Placental Pathology in Down Syndrome–Associated Transient Abnormal Myelopoiesis

In addition to the commonly recognized stigmata of DS, neonates with TAM demonstrate a variety of clinical manifestations. Leukocytosis with increased myeloid blasts and thrombocytopenia are frequent findings in the peripheral blood of affected neonates.¹  As liver is the origin of the abnormal myelopoiesis, infiltration of blasts throughout the organ results in hepatomegaly, jaundice, and elevated liver enzymes.^3,10  Coagulopathy may result from blood viscosity attributable to leukocytosis, abnormal liver functions, and thrombocytopenia. Depending on the extent of their spread to other organs, pericardial and pleural effusions, ascites, and vesiculopustular skin eruptions ensue.^1,11  In skin lesions, blast cells and other leukocytes are present in subcorneal blisters or deposited around blood vessels in the dermis.^10,11  Splenomegaly, seen in about a third of the neonates, is believed to be secondary to portal venous obstruction.¹¹  It should be noted that none of these clinical features are specific to TAM. However, hyperleukocytosis, hepatosplenomegaly, effusions, and skin manifestations are more frequent in TAM neonates compared with DS neonates without GATA1 mutations. Life-threatening complications have been described in 10% to 20% of cases and include fulminant hepatic fibrosis, splenic necrosis, hyperviscosity syndrome, respiratory distress, renal failure, and cardiopulmonary failure. Premature labor and fetal demise could also occur as a result of TAM.¹² 

The median white blood cell count in neonates with TAM is about 40 000/μL, although it is not uncommon for the white blood cell count to exceed 100 000/μL.⁹  Peripheral blood smears show increased myeloid blasts with megakaryoblastic features that include basophilic cytoplasm, cytoplasmic blebs, and coarse azurophilic granules^7,9  (Figure 1). Immunophenotyping by flow cytometry demonstrates variable antigenic expression of CD34 and CD117 (stem cell markers); CD33 and CD13 (myeloid markers); CD41, CD61, and CD42b (megakaryocytic markers); and nonlineage markers such as CD4, CD7, CD36, CD45, CD56, CD71, thrombopoietin receptor (TPO-R), and IL-3 receptor α. Myeloperoxidase is usually negative.^1,2 

Prenatal diagnosis of TAM is not frequently reported. Although most of the clinical features of TAM are identified by physical examination and laboratory testing, some features, such as hepatosplenomegaly and fetal hydrops (where the fetus shows significant edema with effusions in body cavities, increased volumes of amniotic fluid, and thickened placenta), can be detected by third-trimester ultrasonography.^7,12,13  Recent reports have demonstrated that prenatal umbilical artery Doppler interrogation can be useful in recognizing fetal placental vascular abnormalities associated with DS and fetal leukemia.^14,15  Abnormal umbilical artery waveforms and elevation of pulsatility index of the umbilical artery with paradoxically enlarged fetal trunk are described as early signs of fetal involvement. Postpartum placental examination is an often-overlooked diagnostic tool that can also provide remarkable information on this disease entity.

Gross examination of the placenta is mostly unremarkable in TAM, with appropriate placental weight for the weeks of gestation; normal-appearing trivascular umbilical cord, membranes, and chorionic plate vasculature; and no significant parenchymal findings. A few reports have indicated nonspecific placentomegaly in TAM, with increased weights higher than the 90th percentile for weeks of gestation.^16,17  Chorionic villous edema in fetal hydrops and congestion are often associated with placentomegaly. Shorter length of umbilical cord in DS compared with controls has been well documented.^14,18  Several authors^17,19  have described macroscopic occlusive thrombi in chorionic vessels and wedge-shaped pale zones within the villous parenchyma.

A complete histopathologic evaluation of the placenta is emphasized, as most notable findings are observed microscopically. Leukocytosis with left-shifted hematopoietic elements and myeloid blasts can be seen within vessels of the umbilical cord, chorionic plate, and chorionic villi^{12,16,17,19–22}  (Figure 2, A through D). These mononuclear blasts are characterized by large, lobulated nuclei with open chromatin, high nuclear to cytoplasmic ratio, prominent nucleoli, and associated apoptosis (Figure 2, E). Erythroblastosis may also be present, together with abundant immature granulocytes and eosinophils.^16,17,20  Myeloid blasts are generally confined to the vascular lumina, but they can infiltrate the vascular wall and stroma to varying degrees.^16,17,21  Dai et al²⁰  demonstrated extravasation of immature cell aggregates into the perivillous space (Figure 2, D). Although the exact underlying mechanism is unknown, this phenomenon may indicate feto-maternal microhemorrhage and allude to the severity of systemic disease and adverse outcomes in the affected fetus/neonate.

Fetal vascular malformation with organizing thrombi containing blasts and associated downstream avascular villi are the histopathologic correlates to the grossly identified chorionic villous tree thrombi and pale zones within the parenchyma, respectively.^{12,16,17,19–21,23}  It is believed that these fetal vascular malformation changes result from the hypercoagulable state triggered by the myeloproliferative disease in the fetus. Presence of fetal vascular malformation in TAM placentas may also help explain sudden perinatal clinical deterioration caused by thromboembolic spread to the fetal circulation via the umbilical vein. Other placental findings with involvement by TAM include delayed villous maturation, villous edema, and chorangiosis.^16,20,21 

Immunohistochemical staining of formalin-fixed and paraffin-embedded placental tissue is helpful in the delineation of the immature myeloid cells. Strong expression of CD34, CD117, CD42b, CD41, and CD61 (Figure 2, F) confirms the megakaryocytic lineage of the blast cells.^12,20–23  Blasts are negative for TdT, CD3, CD10, CD20, and CD79a. Myeloperoxidase may highlight maturing myeloid cells in the background.

Fixed placental tissue is still amenable to karyotyping for T21 and sequencing of the GATA1 gene. Fluorescence in situ hybridization of placental sections can be used to demonstrate the presence of 3 copies of chromosome 21 in both normal placental cells and intravascular myeloid blasts. This method could also differentiate between constitutional T21 and mosaicism of chromosome 21.^12,19 

GATA1 mutations are specific for TAM in DS and are especially useful when other hematologic conditions are considered in the differential diagnosis. Sequence analysis of GATA1 has been performed successfully from genomic DNA extracted mainly from leukemic infiltrates within the placenta.¹²  Also, DNA extracted from whole placenta sections can be analyzed by next-generation amplicon deep sequencing for the complete coding region of GATA1. This assay is sensitive enough to detect GATA1 mutation loads as low as 3% from placental specimens that may contain a low number of hematopoietic cells.²³  This makes next-generation amplicon deep sequencing assays particularly useful in clinically or hematologically silent TAM in which the mutation load is very low.

The Table summarizes the reports of placental involvement by TAM based on a literature review. In the majority of cases with placental involvement, peripheral blood leukocytosis exceeded 100 000/μL and blast counts ranged from 23% to 95%. The uniform finding of all affected placentas was the presence of circulating blast cells in the umbilical cord and chorionic villous vasculature. Fetal vascular malformation was also observed in nearly half of the cases.

Findings of circulating blasts and leukocytosis in the newborn are not always associated with a leukemic process. Although there is no consensus as to what percentage of circulating blasts in the newborn can be acceptable as normal, it has been reported that up to 8% of blasts may be found in healthy preterm and sick term neonates without DS.⁸  In DS neonates, there is an overall increased frequency of circulating blasts compared with controls, even in the absence of TAM.^8,10  Neonatal leukemoid reaction with white blood cell counts greater than 50 000/μL are seen in a variety of conditions such as sepsis, congenital infections, hemolysis, intraventricular hemorrhage, and bronchopulmonary dysplasia with associated increased morbidity and mortality.

In the same context, finding an increased number of hematolymphoid cells and blastlike cells in the placenta may prompt one to consider infections and metastatic disease. Infection would be the top differential diagnosis, with a variety of bacterial, viral, spirochetal, and parasitic organisms. Acute villitis, intervillositis, and microabscesses caused by listeriosis and E coli, and enlarged, hypercellular chorionic villi with lymphoplasmacytic infiltrations, sometimes with large inclusion cells in cytomegalovirus infection, can all mimic a leukemoid reaction or TAM.

Although neoplasms rarely cross the placenta, both fetal and maternal malignancies can metastasize to the placenta. Interestingly, the distribution of metastatic tumor cells within the placenta appears clearly defined; in fetal malignancies that metastasize to the placenta, tumor cells tend to stay confined to the fetal vessels without villous stromal invasion. On the contrary, in maternal malignancies that spread to the placenta, tumor cells infiltrate the perivillous (maternal) space and spare the fetal villous circulation. In this scenario, tumor cells essentially still occupy the maternal vascular space and hence do not represent true metastasis. Neuroblastoma is one of the most common malignant solid tumors in neonates, and placental metastasis of neuroblastoma can mimic TAM. Involved placentas may show placentomegaly and tumor emboli within fetal vessels of hydropic villi. The tumor cells are larger than myeloid precursors, with a high nuclear to cytoplasmic ratio and pink fibrillar cytoplasm. They usually present as single cells or tissue fragments and may form Homer Wright rosettes.²⁴  Similarly, congenital hepatoblastoma that is otherwise limited to the liver may rarely demonstrate placental involvement with placentomegaly and diffuse tumor emboli in chorionic villous vasculature.²⁵  Immunohistochemistry is useful in differentiating these entities: neuroendocrine markers and anti–α-fetoprotein positivity in metastatic neuroblastoma and hepatoblastoma, respectively. Maternal melanoma among solid tumors and hematologic malignancies are the commonly reported malignancies to spread to the placenta during pregnancy.^26,27  In most of these instances, gross appearance of the placenta is unremarkable and small clusters of tumor in the perivillous space can be identified on careful microscopic examination alone. Although it is exceptionally rare for maternal malignancies to metastasize to the fetus via the placenta, intrauterine growth retardation and death may result when large volumes of tumor occupy the placenta.

Lastly, true congenital leukemia must be ruled out regardless of the patient's karyotype. Rarely, congenital lymphoblastic leukemia can occur in DS neonates and should be distinguished from TAM.²⁸  Mora et al²⁹  reported congenital nonlymphoblastic (myelomonocytic) leukemia in a non-DS neonate with karyotypically normal leukemic blasts that infiltrated the peripheral blood, the skin, and the placenta. Similar to TAM, these immature cells infiltrated the placental chorionic and fetal stem blood vessels, and the neonate had a self-limited clinical course. In contrast, congenital leukemia with karyotypic abnormalities other than T21 may show poor prognosis. Fortunately, these are all rare entities, and yet they underscore the importance of karyotypic or GATA1 mutation analysis and confirmation of diagnosis prior to considering treatment.

The majority of TAM cases will undergo spontaneous resolution without need for treatment. These patients can be managed conservatively with supportive care only. The white blood cell counts normalize and blasts disappear from circulation within several weeks to the first few months of life.^2,7  However, severe cases with associated hydrops fetalis, hyperviscosity syndrome, organomegaly, and cardiopulmonary failure are treated with a short course of low-dose cytarabine (0.5–1.5 mg/kg), which has been shown to decrease the risk of death.^1,7,30  Leukapheresis and exchange transfusions may be performed as temporary treatment for hyperviscosity syndrome.⁷ 

Despite treatment measures, the overall case-fatality rate attributed to TAM is approximately 10%, and the overall mortality rate of DS with TAM approaches 20%. Approximately 20% to 30% of TAM cases will relapse and progress to AML-DS, frequently characterized by acute megakaryoblastic leukemia, within the first 4 years of life.^1,2  Although TAM and AML-DS have similar molecular, morphologic, and immunophenotypic features, AML-DS is not transient and requires intensive cytostatic chemotherapy to achieve complete remission. Nevertheless, AML-DS has a favorable prognosis compared with that of non-DS children who develop AML, with patients having a 3-year 80% survival rate.¹ 

Our review highlights the importance of careful sampling and histopathologic evaluation of placentas, especially those that appear grossly unremarkable. Microscopic placental examination may reveal clinically and hematologically silent TAM, confirm overt cases of TAM, and, depending on the extent of placental involvement (ie, presence of fetal vascular malformation, extravasated blasts in the perivillous space, etc) may sometimes predict the severity of disease. Furthermore, histologic evaluation offers a reliable way to determine the cause of death in macerated stillborn fetuses or when permission for fetal autopsy is not granted. Information obtained from these placentas can be used in parental and genetic counseling. Immunohistochemical studies and modern molecular analyses can be applied to formalin-fixed and paraffin-embedded placental tissue to arrive at an accurate diagnosis in TAM. Herein, we provide a comprehensive review on placental involvement by TAM, which thus far has been described mostly as case reports. We also emphasize the broad differential diagnoses when abnormal cells and cell clusters are encountered in the placenta, whether found within the fetal vasculature or in the maternal space.