with blood clotting. A higher tendency to bleed and clotting disorders are especially typical of APL. They may appear as:

• persistent nosebleeds
• bleeding gums
• unusually many or frequent bruises
• small pinpoint skin bleeds, called petechiae
• in severe cases: internal bleeding, for example in the gastrointestinal tract

These clotting disorders are among the key warning signs of APL. They are the main reason why APL is considered a medical emergency. The bleeding tendency is often linked to disseminated intravascular coagulation, or DIC. In APL, DIC can appear very early.

Other possible symptoms of acute promyelocytic leukemia include:

• bone pain
• weight loss
• night sweats
• in rare cases, an enlarged spleen

Typical warning signs that may point to APL include sudden and increasing bleeding, fever without a clear cause, or a noticeable decline in general health. Early bleeding and clotting disorders can be life threatening. For this reason, suspected APL should be assessed immediately. This helps treatment start as soon as possible and may reduce the risk of complications.

The diagnosis of acute promyelocytic leukemia usually requires fast and precise testing. This is because the disease can cause severe clotting disorders early on. It is therefore a hematological emergency. In many cases, doctors first perform a blood test. This can help assess ongoing tiredness, susceptibility to infections, or increased bleeding. The blood count often shows fewer red blood cells and platelets. It may also show changes in white blood cells and a buildup of immature cells. A blood smear is also performed. In this test, the cells are examined under a microscope. In APL, promyelocytes may already be visible in the blood smear. They often have a typical appearance with many granules and sometimes Auer rods.

The key test for diagnosis is the examination of the bone marrow. A bone marrow puncture is usually taken from the iliac crest. The sample is then examined using cytology and histology. In APL, there is typically a strong increase in promyelocytes. These cells crowd out normal blood formation. This examination also helps distinguish APL from other forms of acute myeloid leukemia. In addition, clotting values such as INR, aPTT, fibrinogen, and D-dimers are measured. Many people already show signs of disseminated intravascular coagulation, or DIC, when the diagnosis is confirmed. DIC is one of the main reasons why treatment must begin so quickly.

Another important part of diagnosis is immunophenotyping using flow cytometry. This test analyzes specific markers on the leukemia cells. It helps confirm that the cells are myeloid promyelocytes. It also helps distinguish APL clearly from other acute leukemias. A more specific molecular genetic test can also detect the PML-RARA fusion. This gene fusion develops through a translocation between chromosomes 15 and 17. The test is important not only for the exact diagnosis. It also helps plan the right treatment, because APL can be treated with drugs that act directly on this fusion protein.

If APL is suspected, treatment is often started before all molecular results are available. This is especially true when the blood count, promyelocytes in the smear, and clotting problems are typical. Early treatment helps reduce the high risk of early complications. Imaging tests such as ultrasound diagnostics or CT may also be used. They can help assess an enlarged spleen or liver, or rule out acute bleeding. In some cases, a lumbar puncture is performed to check whether the central nervous system is involved. Overall, the diagnosis of APL is based on clinical signs, blood and clotting values, bone marrow examination, immunophenotyping, and molecular genetic tests.

Untreated acute promyelocytic leukemia can become life threatening very quickly. At the same time, it is now one of the forms of leukemia with the best chances of cure. This is due to modern treatment approaches. Treatment mainly aims to remove the abnormal promyelocytes, stabilize the clotting disorder, and restore normal blood formation in the bone marrow.

Acute promyelocytic leukemia treatment differs greatly from treatment for other acute myeloid leukemias. Instead of using only classic chemotherapy, doctors mainly use drugs that help the immature cells mature. These drugs act directly on the underlying PML-RARA fusion. The most typical examples are all-trans retinoic acid, or ATRA, and arsenic trioxide, or ATO. In simple terms, ATRA allows the blocked promyelocytes to mature again and eventually die. Arsenic trioxide strengthens this effect and also acts specifically on the fusion protein. Depending on the individual risk profile, ATRA and ATO based treatment may be used alone or together with additional chemotherapy.

Close monitoring and treatment of the clotting disorder are important parts of therapy. So are the prevention and treatment of infections, and the management of side effects from medication. After the intensive treatment phase, many people enter a maintenance phase and long-term follow-up. During follow-up, doctors monitor how well the treatment is working. This is often done by regularly checking PML-RARA transcripts in the blood or bone marrow.

Alongside conventional medical treatment, many people also use complementary medical measures. These can help them cope better with the disease and treatment side effects. Examples include acupuncture, body-oriented therapies, or supportive nutritional counseling. Studies suggest that such methods may help relieve symptoms such as nausea, pain, sleep disorders, anxiety, or exhaustion. They may also improve subjective well-being. It is important, however, that these methods can support quality of life. They do not replace specific APL therapy.

The prognosis of APL has changed greatly in recent years. In the past, APL was linked to high early mortality. Today, cure rates are high, especially when diagnosis is early and treatment is consistent. In many studies, long-term survival rates of more than 80 to 90 percent have been reached. A fast start to treatment is crucial for a good outcome, even when APL is only strongly suspected.

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